7552 Architecture

AT/MCA Compatibility
Backplane Connectors
Backplane Modifications
7552 Card in Model 60?
Programmable Option Select
BIOS Origins

The vast majority of the information presented here comes from the following source:
"Surface Computer System Architecture for the Advanced Unmanned Search System (AUSS)"
Technical Report 1538, Dec 1992, unclassified NCCOSC document - PDF, alternative PDF.

AT/MCA Compatibility (from NCCOSC)

In theory, the 7552 PCs were supposed to be "AT compatible." After receiving our first unit, we discovered that the machines were in fact only partially compatible. The bus design turned out to be a hybrid of the AT Industry Standard Architecture (ISA) design and the new PS/2 Micro Channel (MCA) design. The biggest compatibility problem with the ISA specifications was that some signals were left off the bus. In addition, certain details, such as the address location of the keyboard port, were changed in such a manner that the 7552 was incompatible with some operating system software. These incompatibilities are covered in detail in appendix A, which discusses bus modifications. After the initial evaluation of the first 7552 was completed, it was decided to remove the 10 MHz 80286 CPU and memory cards and replace them with third-party 3 20 MHz 80386 CPU/memory cards. The net result gave us ISA bus compatibility and software compatibility with our desktop machines, plus a significantly higher processing speed. The 7552 enclosure, passive backplane, power supply, and card shrouds and adapters have been kept and have proven to be reliable and trouble free.

Backplane Connectors

The IBM 7552 industrial computer uses a passive backplane design. Feature cards plug into the bus via two 96-pin (3 x 32) DIN connectors. The signals assigned to the backplane bus are composed of signals from three sources:

  • IBM Micro Channel (16-bit)
  • IBM PC AT bus subset
  • IBM 7552 unique signals

Ed. Tom: The IBM 7552 backplane features 9 card positions, One of which is typically unusable because the DASD module physically occupies two slot positions but uses only one pair of backplane connections.

Feature Adapter/Backplane Connectors

System Bus Backplane Connector J01-J09

PinSignal NamePinSignal NamePinSignal Name
C30+5 VDCB30-DACK7 (1)A30-DO 1
C29-PR PWR CKB29DRQ6 (1)A29-IRQ8
C28+12 VDCB28-DACK6 (1)A28-IRQ1
C27-TEMP CKB27DRQ5 (1)A27-IRQ15
C26-12 VDCB26-DACK5 (1)A26-IRQ14
C25-P/S CKB25-XMEM W (1)A25-IRQ12
C24+5 VDCB24DRQ0 (1)A24-IRQ11
C23ReservedB23-XMEM R (1)A23-IRQ10
C21-CD DS 16B21IRQ15 (1)A21DPAR 1
C20ReservedB20IRQ12 (1)A20D15
C19-SBHEB19IRQ11 (1)A19D14
C18-REFRESHB18IRQ10 (1)A18D13
C17-DS 16 RTNB17-IO CS16 (1)A17D12
C16-SFDBK RTNB16-MEM CS16 (1)A16D11
C15ReservedB15-BHE (1)A15D10
C13GNDB13LA1 (1)A13D8
C12CD CHRDYB12LA2 (1)A12D7
C10-CD SFDBKB10LA3 (1)A10D6
C09+5 VDCB09T/C (1)A09D5
C07+5 VDC CONTB07LA5 (1)A07D3
C06M/-IOB06IRQ3 (1)A06D2
C05+12 VDCB05LA6 (1)A05D1
C04-CMDB04IRQ4 (1)A04D0
C03+5 VDCB03LA7 (1)A03DPAR 0
C01GNDB01LA8 (1)A01GND

System Bus Backplane Connector J10-J18

PinSignal NamePinSignal NamePinSignal Name
F32GNDE32LA9 (1)D32GND
F31-TCE31LA10 (1)D31-S1
F30+5 VDCE30IRQ7 (1)D30-S0
F29ARB/-GNTE29CLK (1)D29A0
F28+12 VDCE28LA11 (1)D28A1
F27ARB3E27LA12 (1)D27A2
F26-12 VDCE26DRQ1 (1)D26A3
F25ARB2E25LA13 (1)D25A4
F24+5 VDCE24-DACK1 (1)D24A5
F23ARB1E23LA14 (1)D23A6
F21ARB0E21LA15 (1)D21A7
F20ReservedE20-DACK3 (1)D20A8
F19-IRQ7E19LA16 (1)D19A9
F18-IRQ6E18-IOR (1)D18A10
F17-IRQ5E17LA17 (1)D17A11
F16-IRQ4E16-IOW (1)D16A12
F15-IRQ3E15LA18 (1)D15A13
F14-IRQ9E14-SMEM R (1)D14A14
F13GNDE13LA19 (1)D13A15
F12-BURSTE12-SMEM W (1)D12A16
F10-PREEMPTE10IRQ9 (1)D10A17
F09+5 VDCE09-SETUP 1 (1)D09A18
F08-ADL(ALE)E08-SETUP 2 (1)D08A19
F07-5 VDCE07-SETUP 3 (1)D07A20
F06MADE 24E06-SETUP 4 (1)D06A21
F05+12 VDCE05-SETUP 5 (1)D05A22
F04-CD SETUPE04-SETUP 6 (1)D04A23

