ICUS II Hard Disk Upgrade
Written by Gil Kloepfer, Jr. & Lenny Tropiano

The following text was originally published as the Revision 2.0, January 1, 1992 ICUS Hard Disk Upgrade, and contains information for upgrading the AT&T UNIX-pc (3B1) to support larger MFM hard disks and an additional disk as needed. It was originally sold as a kit, and is now being made available for no charge. However, there is ABSOLUTELY NO WARRANTY for the use or misuse of this information. There is NO SUPPORT available from the author, since he no longer has a 3B1 system for testing. Some of the text has been modified to make sense in this HTML format.

This upgrade guide and its contents contains information that is proprietary to Gil Kloepfer, Jr. and Lenny Tropiano, and may not be used for or sold for profit of any kind without the express, written permission of the above mentioned persons. Any use of the information contained herein is at the user's own risk. ICUS Software Systems and the authors of this guide bear no responsibility for the use or misuse of this information, nor do they bear any responsibility for any damages or injuries resulting from the installation of the upgrade described herein. Furthermore, this document carries with it no warranties, express or implied. ANY PORTION OF THIS DOCUMENT DUPLICATED AND/OR DISTRIBUTED MUST HAVE THE ABOVE PARAGRAPH INCLUDED WITHOUT MODIFICATION.


The SSDL/ICUS UNIX-pc Hard Disk Upgrade is a hardware addition and modification which provides a simple, cost-effective method of providing three additional functions to the UNIX-pc: to upgrade a P3...P5 motherboard to a P5.1 motherboard, add a fourth head-select line, and add a second drive select line. This upgrade, together with the proper hardware additions, allows larger disk drives and a second hard disk drive to be used with the UNIX-pc.

The circuitry developed for this upgrade is based on information contained in an unpublished field upgrade designed at Convergent Technologies/AT&T before the UNIX-pc line was discontinued. The hardware portion of this modification defines what is known internally at AT&T as a P5.1 motherboard - that is, a motherboard which contains a new external register known as MCR2 (miscellaneous control register 2). MCR2 provides the hardware/software interface necessary to enable the additional hardware capabilities described above.


This upgrade and its information are mainly for those who feel comfortable with a soldering iron and wiring circuits, and who don't feel at all funny about poking around on their UNIX-pc motherboard. The nature of the upgrade requires a motherboard modification and approximately 18 wires soldered from various points on the motherboard to a socketed location. It isn't for the squeamish or casual computer user.

Even if you cannot perform this upgrade, you may wish to read this guide to gain further knowledge about how additional hardware may be added to your UNIX-pc, or save the guide for future reference when you may desire these modifications.

Someone experienced in the construction of electronic circuitry, either through a hobby or by profession, should note that the construction and installation of the upgrade will take approximately 3-5 hours to complete. Even the most ambitious readers should allocate this time to assure that all steps of the upgrade are performed correctly and tested completely before turning on the system power supply. It is strongly recommended that this entire guide be read completely before attempting to perform the upgrade.


The following changes and enhancements are incorporated into this manual, version 2.0:


The upgrade itself involves the following signals:

Signal Description
Vcc, GND Power for the PAL. In version 1.0, this also was the power supply connections for the extension board.
Tel Status* Telephone status port request. Part of this port is used to sense the P5.1 feedback signal.
MCR2* MCR2 port request. This signal is active low when data is written to the external MCR2 register.
RST* Reset. Required to reset the MCR2 register to re-select the primary hard disk drive and deselect the 4th head select line.
D0 Data bus bit 0. D0 on MCR2 is the 4th bit of the head select nibble.
D1 Data bus bit 1. D1 on MCR2 is the second hard disk select bit.
D4 Data bus bit 4. D4 on MCR2 is asserted and deasserted by the operating system and sensed through bit D4 of the Tel Status signal. This is what tells the operating system that the motherboard is a P5.1 revision.
HDSEL3* Head select bit 3 (4th head select). Part of the 34-pin disk control cable.
DDRIVE1* Drive select 1 (2nd drive). Output from the PAL to select disk drive 1 (connected to pin 28 of the 34-pin disk control cable).
DSKDAT0 Disk data from drive 0 (Called Rd Dta, Dr 0 in version 1.0). The disk data received from the first hard disk (through a 26LS32) is connected to a 2:1 multiplexer at this point. This is necessary to arbitrate contention for the disk data separator. This signal is obtained from the pin lifted (disconnected) from the motherboard at 13K pin 3.
DSKDAT1 Disk data from drive 1 (Signal was internal to the extension board in version 1.0). The disk data received from the second hard disk (through a 26LS32) is connected to the 2:1 multiplexer at this point. See DSKDAT0 for additional details. The signal DSKDAT1 is obtained from the lifted pin (pin 11) on IC 13K.
DDRIVE0* Drive 0 select (not used in version 2.0). Signals the circuitry on the extension board that drive 0 is being selected.
DSEPIN Data separator input (was called Dta Sep In in version 1.0). This is the output of the 2:1 multiplexer and is connected to the input of the disk data separator. The entry point for the data separator is the pad where the pin was lifted on the motherboard at 13K pin 3.
Disk Write Dta Disk write data (no longer used in version 2.0). The TTL-level data was transferred to the extension board through this pin. The signal is buffered and transferred through the disk data cable. This function is now performed on the motherboard.
SYSCLK System clock (not used in version 1.0). The system clock provides some timing functions within the PAL for certain asynchronous operations.

