Case Study - Restoring an Intel MDS-800

A horizontal ribbon.

Introduction

This is the story of my work to take a bunch of boards from Richard Main that came out of some MDS-800s and make them work. I have always wanted to get my hands on a real MDS-800 but never did get one. Richard was getting frustrated trying to get any of his to work so I offered to do the testing and debugging for him.

The Hardware Reference Manual (HRM) for the Intel MDS-800 I have is Intel Order Number 9800132C.
The Schematic Diagrams for the Intel MDS-800 I have is Intel Order Number 9800147A.
The Hardware Reference Manual (HRM) for the iSBC 064 I have is Intel Order Number 9800488B.
The Hardware Reference Manual (HRM) for the Zendex ZX-200A I have here.
The Hardware Reference Manual (HRM) for the iSBC 202 I have is Intel Order Number 9800420A.
The ISIS-II User's Guide I have is Intel Order Number 9800305F.

To do this I initially had:
1) An unknown MDS-800 CPU module 1 (Picture).
2) An unknown MDS-800 CPU module 2 (Picture).
3) An unknown MDS-800 Monitor module 1 with Monitor ROM (Picture).
4) An unknown MDS-800 Monitor module 2 with no Monitor ROM (Picture).
5) An unknown MDS-800 Front Panel Control module (Picture).
6) An unknown iSBC 064 RAM board (Picture).
7) A known good iSBC 064 RAM board (Picture).
8) A known good Zendex ZX-200A (Picture).
9) A new ATX Power Supply. (Picture)
10) An Electronic Solutions 7-slot Card Cage and Back Plane. (Picture)
11) The manuals listed above.

Configure the Card Cage/Back Plane

Back Plane Power

This Back Plane has no -10VDC. Early Intel Back Planes provided +/-5VDC, +/-12VDC, and -10VDC. Newer products had only +/-5 VDC and +/-12 VDC. The CPU module, Front Panel Control module, and Monitor module all require -10VDC from the Back Plane so I added the -10VDC modification to the Back Plane. That modification is shown here.

The Back Plane has a -5V regulator to supply the -5V need. Current ATX power supplies no longer provide -5V on pin 18. That is shown here.

I have cable harnesses to attach ATX power supplies to older IBM mother boards. I modified one of these cable sets to power the 9-slot Back Blane. See the wire lists below.

Modified ATX to PC Power Connector. Removed the white wire from the ATX connector (-5V) which new power supplies don't provide. Extended the gray wire to reach the Front panel bracket for the Power On LED and resistor. That modification is shown here.

        ATX Connector          J1      J2       Front Panel
        Pin     Function      Pin     Pin
        1       +3.3V --\/\/\/-+                  
        2       NC             |
        3       GND -----------+ 
        4       +5V ................... 4       
        5       GND ................... 1          
        6       +5V ................... 5           
        7       GND ................... 6      
        8       PWR OK ........................ PWRLED        
        9       NC        
        10      +12V .......... 4         
        11      NC
        12      -12V .................. 3
        13      GND ........................... PWRLED/PWRSW   
        14      PS ON ......................... PWRSW
        15      NC
        16      GND ........... 6
        17      GND ........... 1
        18      NC
        19      +5V ........... 2
        20      +5V ........... 3
                NC ............ 5
                NC .................... 2
        

Power Tests with No Boards

Time for the 60hz smoke and fire test! No Smoke! No fire! I verified all 5 voltages are correct on the Back Plane.

P2 Connectors for Back Plane

I removed the P2 Back Plane from the 7-slot chassis. Then I installed 2 each P2 connectors in the top two slots of the Back Plane. That is shown here.

The first part of the top P2 connector is used to connect the BPRN/ and BPRQ/ signals from the other multibus P1 connectors to the Front Panel module. This allows the use of the parallel bus priority circuit on the Front Panel Control module to work with the multibus. Since I have a 7-slot Back Plane, the priority 6 BPRN and BPRQ lines will not be implemented. The slots in the Back Plane are numbered from top to bottom. And the wiring of pins 25, 26, and 18 reqire the modules be inserted from top to bottom. The daisy chained connections between pins 16 and 15 need to be cut between pin 16 and the wirewrap post for all the connectors.

The only other pins (17-60), of this connector are wired one-to-one to the P2 connector in slot 2 (second slot down from the top) which connects to the CPU module, are 21, 30, 31, 48, 50, 52, and 54. The trace betweem pin 16 on each connector and the AX pin each need to be cut! See the wire list below.

