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motherboard

In this image, you can see, from the right, a 16-bit ISA slot (occupied by a controller card), an 8-bit ISA slot, four more 16-bit ISA slots, and two of these strange slots that look like a 16-bit ISA slot, except with an 86-contact extension rather than the normal 36-contact extension. These are definitely not VLB extensions. I found this page on the motherboard, which says they are some sort of "32-bit external memory card." What are these slots? Are they compatible with 8-bit or even 16-bit ISA?

This is only one of this computer's oddities. The controller card, while sporting a typical floppy connector (the solder ends of which are visible at the top right), also has a strange 34-pin connector (keyed differently from the floppy connector and visible immediately below it) labeled "winchester", as well as two 20-pin connectors (visible below the "winchester" connector) labeled "drive 0" and "drive 1". The winchester and drive 0 connectors both connect to a very (physically) large Seagate? ST-251 hard drive.

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I found this page on the motherboard, which says they are some sort of "32-bit external memory card." What are these slots?

They are exactly that, memory expansion. This is a rather early 386 board from before memory modules became a thing.

The mainboard can be fitted with 1 MiB using 256 KiBit chips (41256), so any expansion has to go on cards. With (AT) ISA being only 16 bit, using an unmodified Bus would slow down the 386 considerable. Not cool for someone who just payed an upper end price tag. So they had to come up with some way to allow 32 bit wide memory expansions. Instead of making a whole proprietary slot or pinhead, they decided simply to expand the 16 bit ISA layout to 32 bit - so in some way it is like VLB. Going that way allowed to use these slots as well for regular I/O cards.

Are they compatible with 8-bit or even 16-bit ISA?

Im pretty sure they are fully compatible at least with 8 bit cards - it might not be for 16 bit cards. All educated guesswork from the visible traces.

This is only one of this computer's oddities. The controller card, [...] also has a strange 34-pin connector [...] labeled "winchester", as well as two 20-pin connectors [...] labeled "drive 0" and "drive 1". The winchester and drive 0 connectors both connect to a very (physically) large Seagate? ST-251 hard drive.

(No, the ST-251 is not a large disk drive, it's only a 5.25 and in addition half height. Those were considered quite small at the time, top notch miniaturisation)

Again, it's exactly whats written on, it connects to a Winchester Drive - slang name for a hard disk - and a ST-251 is much like the prototypical 40 MiB drive of that time. The interface is called ST-506 after the first Seagate drive using it. The 34 Pin connector holds all drive control signals, while the 20 pin delivers data.

So, nothing special here, just the regular disk controller in pre-IDE times.

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  • And as usual a far more comprehensive answer than mine, which I'll delete. – manassehkatz-Moving 2 Codidact May 15 at 4:05
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    More info about the memory slots: vcfed.org/forum/… – snips-n-snails May 15 at 4:13
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    @manassehkatz-Moving2Codidact I am sorry, that was not my intention. Yours is quite valid on its own. It points out the most important information about the controller/drive interface. Worth keeping. – Raffzahn May 15 at 11:23
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    Before I found out that the 'winchester' name came from an American rifle, I assumed it had something to do with IBM Hursley, located just outside Winchester in the UK :-) – another-dave May 15 at 11:24
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    My answer was 100% valid. But it was not super-innovative (i.e., your including the ST-506 reference is not something you wouldn't have come up with on your own - anyone who has been in the business since the 1980s would know that part) and you provide far more detail. I've had my "great" answers in the past - but that wasn't one of them. – manassehkatz-Moving 2 Codidact May 15 at 14:05
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To add to Raffzahn's answer: I strengthen the "might not be compatible to 16-bit cards" to "are most likely not compatible to 16-bit cards". As I see the traces on the photo, there is nothing that supports the impression these slots are ISA 16-bit compatible - but also no proof they are not.

A reliable tool to learn more about that board is a continuity tester. I already probed a lot of computer hardware using continuity beepers of multimeters, and despite randomly applying open-circuit voltages of up to 3V to undefined pins, I never damaged any electronic component by it. If you happen to have a quickly responding continuity tester, you can easily probe connections to a pin by having one probe at the connector you are interested in, and sliding the other probe alongside possible pins that might be connected to your connector.

For your board, I see two likely possibilities on how the memory expansion slot is connected:

  • It might be some kind of local bus slot, i.e. directly connected to the 80386 address and data pins. Address decoding (like row/column multiplexing, determining whether the board should respond to that address) would be on the memory card in this case. You should be able to beep out address and data to the 80386 pins directly or connected through 74F245 or 74F244 buffer chips.
  • It might be some kind of 32-bit memory module connector (like the 72-pin PS/2 SIMMs) with already decoded address and RAS/CAS lines. In this case, you should be able to beep out the data pins again to the 80386 local bus (and again possibly through the buffer TTL chips), whereas the address pins and RAS/CAS pins are connected to a memory decoder, which is often part of a VLSI (the technology "very large scale integration", not necessarily the manufacturer with the same name) chip, on which you might find a datasheet in the internet.

The only thing to add to the hard drive controller information is that there were different kinds of "winchester" controllers, most prominently MFM and RLL controllers. The hard drive model number indicates it should be connected to an MFM controller, and have a capacity of 40MB. If your computer shows a 60MB hard drive, someone swapped the original MFM controller by an RLL controller, which manages to write data more densly to the drive, with only moderately higher requirements on the drive itself. This is an unsupported configuration, and while it works for some controller/drive combinations, there are numerous reports of reliability problems around the internet caused by using RLL controllers with MFM drives.

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  • Yeah. I’d be absolutely shocked if the initial section was not 8-bit ISA given the trace layout. But the memory expansion part appears to have zero traces in common with the 16 bit extension connectors, so a more direct connection to the main bus seems likely, especially since it was designed for memory expansion. – Kevin Cathcart May 16 at 13:15
  • This is in stark contrast to the other 8 bit ISA location, where there exists through holes to support the 16 bit extensions. I’d be absolutely shocked if that would not work if populated. Indeed, I’d not be surprised if the reason it was unpopulated was merely because the system shipped with a factory installed 8 bit card in that slot that they were not anticipating that users would remove. Hence no need to populate. – Kevin Cathcart May 16 at 13:20
  • @KevinCathcart Wish I knew more about what this machine went through! This was my mom's 386 that she bought new in 1989. It must've had a video card at some point but none of us know what happened to it. The 8-bit slot is also one of two that's missing a placeholder shield, so I do agree that it was probably a factory card that was removed. – robbie Jun 16 at 5:38

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