These signals are processed in a preprogrammed PAL (Programmable Array Logic) device which performs decoding of the extra signals needed in the upgrade, contention for the data separator, and feedback for P5.1. The write data to the second disk is provided by a shared bus driver (used for the first hard disk). The read data from the second drive is buffered through a spare section of the 26LS32 (at 13K) already on the motherboard. Three pins (in addition to the one that is lifted to decouple the first drive's data directly from the data separator) will be lifted on this chip so that access may be gained to an unused portion of the chip.

The PAL's internal operation may be seen in block form in Appendix VI. More detailed information about the operation of the upgrade may be understood by looking at the schematic diagram for the version 1.0 upgrade in Appendix VII.

The reason why the number of chips were able to be reduced relative to the chip-count in version 1.0 is due to the functionality provided by the PAL, and some utilization of line drivers and receivers already on the motherboard. The line driver used by the first hard disk drive may also be used to drive the second hard disk. Originally, we did not consider this option. Since the "write" data is the same for both drives (it will only be recognized when the appropriate drive has been selected), there is no need for an additional line driver. The line receiver at 13K is only half-utilized as configured on the motherboard. By lifting 3 pins for the unused line receiver, one is able to utilize the receiver in other circuitry.


Step one - BACK UP YOUR HARD DISK! If you goof, you could erase all the data on the drive. No further explanation here - if you are confident enough to work with an unbacked-up drive, good luck.

Note that the following procedures and equipment list only refer to the version 2.0 upgrade. If you are interested in building the extension board and performing the upgrade as in V1.0, you will need to form your own parts list and proceed with the sketchy comments in Appendix VII.

Before beginning construction, be sure that you have the following supplies on-hand (at LEAST):

These are the minimum supplies you'll need to get started on this project. Don't start without any of the above or you'll be stopping halfway through to wait for parts. Also note that you will be working with static-sensitive materials throughout the upgrade. Take necessary precautions (in addition to those listed here) where appropriate. Also note that all the connectors are the IDC-type. These are the type that require no stripping or soldering -- you simply insert the wire and crimp the connector closed. Well, kind of simply. These connectors are fragile, and it is important to treat them with care.

Before starting to work on this project, choose a suitable work area with plenty of flat, well-lit space and a comfortable chair. When you have found this place, USE THE OUTLET TESTER TO CHECK ALL OUTLETS THAT YOU WILL BE USING! No matter how confident you are about the electrical wiring in your work area, DO NOT SKIP THIS STEP. If your outlets are miswired, you may burn-out your computer and hard disk drives, at least. You may also electrocute yourself and die. When you are confident that the outlets are wired correctly and grounded, prepare the work surface by placing the grounded anti-static mat on the table and begin.

The motherboard modifications must be made with extreme care. If you have never soldered wires to a PC board or feel funny about soldering jumpers between solder pads, get a broken transistor radio and practice on that a while. We (probably wrongly) assume that most users performing this upgrade will have some experience with this kind of procedure. Do not try to use telephone wire or any other type of wire except for the type listed in the parts list. Although wire-wrap wire is thin and sometimes tricky to use to perform point-to-point soldering, it is more appropriate for the kind of wiring you will need to perform.

