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Unknown IBM 5150 Card

O

Oddsocks

Member
Joined
Mar 3, 2024
Messages
36
Hi,

Does anyone recognise the following card. It came out of an IBM 5150. It looks IBM branded as it has the same kind of design language as their other cards but basically no ID on it. It has a female DB25pin connector, a microcontroller and some DRAM. I thought it might just be a serial card, but I'm not entirely sure.

20240308_135931.jpg
 
I can just about make out the frequency of the crystal as 3.579545 Mhz which is precisely the NTSC colourburst frequency, and that fact combined with the sockets for eight DRAM chips leads me to believe it must be some kind of video interface. Then again... the markings on the Intel chip are difficult to make out but it looks to be an Intel MCS-48 microcontroller. The MCS-48 takes 15 clock cycles to execute a single instruction! Which is far far too slow to be a video controller, right?
 
I did notice the frequency too but assumed it may have just been coincidence that it was close to the NTSC colourbust frequency and didn't see any obvious BIOS chip for a video card. If it counds for anything, this 5150 comes from a PAL region. The DRAM chips I thought may have been a FIFO if it was a serial port but maybe SRAM would make more sense. The micro-controller is an Intel D8748 which reading the specs more closely, does contain an internal 1KB EPROM. maybe there's a window hiding under the sticker. I'll have to examine the pinout and see if it's possible to read it with the T48. The datasheet suggests all instructions execute in either a single, or 2 clock cycles
 
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The other thing is the IC socket near TP2 appears to be installed incorrectly - too high on one side but looking at the soldering it doesn't appear to be anything done after-market so I think it left the factory like it
 
Until the firmware is dumped it can be anything from a terminal emulator to a disk interface card... my bets are this is some sort of external disk controller but this is pure speculation. It is known of other IBM computers to have interfaces to external, intelligent disk devices - I haven't heard about the IBM PC having one but...
 
I suspect it's aftermarket. Maybe a hardware print spooler?
 
Plasma said:
I suspect it's aftermarket. Maybe a hardware print spooler?
Click to expand...
Ah, a print spooler makes a lot of sense! The MCS-48 would be plenty fast enough to marshal data into and out of the print buffer. And in that case, the unpopulated 36 pin header could perhaps be for attaching a Centronics connector?
 
kdr said:
Ah, a print spooler makes a lot of sense! The MCS-48 would be plenty fast enough to marshal data into and out of the print buffer. And in that case, the unpopulated 36 pin header could perhaps be for attaching a Centronics connector?
Click to expand...

Could well be the case. The only thing about after market I guess, is that the card looks so similar to genuine IBM cards it makes me think it was made by IBM, but that is subjective. The only real history I know about this 5150 that it came out of, was it was used in a pharmacy in the 80s
 
RetroAND said:
Until the firmware is dumped it can be anything from a terminal emulator to a disk interface card... my bets are this is some sort of external disk controller but this is pure speculation. It is known of other IBM computers to have interfaces to external, intelligent disk devices - I haven't heard about the IBM PC having one but...
Click to expand...

For sure, unfortunately the T48 I have won't read the firmware from the MCS-48 natively and I'm not smart enough to figure out how to produce an adapter to at least allow the EPROM contents to be read. However plugging the card into an AT box, I don't see any option ROMs present in the memory map, so either the card isn't AT compatible in any way or the firmware doesn't present itself to the BIOS as an option ROM
 
Plasma said:
I suspect it's aftermarket. Maybe a hardware print spooler?
Click to expand...
I would actually assume it's a serial card with FIFO buffer.

A print spooler makes not sense to me. For driving a parallel-port printer, you do not have to maintain a certain speed. And printers at that time already had internal memory so you had not to wait for the last character reaching the printer before being able to use the OS again.

It's also clear to see that this card is not from IBM.
 
Timo W. said:
I would actually assume it's a serial card with FIFO buffer.

A print spooler makes not sense to me. For driving a parallel-port printer, you do not have to maintain a certain speed. And printers at that time already had internal memory so you had not to wait for the last character reaching the printer before being able to use the OS again.

It's also clear to see that this card is not from IBM.
Click to expand...
Could be serial, but the female DB25 isn't the typical gender. It looks like this card can have up to 64K RAM. I can't see a serial port needing that much. The 16550 had a 16-byte buffer.

The IBM 5152 printer sold with the 5150 only had a 256-byte buffer, and would benefit from a spooler.
 
g4ugm said:
No FCC sticker on the back? What is the type number of U2? Almost certainly not serial, nothing that would do RS232 or Current Loop on the board.
Click to expand...
Sadly no FCC sticker. The only ID I see on the rear is "GX-211-VO" which I don't think has anything to do with the card itself but maybe something involved in the manufacture. U2 is a Mitsubishi logic chip - M74LS74AP - flip flops & latches
 
Could you check the pinout for the ribbon connector, please? At least the power supply lines...
I have the feeling this is a floppy controller, not for the standard 5 1/4 with Shugart interface but for the older IBM 8" like the S/23 and S/36 have. These drives use 36-pin ribbons. It would make sense if the computer it was attached to was in some sort of data center with other IBM midrange/mainframe computers.
 
RetroAND said:
Could you check the pinout for the ribbon connector, please? At least the power supply lines...
I have the feeling this is a floppy controller, not for the standard 5 1/4 with Shugart interface but for the older IBM 8" like the S/23 and S/36 have. These drives use 36-pin ribbons. It would make sense if the computer it was attached to was in some sort of data center with other IBM midrange/mainframe computers.
Click to expand...

I can certainly do that. I'll grab the multimeter and report back. The only 'history' of this particular machine I know was that it was used in a pharmacy in the 80s, in case that adds any context
 
Ditto above, I wonder if it adapts to GPIB / HPIB or relates to old pos scanners / equipment.

Hmm digital I/O?
 
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OK, here's what I've found so far.

36 Pin Internal Connector
Pins 18-26 GND
No voltage pins

25 Pin External Connector:
Pins 18-25 GND
No voltage pins

Card Pins
IRQ 7 Only
+5V (no +12V, -12V or -5V connected)
Only really I/O read and write connected with some address and data lines

So to me, the external connector appears to be a parallel port, given the IRQ and ground pins. I thought the 36pin connector might have been a header for a centronics connector but the pinout doesn't seem to match so it could be something else. So to me the MCS48 and the small amount of RAM is sounding more like a print spooler. Any other thoughts?
 
Oddsocks said:
The other thing is the IC socket near TP2 appears to be installed incorrectly - too high on one side but looking at the soldering it doesn't appear to be anything done after-market so I think it left the factory like it
Click to expand...
I've think they just shifted it over to maintain a certain clearance for the hole through the card in the top left.

Printer buffer would make sense to me except for the connector being the wrong gender. Printer buffers were a whole product category back then.

My first guess would be something like an SDLC or other 'intelligent' interface controller, but I don't see any level shifters. So perhaps some sort of industrial interface at TTL levels?
 
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