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2May/170

V86P Expansion

Retrochallenge RC2017/04 might be over, but I'm going to try and keep moving on this project. I'd already started on the pinout for the internal connectors used by the hard disk controller, and finishing that off seemed like a fairly straightforward task. Having done that, the Expansion Bus header on the back of the system was just a matter of following the same process.

I started off with the hard disk controller, a multimeter, and the datasheet for the OMTI 5090 IBM PC Bus Interface controller. The datasheet has the pinout for the IC, along with a table identifying which pins were for the system bus, and which were for the system interface. For each of the system interface pins, checked for continuity to each of the pins on the header connecting to the motherboard. This got most of the header identified, with just a few unlabelled pins. To get those, I basically did the same thing on the motherboard, except in this case identifying the signals was more difficult. On the motherboard, many of the signals need to be buffered (or in the case of address lines, latched). For that, Victor just used 74-series logic - 74HCT244 and 74HCT373 specifically, which is well documented. I traced the signals from the connector to the appropriate buffer output, looked up the input on the datasheet, then traced the input back to the main Chips & Tech 82C100. From there I could identify the signal.

There are three headers of interest - two which connect to the motherboard, and another labelled TP1, presumably test points. After ringing out the connectors, the latter may actually be a jumper-block used during testing.

After getting the pinouts for the connectors to the HDD controller, I repeated the same process for the expansion bus connector - the results of which were pleasantly surprising. It turns out to be essentially the same pinout as a regular 8-bit ISA connector, but with a couple of differences. Firstly, the pins that would typically provide power rails are replaced with ground pins; pins A1 and B1 are not present, with /IO CH CHK is relocated to pin 13; and pin 7 is replaced with a 4.7k pull-up resistor to Vcc. 

With all that said, here are the pinouts:

CN1 - to motherboard

A_19 1 2 A_18
Gnd 3 4 Gnd
Vcc 5 6 Vcc
AEN 7 8 IRQ_5
A_17 9 10 A_16
A_15 11 12 A_14
A_13 13 14 A_12
A_11 15 16 A_10
A_9 17 18 A_8
A_7 19 20 A_6
DRQ_3 21 22 RSTIN
/DACK_3 23 24 A_5
/IORD 25 26 A_4
/IOWR 27 28 A_3
A_2 29 30 A_0
A_1 31 32 Gnd
  33 34  
Vcc 35 36 Gnd
Gnd 37 38 Vcc
/MEMR 39 40 Gnd

 

CN3 - to motherboard

D_0 1 2 D_1
D_2 3 4 D_3
D_4 5 6 D_5
D_6 7 8 D_7

 

TP1

Gnd 1 9 DT_0
Gnd 2 10 DT_1
Gnd 3 11 DT_2
Gnd 4 12 DT_3
Gnd 5 13 /CNTA
Gnd 6 14 /CNTB
Gnd 7 15 /ROMDIS
Gnd 8 16 /RADR

 

CN2 - to hard disk

Gnd 1 2 /READ
Gnd 3 4 /WRITE
Gnd 5 6 Reserved
/SELECT 7 8 /SHIP_READY
Gnd 9 10 /WRITE_GATE
/MOTOR_ON 11 12 /HEAD_1
/DIR_IN 13 14 /STEP
/WRITE_FAULT 15 16 /SEEK_COMPLETE
/SERVO_GATE 17 18 /INDEX
/TRACK_0 19 20 /READY
Gnd 21 22 5V
Gnd 23 24 5V
Gnd 25 26 12V

 


Expansion Bus Connector

RESET 1 2 D7
Gnd 3 4 D6
IRQ2 5 6 D5
Power On 7 8 D4
DRQ2 9 10 D3
Gnd 11 12 D2
/IO CH CHK 13 14 D1
Gnd 15 16 D0
Gnd 17 18 IO CH RDY
/SMEMW 19 20 AEN
/SMEMR 21 22 A19
/IOWR 23 24 A18
/IORD 25 26 A17
/DACK3 27 28 A16
DRQ3 29 30 A15
/DACK1 31 32 A14
DRQ1 33 34 A13
/REFRESH 35 36 A12
CLOCK 37 38 A11
IRQ7 39 40 A10
IRQ6 41 42 A9
IRQ5 43 44 A8
IRQ4 45 46 A7
IRQ3 47 48 A6
/DACK2 49 50 A5
TC 51 52 A4
ALE 53 54 A3
Gnd 55 56 A2
OSC 57 58 A1
Gnd 59 60 A0

Italics indicate differences in pinout to the standard ISA pinout

Pin 7 appears to be a 4.7k pull-up to Vcc, presumably to indicate when the system is powered on

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