Removing the motherboard requires that the UNIX-pc be opened completely, that any expansion cards be removed, and that many screws be removed from the cabinet. Keep these screws in a safe place, organized in such a way that you can put them all back properly. Strip the machine down to the point where the motherboard/backplane (bottom) section is completely separated from the upper (screen/disk/power-supply) section of the machine. Note the orientation of the disk drive cables on the motherboard! If you have never removed the cover of a UNIX-pc, you will need to talk to someone who has. The procedure is too complex to cover in this manual (suffice to say that you can figure it out, but you should be careful to unplug the power cable from the power supply and the monitor connector from the motherboard BEFORE lifting the top shield from the motherboard assembly). The motherboard is physically removed from this assembly by removing the dozens of screws around the perimeter of the motherboard, removing the four metal stand-off posts with a nut driver or adjustable wrench, and removing the four screws on the edges and bottom of the cabinet which retains the connector mounting bracket in the back. The motherboard should slide out (toward the back) from the backplane edge connector. When you have the motherboard out, place it on the anti-static mat and become familiar with the IC designations and any peculiarities with the board. Note that this is a multi-layer PC board.

The way the modifications were done in the prototype, and the way it is suggested that you do them, is to insert a 20 pin machined socket to the motherboard at location 27P (unused) and terminate all motherboard jumpers at that point. This procedure is the same as that used for the field upgrade P5.1 that Convergent/AT&T is currently using. The V2.0 upgrade PAL pinout shown in Appendix I is designed such that very few changes will be necessary if you are converting a motherboard which has the P5.1 PAL modification installed. If you have the P5.1 PAL installed, you should see Appendix IV for a step-by-step procedure for upgrading from this level. If you are installing the upgrade new, you will need to clean the solder out of the 20 holes where the socket will go, and the two holes above that where a bypass capacitor will be installed. This is done using your favorite desoldering method. Be careful not to damage the PC board. Install the IC socket in the holes, and only solder pins 10 and 20 onto the PC board at this time! Doing this will hold the socket in place, and not soldering the others yet will make the jumpers easier to connect at those places. NOTE: Pins 10 and 20 are GND and Vcc respectively, and are permanently part of the PC board.

Before beginning installation of any of the jumpers, all modifications at IC 13K should be completed. These modifications must be VERY carefully performed. Extract the solder from pins 3, 9, 10, and 11 of IC 13K. Using an X-acto knife or similar, cut these pins as close to the PC board as possible, and lift (WITHOUT BREAKING!) the remaining part of the pin halfway up. Any existing jumpers on pin 3 (some motherboards have been modified at the factory like this) should be removed and soldered to the PC board in the hole where the pin was lifted from. As per Appendix I, one of the expansion jumpers will be soldered to the part of pin 3 that is lifted, another will be made to an location on the motherboard electrically connected to the place where the pin was lifted from. If you break IC 13K, you will need to extract the entire chip and replace it. Be careful. See Appendix II to see how this modification is electrically involved in the circuit. At this time, you should fill-in all of the holes in the motherboard at 13K where pins were lifted with solder.

Install all the jumpers according to Appendix I. Note that the V1.0 photos (if you have them) do not reflect the wiring as it is in the V2.0 upgrade, but they may be useful to observe how the wiring can be routed. Try to keep the wiring neat, off solder pads, unobstructed, and "kind of" together. This will allow easier testing and less adhesive later. When all the jumpers are installed, turn the board to the component side of the motherboard and test each connection according to the jumper list. The test should be performed with an ohmmeter or a low-current/voltage continuity tester (as available on some multimeters). Double check the connections for their integrity. When you are satisfied that the connections have been made properly, epoxy the wire-wrap wire to the PC board. Use electrician's tape to hold the wire temporarily still while applying the epoxy and while waiting for it to set. Use the epoxy sparingly, and only "dot" it in appropriate spots where the wire needs support to the board. While the epoxy sets, solder the bypass capacitor to the motherboard in the two holes above the 20 pin socket you installed.

When the epoxy is sufficiently dry (about a half-hour to 45 minutes, take a break while waiting), carefully replace the motherboard in the bottom assembly noting the alignment of the motherboard to the backplane edge connector, the holes where the screws go in the motherboard, and the tabs in the back for the connector mounting plate. Be careful here, and make sure you don't pinch or catch the motherboard jumpers on extruding items from the cabinet.