Here is a picture of the completed Back Plane.

        Slot 1 P2   Function    Multibus    Slot 2 P2 
        Pin                     Pin         Pin
         1 ........ BREQ4/ .... B5          NC  
         2 ........ BPRN5/ .... A6          NC
         3 ........ BPRN4/ .... A5          NC
         4 ........ BREQ5/ .... B6          NC
         5 ........ BREQ3/ .... B4          NC
         6 ........ BPRN6/ .... A7          NC
         7 ........ BPRN3/ .... A4          NC
         8 ........ BREQ6/ .... B7          NC
         9 ........ BREQ2/ .... B3          NC
         10 ....... BPRN7/      NC          NC
         11 ....... BPRN2/ .... A3          NC
         12 ....... BREQ7/      NC          NC
         13 ....... BREQ1/ .... B2          NC
         14 ....... BPRN8/      NC          NC
         15 ....... BPRN1/ .... A2          NC
         16 ....... BREQ8/      NC          NC
         17 ....... BPRN0/ .... A1 (CONN ON BD) NC
         18 ....... BREQ0/ .... B1 (CONN ON BD) NC
         21 ............................... 21                                           
         30 ............................... 30                                           
         31 ............................... 31                                           
         48 ............................... 48                                           
         50 ............................... 50                                           
         52 ............................... 52                                           
         54 ............................... 54   
        

CPU Module

The CPU module contains the 8080 CPU for the MDS-800.

Visual inspection of the CPU module found no significant problems. Visual inspection of CPU module 2 found no significant problems.

J1 - In for Serial Priority is open on both modules.

Front Panel Control Module

The Front Panel Control module contains the BOOT EPROM for the MDS-800. It also interfaces the Front Panel to the MDS-800. It provides a 100-pin card edge connector to interface the Front Panel.

Visual inspection of the Front Panel Control module found no significant problems.

Minimal Front Panel Connections

I built a small adapter board which plugs into the P1 connector and contains a 40-pin connector for the actual front panel to plug into. The wiring between the P1 connector and the front panel elements is shown below:

P1	Function	40-pin
All Odd	Ground		1
All Odd	Ground		2
42	LED 0		3
90	SW0-O		4
94	SW0-C		5
44	LED 1		6
82	SW1-O		7
88	SW1-C		8
46	LED 2		9
86	SW2-O		10
84	SW2-C		11
52	LED 3		12
92	SW3-O		13
96	SW3-C		14
48	LED 4		15
66	SW4-O		16
68	SW4-C		17
50	LED 5		18
62	SW5-O		19
64	SW5-C		20
40	LED 6		21
74	SW6-O		22
76	SW6-C		23
38	LED 7		24
72	SW7-O		25
70	SW7-C		26
22	HLT LED		27
30	BOOT-O		28
28	BOOT-C		29
24	RUN LED		30
32	RST-O		31
36	RST-C		32
	NC		33-38
2-10	+5VDC		39
2-10	+5VDC		40
That is shown here.

1-2 T-OUT INTRPT are open. 3-4 T-OUT ACK pins are connected. 'C' CLK and 'B' CLK are connected.

Monitor Module

The Monitor module contains the Monitor EPROM and provides all the I/O for the MDS-800. It provides a 100-pin card edge connector to interface to the Back Panel I/O connectors.

The 2316 PROM used on the Monitor module does not match the pins on a 2716 EPROM. To use a 2716 in the socket an adapter must be built. See the wire list below.

        2316 PROM   Function    2716 EPROM
         1 ........... A7 .............. 1
         2 ........... A8 ............. 23
         3 ........... A9 ............. 22
         4 .......... A10 ............. 19
         5 ............A0 .............. 8
         6 ............A1 .............. 7
         7 ............A2 .............. 6
         8 ............A3 .............. 5
         9 ............A4 .............. 4
        10 ............A5 .............. 3
        11 ............A6 .............. 2
        12 .......... GND ............. 12
        13 ........... E2 ............. 20
        14 ........... E1 ............. 18
        15 ........... E0 ............. NC
        16 ........... D7 ............. 17
        17 ........... D6 ............. 16
        18 ........... D5 ............. 15
        19 ........... D4 ............. 14
        20 ........... D3 ............. 13
        21 ........... D2 ............. 11
        22 ........... D1 ............. 10
        23 ........... D0 .............. 9
        24 .......... VCC .......... 21/24
        

Visual inspection of Monitor module 1 found that R7 has burned. Visual inspection of Monitor module 2 shows R7, R14, and R17 are burned. R7 tied +12VDC to P1-26 - CRT CTS. R14 tied +12VDC to TTY RXD return. R17 tied -10VDC to TTY RDR CTL return. These appear to be caused by incorrect wiring of the CRT and TTY ports. All bad resistors were replaced. Monitor module 1 also has 1 engineering change on the botton of A5 to repair a missing trace. The trace is present on Monitor module 2.