   (1) IBM PC AT Unique Signals

Backplane Modifications (from NCCOSC)

To work around software and hardware incompatibilities, the standard IBM 7552 bus was modified to make it capable of being 100 percent compatible with the full IBM PC AT Bus standard. To accomplish this, modifications were made to the IBM PC feature adapter. A side view of the adapter is shown below in figure A-1. This adapter accepts a standard IBM PC AT accessory card and then maps its bus signals to lines on the 7552 bus. A standard AT card mates to a 98-line bus via a 62-pin and a 36-pin edge card connector. The feature adapter has the appropriate mating edge connectors on one side into which the AT card is inserted. On the other end of the feature adapter are two 96-pin DIN connectors that mate to the 7552 backplane. In between, traces are etched that map AT bus signals to 7552 bus signals. For whatever reason, IBM chose not to support all the AT bus signals on the 7552 bus. The signals left off were:

  • DMA level 2 (DRQ2 and -DACK2)
  • Interrupt levels 6 and 14 (IRQ6 and IRQ14)
  • AT adapter cards that are bus masters (use the -MASTER line)
  • Zero wait state bus cycles

To reinstate these signals and achieve sufficient compatibility to the IBM PC AT standard for our hardware and application software, the original 7552 CPU and memory cards were removed and replaced with third-party CPU/memory cards designed for operating in AT-compatible passive backplanes. The standard 7552 CPU cards incorporated changes that used some AT signals and some Micro Channel signals. This hybrid architecture created hardware and software incompatibilities with our surface computer designs, necessitating replacement of the 7552 CPU and memory cards. Once replaced, the 7552 bus was made compatible with the AT standard by adding the necessary missing signals back onto the bus via changes to the feature adapter signal mappings. The bus signal definitions as delivered for the 7552 are shown above.

7552 Card in Model 60? (by Major Tom)

In the "Bus Wars" episode of the Computer Chronicles Chet Heath showed a modified Model 60 with a special processor board to demonstrate the possibility and benefits of multiple processors on the Micro Channel bus.

A modified Model 60 with the "test tool" processor board installed

Compare the form-factor of this test card to the 7552 Processor Board (pictured below). They seem to be approximately the same size and the two backplane connectors appear to be situated in the same area on both boards. Even the position of the PCB mounting holes is a close match...

7552 Processor Board

In the video, Chet says: "This is a Model 60 with a test tool built by Ernie Mandese at Boca Raton... that was built in 1986 to demonstrate - to our satisfaction - that the Micro Channel could support a number of processors."

Hmm, has the 7552 design started its life as a development/test platform for the Micro Channel Architecture?

Programmable Option Select (by Major Tom)

The POS functionality known from the other x86-based MCA systems is implemented somewhat differently here. There are 9 device positions in total (compared to the usual 8 or less) - one of which is used by the CPU card. The last (rightmost) position is typically unused as the connector is physically blocked by the two-slot-wide DASD module. Similar to the standard MCA the 7552 uses 2-byte adapter IDs to identify the individual cards (this possibly applies to the CPU card as well). The "Setup" option of the Diagnostic Diskette can be used to view the installed adapters/devices:

IBM 7552 "Setup" Device List (all slots empty, incompatible system used)

The system doesn't use the standard ADF files known from the PS/2 world. The supported device IDs and description/configuration data are likely all "hardcoded".

When Int 15h Func. C0h is called on the 7552, the machine identifies itself as a Dual-Bus System and reports the presence of the Micro Channel bus. It supports the standard Int 15h Func. C4h - "Programmable Option Select" call as well (incl. subfunctions 00h, 01h, and 02h). The base POS register address is 100h as in the case of the standard PS/2s. The 7552 uses a non-standard port E1h to enable/disable setup mode for one of the 9 devices (PS/2s use port 96h for adapter setup and 94h for planar setup).

Why does the 7552 use a unique MCA/POS implementation and why IBM never widely marketed the feature? Maybe they modified it to separate the industrial implementation from the consumer PS/2 world? Or perhaps what we see here is an *earlier* Micro Channel iteration that started as a development platform and IBM later decided to market it? We can only speculate...

The later 7568 "Gearbox 800" shares the same form-factor with the 7552 but the underlying MCA/POS implementation has been reworked to closely match the rest of the PS/2 world.

BIOS Origins (by Major Tom)

The 7552 system BIOS is rather unusual as well. Unlike all the other x86-based MCA machines, the 7552 has a single-segment BIOS - 64 KB. One half of this space is consumed by the 32 KB ROM BASIC. And the BIOS code itself is much closer to the IBM AT codebase, than to the earliest known PS/2 firmware - the Model 50/60 ROM revision 0. It also completely lacks the ABIOS code...

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