Place a small bare wire jumper from pin 3 (DSKDAT0) to pin 13 (DSEPIN) on the socket you installed. This will configure the motherboard as if there were no upgrade installed. Assemble the unit as much as you feel comfortable to be able to turn it on, and boot the system. The system should operate as it always does. If everything works up to this point, power-down again and get to the top of the motherboard. Remove the jumper in the socket and plug-in the PAL. Power the machine up again. The machine should, again, operate normally with the exception that you should see the message System board is P5.1 on bootup. If this works OK, bring the system down again and proceed to the next section. If there were any problems with the above steps, stop and re-check all your work. Especially make sure that all pins on the PAL are properly in-place.

You will now perform the last of the motherboard modifications - a 20 pin male jack must now be installed for the second disk's data cable. Note that unlike the 34 pin control cable, the 20 pin data cable may not be daisy-chained. There are two ways to perform this step, and both methods will be presented. First, common to all methods, is to cut a 5 inch piece of 20 conductor ribbon cable. Basically, of the 20 pins on the data cable, only 10 are used. The other 10 are left disconnected. The normal way to prepare the connector is to insert an entire 20 conductor strip of cable into the 20 pin male DIL (dual in-line) IDC (insulation displacement connections) connector, and leave the first 10 disconnected on the other end. However, if you feel confident in handling IDC connectors, it is recommended that you strip-off 10 conductors and only insert the cable into the last 10 (5 in-line) connector positions. This way there are no disconnected wires hanging off on the other side. Take a close look at the connector. On one of the edges should be a notch, and if the open end of that notch is pointing up, you should be able to locate an arrow to the left of this notch. This arrow signifies pin 1 of the connector. Unlike DIP chips, DIL connectors number the pins in a zigzag pattern, for example pin 2 is above pin 1, pin 3 is next to pin 1, and pin 4 is next to pin 2, and so on. It is important to understand this so that you can check your wiring, and so you understand why the numbering of the ribbon cable is still sequential. Holding the 20 pin male DIL IDC connector with the pins pointing out toward you, and the notch on the bottom, insert the ribbon cable into the designated place at the back of the connector up from the bottom, with the colored stripe toward the left side (pin 1). If you have stripped off 10 conductors from the cable (performing this step the alternate way), the 10 remaining conductors should be right-justfied, leaving the leftmost 10 positions empty. At this time, carefully crimp the cable onto the connector. If you are at all unsure of whether you have properly performed this step, stop and re-trace your steps. If you used all 20 conductors, when you're done installing the connector on one side of the cable, go to the other side and zip the cable down the middle about 1.5 inches (so that conductors 1-10, and 11-20 are separated from each other). Take a pair of scissors and cut off conductors 1-10 at the 1.5-inch mark, so that conductors 11-20 extend further than 1-10. In the next procedure, you will only work with conductors 11 through 20. Those who have only installed conductors 11-20 (half of the 20 conductor cable) should realize that the first conductor is 11, and so on.

Soldering various conductors of the ribbon cable is made easier if conductors 11-20 are zipped in groups of two. Zip each group of two (eg. 11-12, 13-14, etc.) down to the 1.5-inch mark from before. Strip a very small amount of insulation from conductors 11-12, twist them together, tin the leads, and tack-solder them to pin 8 of IC 14H (26LS31) on the motherboard. The tack-soldering refers to tinning (wetting both sides with solder) then just soldering the wire to the connection. It may help to use a pair of needle nose pliers to hold the wires while the solder cools. Be conservative with the amount of heat and solder used to prevent damaging a chip or causing a short-circuit. Next, strip and tack-solder conductor 13 to pin 10 of 14H, and conductor 14 to pin 11 of 14H. Next, strip a small amount of insulation from conductors 15-16 and 19-20, and twist all 4 wires together and tin the wires. Tack-solder these wires to pin 8 of IC 13K (26LS32, the IC with the 4 lifted pins). There should be 2 more conductors remaining, 17 & 18. Take the 100 ohm resistor and cut the leads so that a quarter-inch remains on each side. Strip a small amount of insulation from conductors 17 and 18, and connect a conductor to each side of the resistor, pretty close to the resistor itself (away from the ends). Tack-solder the resistor lead with conductor 17 attached to it to pin 10 of IC 13K (a lifted pin). Solder the remaining resistor lead (with conductor 18) to pin 9 of 13K (also lifted). A good idea is to make sure that resistor is laying as close to the top of 13K as possible, to prevent it from shorting against the metal shield. Check all wiring of the socket, against the following table, and double-check that no wires have broken off while work was being performed.