CRT Serial Port

The CRT port level shifters can be configured for either RS-232 or TTL levels in and out. I verified that the port is configured for RS-232 on both Monitor modules.

The CRT port can be configured for various baud rates via jumpers. Both modues are jumpered 19-20 which provides 153.6 KHz to the CRT 8251. The manual says the CRT 8251 is set for a divide by 64. This means the CRT port is currently set for 2400 baud.

The MDS-800 requires the boot switch be hit to begin the boot proces after a reset. And like the MDS-225 IPC Monitor, the MDS-800 monitor requires a space (020h) be sent to start the Monitor program.

I connected one DE-9 connector to P1 (100-pin card edge connector) to provide RS-232 connectivity to the system for testing. I used a DE-9 connector rather than a DB-25 connector to match my terminal server cables. Actually, I only connected Ground, TxD, and RxD as those were the only signals provided in RS-232. I looped CTS to RTS on the P1 connector. See the wire list below. That is shown here.

        P1 Pin      Function            10-Pin Header   CRT DE-9 Pin
         3 ........ CRT DTR 
         8 ........ CRT RTS Loop to P1-20
        19 ........ CRT DSR 
        20 ........ CRT CTS Loop to P1-8
        26 ........ CRT CTS 
        27 ........ CRT GND ........... 5
        28 ........ CRT RXD ........... 2
        29 ........ CRT GND ........... 5
        30 ........ CRT TXD ........... 3
        

Other I/O ports

I will also ignore the other I/O ports for now.

iSBC RAM Board

This board provides all the RAM for the MDS-800. It differs in board layout from my two iSBC 064 in the E36-E37-E50 jumpers at the lower left side of the board. It also contains an engineering change on A103 pin 1 and A92 pin 1.

Visual inspection of this board shows no significant problems.

The board jumpers are identical to my operational board except for E31-E29-E30 which is strapped E31-E29 and the configuration of ADR SEL jumpers. It appears only 32KB of the board are configured to operate. Also E39-E40 is E39-E44 and E38-E41 is E38-45 on the unknown board.

I will leave this board as jumpered to see if this is what is needed for the MDS-800. I did try this board in my iSBC 80/10 machine and it works. I was able to test most of the RAM and it tested ok. I discovered a problem with my modification to the iSBC 80/10 board as the onboard ROM and RAM were not being disabled buy the i8255. I have corrected MonV40X.asm so the ROM and RAM can be disabled.

Initial Operational Attempt

At this point I can place the Front Panel Control module in slot 1, the CPU module in slot 2, the Monitor module with the ROM in slot 3, and RAM module in slot 4. I will connect the system up to my PC with Teraterm and lets see what happens.

This attempt failed. COU module #1 seems to be behaving correctly. Instead of the INT2 LED lighting, I get the INT3 LED. But there is no serial data from the Monitor module being sent. There must be a problem with my adapter for the CRT port. The other CPU boards fail to work.

In the mean time, Richard has gotten two of his MDS-800's to work. So I sent the boards back to him to test in his operational MDS-800. He has sent me an operational set of boards and all the failed boards. Now to get my poor mans MDS-800 to operate.

Second Operational Attempt

After the return of the tested modules in Richards MDS-800 I again tried the three good modules in my system. Still no ASCII output to the terminal. I reinspected the P1 adapter for the Monitor module and found I had jumpered pin 6 to pin 20. After connecting pin 8 to pin 20, I got output on the terminal. The Monitor is working correctly. However, I seem to have a problem with the backplane and the ZX-200A disk controller. I am not getting the Interrupt 2 LED to light when I boot the system. More debug to do!

Multibus Test Adapter

I intend to make an adapter to pass all the multibus bus signals to my HP-1650 Logic Analyzer. I can use this adapter on any multibus I system I have, so it will be a good addition to my test equipment. And with the HP reverse assembler, I should be able to display Intel mnemonics on the Logic Analyzer.

A horizontal ribbon.

Last updated: 16 October 2016