(pins 1-10)No connection
11GND (14H pin 8)
12GND (14H pin 8)
1314H pin 10
1414H pin 11
15GND (13K pin 8)
16GND (13K pin 8)
1713K pin 10 & 100 ohm resistor
1813K pin 9 & 100 ohm resistor
19GND (13K pin 8)
20GND (13K pin 8)

Connections to 20 pin data cable for second hard disk drive

You have now completed all modifications to the motherboard. The 20 pin connector you have just installed should lay flat on the motherboard in a place where it will not interfere with the functioning of another part. Optionally, you may locate this connector outside the machine (additional cable may be used for this purpose. It is not recommended to crimp the 20 pin card-edge connector directly to this cable because it will mean performing the above soldering procedure every time the cable length needs to be changed.

In order to make the new disk cables, you will need to have the old cables out of your machine for comparison. To do this, remove the internal hard disk drive by removing the 4 screws holding it in place on the shield that the drive is mounted on. Carefully observe the direction that the cable is attached to the drive, then remove the two ribbon-cables. Use the remainder of the 20 conductor cable to make a cable which looks exactly (observe pin numbers and cable orientation when installing the connectors) like the 20 conductor cable currently in the machine, except the new cable will be longer. For the 34 conductor cable, place the 34 pin DIL connector on one end. Place the first card-edge connector approximately 8 inches from the DIL in the orientation of the old cable (use the old cable as a measuring "tape"). Install the other 34 pin card-edge connector on the other end of the cable, again assuring that it is in the same orientation as the first cable.

The original 34 conductor cable is not used further, and can be stored away in the event it is needed again. Install the internal hard disk using the original 20 conductor cable, and the first card-edge connector on the new 34 conductor cable. The new 20 conductor cable plugs into the 20-pin male DIL connector that was installed above and should be laying on top of the motherboard. Both the 20 and 34 conductor cables extend out the back of the UNIX-pc.

You should now prepare to put the UNIX-pc completely together again. There is plenty of clearance for two ribbon cables to slip out the case in the back. Be careful that the cables as routed through the back of the machine don't get pinched when the cover is replaced on the UNIX-pc. It is recommended that a layer of electrician's tape be put over the ribbon cables at the point where they may be temporarily squeezed in the case in order to prevent damage to the cables. It may also help to cut a very slim notch in the cover at this point so when the cover is replaced it is less likely that the cables will be pinched. Do not connect the second hard disk yet. Again, try booting the machine to make sure everything works.

Shut the machine down one last time. Install the second hard disk drive in its cabinet with power supply, making sure that the disk select jumper on the hard disk is set to the second drive designation, which may be 1 on some drives and 2 on others. This is usually a small jumper block in close proximity to the 34 conductor disk control cable connection. Although in the prototype, the terminating resistor pak for the internal drive was left intact, it is recommended that the terminating resistor pak for the internal drive be removed when operating the system with two hard disk drives. If you have decided to locate both hard disk drives external to the UNIX-pc, the drive in the center of the daisy-chain should have the terminating resistors removed. The last hard disk on the chain should always have the terminating resistor(s) installed. Plug both ribbon cables into the hard disk, paying close attention to the orientation of the connectors and cable-ends with respect to the hard disk drive (they should be oriented in the same way as the internal hard disk).

Check the outlet(s) you are going to plug the UNIX-pc and hard disk supplies one last time with the outlet tester. If all checks OK, then turn on the second hard disk supply, then the UNIX-pc. The system should boot normally.

If the second disk has never been formatted, or you wish to reformat the disk, use the following procedure:

Using the default partitioning, you can use both the "swap" partition and the user partition on the second drive as filesystems (these are [r]fp011 and [r]fp012). To get the second disk going, log in as root, and type:

     # mkfs /dev/rfp011  BE VERY CAREFUL WITH THESE
     # mkfs /dev/rfp012  DEVICE NAMES!!!

Now: # mount /dev/fp011 /mnta
     # mount /dev/fp012 /mntb

Try: # df -t

You should see three mounted disk devices and lots of free space on the /mnt* filesystems. Try copying things to these filesystems and using them. It should work OK.

If all worked normally, congratulations. You now have two hard disks on your machine. You may also install disk drives with more than 8 heads, and you have any other hidden goodies that come with a P5.1 motherboard.


The upgrade procedure described above is a generalized procedure which assumes that your motherboard looks like ours, and which assumes that your experience in wiring is sufficiently advanced to handle wiring problems which may occur. This is, perhaps, the most serious drawback in having an upgrade of this type. The benefits, however, are that the technical information will apply to all UNIX-pcs, and adaptation to other motherboards (if any problems occur) should be trivial. As of this writing and the publication of the UNIX-pc Technical Reference Manual, there are only 3 major revisions of the motherboard, all of which will work with this upgrade procedure.

By the very nature of using unshielded disk cables, a good deal of RF interference may be generated which tends to cause a large amount of television jamming to occur. As of the writing of this guide, we have not solved this problem and probably will not. Unfortunately, this makes your modified UNIX-pc non-FCC-compliant, and may destroy good relations with your neighbors. Our suggestion is to move ahead with the upgrade, experimenting with possible shielding methods after the project is complete. An area that should help is that the cables can be shielded by using a metal-encased ribbon cable. There still might be interference, but it should be reduced significantly.

Finally, the 3.51 and 3.51a revision of the OS only allows a maximum of 4 partitions (including the floppy partition) to be mounted simultaneously. This has been fixed in the 3.51m revision of the operating system, where 8 filesystems may be mounted.


We have been using the upgrade described in this manual for a while with a good deal of success. Throughout the upgrade, we have run-into various problems which we have tried to explain in this guide.

A prototype configuration has operated as a small /tmp partition and a /usr/spool/news partition (for a full news feed). Because of the enormous turnover of large quantities of data inherent in handling a news feed, it is felt that this has been a good overall test of the circuit's performance and stability. This in itself makes the concept of successfully performing this upgrade on other UNIX-pcs promising.

In addition to the extra disk space, this upgrade has also provided a learning experience in the means by which disk circuitry operates and how the disk drives are physically connected together. It is hoped are that others find this to be true, in addition to reaping the benefits of expanded disk storage.


Although we did the overall upgrade circuit design, as well as taking all photographs in V1.0, writing this manual, and performing the PAL design and duplication, there are others who helped in making this whole idea possible. We would like to thank John Milton for providing us with the fever for getting this idea on the road, to Jan Isley for his contributions and assistance in working with Lenny to get to the right people, and to all those on usenet who have taken the time to offer their assistance in various capacities. It is this sharing of expertise and knowledge which helps us all to grow professionally.


Pin #Jump-toSignal Name
127P pin 12 (see pin 12)CLK (PAL master clock)
226G pin 10TELSTAT* (Telephone status port)
3Lifted pin on 13K pin 3DSKDAT0 (Disk data from drive 0)
4No connectionNC
5Lifted pin on 13K pin 11DSKDAT1 (Disk data from drive 1)
622H pin 10 (pad, see diagram below)D1
722H pin 11 (pad, see diagram below)D0
827G pin 13MCR2SEL* (MCR2 select)
921M pin 2RESET* (System reset)
10(internally connected)GND
1127P pin 10 (permanently tied to GND)OE* (PAL output enable)
1227P pin 1 (see pin 1)CLKOUT (generation of PAL master clock)
1314M pin 3DSEPIN (Data separator input)
14No connectionNC
15No connectionFBACK (Internal P5.1 feedback latch)
16JH1 pin 28DDRIVE1* (Disk drive select 1)
17JH1 pin 2HDSEL3* (Head select 3)
1822M pin 24SYSCLK (System master clock)
1922H pin 7 (pad, see diagram below)D4
20(internally connected)Vcc

The following diagram shows which pads are internally connected to D0, D1, and D4. Although utilizing these solder pads during assembly will help to reduce the overall length of connections (which can cause timing problems), it is easy to confuse one solder pad with another. To avoid this problem, be sure to check to see that you are connecting to the proper solder pad by checking continuity between the solder pad and actual IC location noted above.

[Diagram of solder pads]


[Schematic at 13K pin 3]


The following additional commands are available using the expert-level diagnostics mode. These diagnostic commands are not listed in the expert-level command help, but are useful when performing this upgrade.

To utilize the additional diagnostics:

  1. Boot the UNIX-pc diagnostic floppy disk
  2. Enter "expert-mode" (enter s4test in response to the numeric menu prompt). Documented commands may be listed by entering ? in response to the expert mode prompt.

The following undocumented commands are available:

      2,n Do something to the second drive
      6,n Do something to the first drive

      n=  2:Perform format (no surface test)
           4:Sequentially check sectors (non-destructive)
           5:Randomly check sectors (non-destructive)
           8:Enter bad blocks
           12:List VHB and bad block information
           23:Perform destructive surface test (no format)


This appendix is a guide for those who have already installed the P5.1 Field Upgrade PAL (dated 4/9/86 and developed at Convergent Technologies). It will still be necessary to read this entire guide and check all work against the main text of the manual, however this text will help to proceed faster through some of the motherboard wiring. This appendix is organized as a step-by-step procedure that assumes familiarity with the remainder of this manual.

Conversion Procedure:

  1. Remove the P5.1 upgrade PAL. It will no longer be needed. Be sure that you do not confuse this part with the V2.0 upgrade PAL.
  2. Remove jumper at 27P from pin 4 to pin 19
  3. Move jumper currently at 27P pin 4 to 27P pin 19. The jumper originates from a solder pad on the motherboard electrically connected to 22H pin 7 (see Appendix I).
  4. Lift IC pins on 13K as described in the main procedure
  5. Add the following jumpers:
    Lifted pin at 13K pin 3 to 27P pin 3
    Lifted pin at 13K pin 11 to 27P pin 5
    Pad (from 22H pin 10) to 27P pin 6 (see Appendix I)
    14M pin 3 to 27P pin 13
    JH1 pin 28 to 27P pin 16
    22M pin 24 to 27P pin 18
  6. The wiring for the socket at 27P is now complete. Continue the main procedure from the point where the wiring for the 20 pin Male DIL IDC connector is installed.

It is now extremely important to check all of your work against the layout shown in Appendix I. Check all connections with the continuity tester to assure that all connections are correct. Also be sure that all jumpers removed in the above procedure are completely removed from the motherboard. Do not leave dangling wires laying inside the motherboard assembly. These could short against a pad on the motherboard and cause severe damage to the unit.

Although the procedure outlined in this appendix has been carefully checked, it has not been tested on a real 3B1 for integrity. If you are at all unsure as to the location of a jumper, Appendix I should be viewed as the ultimate authority.


This appendix is a guide for those who have already installed the ICUS V1.0 hard disk upgrade. It will still be necessary to read this entire guide and check all work against the main text of the manual, however this text will help to proceed faster through some of the motherboard wiring. This appendix is organized as a step-by-step procedure that assumes familiarity with the remainder of this manual. Note that following this procedure requires that the following be true:

  1. You have followed the Motherboard Socket Layout & Jumper Notes as shown in Attachment 2 of the ICUS upgrade V1.0 manual. If you used a different socket layout, you will need to derive the changes necessary on your own.
  2. Many of the jumpers to the V1.0 extension board socket will need to be extended to a pin further away from the socket than the originals. If you have not left enough slack in the wiring to accommodate this, you will likely need to remove all existing wiring and proceed with this upgrade from the beginning. Note that this appendix may still provide some useful information, so it may be helpful to skim-through it.

Conversion Procedure:

  1. You may unplug the V1.0 extension board. It will no longer be needed.
  2. Remove the jumper from 17K pin 9 to 27P pin 12
  3. Move the jumper at 27P pin 19 to 27P pin 2. This jumper originates from 26G pin 10.
  4. Move the jumper at 27P pin 4 to 27P pin 19. This jumper originates from a solder pad which is internally connected to 22H pin 7 (see Appendix I).
  5. Move the jumper at 27P pin 5 to 27P pin 16. This jumper originates from JH1 pin 28.
  6. Switch the jumpers at 27P pin 8 and 27P pin 17 (ie. put the jumper currently on pin 8 to pin 17, and the one on pin 17 to pin 8). When this operation is complete, the jumper originating at 27G pin 13 should be connected to 27P pin 8, and the jumper originating at JH1 pin 2 should be connected to 27P pin 17.
  7. Remove the jumper from 14H pin 9 to 27P pin 14
  8. Lift the three additional pins (pins 9, 10, and 11) on IC 13K (26LS32) according to the steps shown in the main procedure.
  9. Insert a jumper from lifted pin 11 on 13K to 27P pin 5
  10. Insert a jumper from 27P pin 1 to 27P pin 12
  11. Insert a jumper from 27P pin 11 to 27P pin 10
  12. Insert a jumper from 22M pin 24 to 27P pin 18
  13. Check to see that pins 4, 14, and 15 on 27P are left unconnected

It is now extremely important to check all of your work against the layout shown in Appendix I. Check all connections with the continuity tester to assure that all connections are correct. Also be sure that all jumpers removed in the above procedure are completely removed from the motherboard. Do not leave dangling wires laying inside the motherboard assembly. These could short against a pad on the motherboard and cause severe damage to the unit.

Although the procedure outlined in this appendix has been carefully checked, it has not been tested on a real 3B1 for integrity. If you are at all unsure as to the location of a jumper, Appendix I should be viewed as the ultimate authority.


The PAL equations are:

chip icusup pal16r4

; 1    2         3    4    5     6  7     8      9    10
clk /telstat dskdat0 nc dskdat1 d1 d0 /mcr2sel /reset gnd
;11   12     13   14   15      16      17      18   19 20
/oe clkout dsepin nc /fback /ddrive1 /hdsel3 sysclk d4 vcc


hdsel3 := /reset * d0

ddrive1 := /reset * d1

fback := /reset * d4

/d4 = /fback
d4.trst = telstat

/dsepin = ddrive1 * /dskdat1 + /ddrive1 * /dskdat0
dsepin.trst = vcc

/clkout = reset * /sysclk + /reset * /mcr2sel
clkout.trst = vcc
[Schematic of PAL function]


The following information (and the schematic following) is provided to those who are not familiar with the V1.0 upgrade. It outlines some of the theory behind the operation of MCR2, and presents a non-PLD alternative for performing the upgrade. Although it is not recommended that this upgrade be performed any longer, it has educational value.

The 5 chips used in the V1.0 upgrade are: 26LS31 (write data buffering), 26LS32 (read data buffering), 74LS273 (latches the data for MCR2), 74LS240 (provides input port for P5.1 signal to Tel Status port, second set of 4 gates used as inverters/buffers for other signals in the circuit), 74LS02 (provides additional logic necessary to handle disk data separator data contention). The 5 chips and a 20 pin disk data cable header may be perfboard or PC mounted, and are connected to the motherboard via a ribbon cable.

The circuit theory is best understood by studying the schematic in conjunction with the above signal information. In short, the 74LS273 D flip-flop is used as a latch to hold the data written to the address location referred to in the UNIX-pc operating system as MCR2. The flip-flop is reset (cleared) to a known state by sensing the hardware reset signal (RST*) upon power-up or by pressing the reset button. One half of a 74LS240 inverting tri-state buffer is configured to act as a single-bit input port designed to feedback the status of bit 4 of the MCR2 port on the telephone status port. This sounds confusing because it was a kludge developed to save hardware and to use existing input port locations in the UNIX-pc memory map. The P5.1 upgrade is sensed by writing a certain value to bit 4 of MCR2 and checking bit 4 of the telephone status port to see if the values match. When this occurs on system boot, the OS writes the "Main board is P5.1" message to the display, and an internal OS variable is set that signals to various drivers that this modification was installed on the motherboard.

The remainder of the circuitry performs electrical buffering of the disk drive signals, decoding of the select signals for the fourth head select and second drive select, and contention handling for the hard disk data separator located on the motherboard. The 26LS31 and 26LS32 handle the buffering of the disk read and write signals for the second hard disk drive. Because the data separator circuit is shared between the first and second hard disk drive, a circuit forming a single-bit multiplexer (constructed using 3 NOR gates from a 74LS02) is used to select whether the input buffer from the first or second drive should be routed to the data separator. The remaining half of the 74LS240 is continuously enabled and its gates are used as inverter/drivers for the remaining parts of the circuit. Note the use of higher fan-out gates for the head select and drive select lines. Although they were not specified in the original Convergent/AT&T field-upgrade, we noted an exceptionally high loading of the gates and ultimately decided to keep the buffer/inverter outputs for the signals directly connected to the disk drive isolated from other parts of the circuit. [Schematic of V1 upgrade]

Last revised: Mon Sep 12 09:37:01 CDT 2022
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Gil Kloepfer, Jr.,