VOGONS


First post, by T-Squared

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I was able to get a clone system to work with chips it normally doesn't use (256kbit, along with the normal 64kbit memory), and everything seems to be in-place for it to work properly (I've traced a bit of the system using the original 5160 schematics, and a multimeter). But the system is not detecting the full 640k I want to use. I am not sure where to go from here, though.

The one reason I'm assuming that the system isn't seeing the extra 384k of memory is that the A8 line (Pin 1 on the 256kbit memory) isn't being driven (I don't have an oscilloscope), even though it's connected to the relevant similar components that it uses in-place of the ones on the original 5160 schematic.

I'm also assuming that the system is detecting the basic structure of the memory (somewhat) correctly, and that the logic for multiplexing the memory is working too, because if it weren't, the system wouldn't even start up.

i.e. it sees only 256kBYTE because that's is all that is being activated and run-through by the POST because of the not-working A8 switching pin, but neither type group of memory individually adds up to 256kBYTE (2 banks of 256kbit; 512kBYTE total, 2 banks of 64kbit; 128kBYTE total = 640kBYTE)

Should I go farther down the A8 Line to make sure that it's connected properly, or am I on a wild goose chase and should check the multiplexing logic instead?

Reply 1 of 14, by weedeewee

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Could it be switch 3 & 4 ?
http://www.minuszerodegrees.net/5160/misc/516 … ch_settings.htm
Switches 3 and 4: Enabled motherboard RAM.

3=ON , 4=ON : Enable only bank 0
3=OFF, 4=ON : Enable only banks 0/1
3=ON , 4=OFF: Enable only banks 0/1/2
3=OFF, 4=OFF: Enable banks 0/1/2/3

also, how the 256kbit chips neeed to be refreshed could an issue.

Reply 2 of 14, by T-Squared

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I already looked at the DIP switches. They looked correct. (Even setting both 3 & 4 to ON gave me just 64kb of memory, which is exactly 1/4 of what the original count should be.)

I've also taken the refreshing into account as well (sort of). I adjusted the A8 delay line to make sure that it matched the original IBM specs in the schematics, but it didn't seem to do anything.

Also, the chips may not have to be refreshed in a certain way. Like I said, the system counts up to its normal count of 256kBYTEs, yet neither type group of memory individually adds up to 256kBYTEs. This tells me that the 256kBIT memory is being accessed in a similar way to the 64kBIT, without the A8 line being activated. Without that A8 line, it might as well all be 64kBIT memory.

Reply 4 of 14, by Plasma

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Your motherboard is just using the 256k chips as 64k chips.

http://minuszerodegrees.net/5160/motherboard/ … d_revisions.htm

It is possible to upgrade a 256KB 5160 motherboard to support 640KB. An easier alternative is a memory expansion card like the Lo-tech 1MB card.

Reply 5 of 14, by T-Squared

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I had thought about that card, but I am so close to getting this to work. Partly why I want to put 640k on the motherboard is because I have limited space for cards, only two slots, one of which is being taken up by an XT-IDE drive, so I can use the free slot for whatever card I want.

I'm going to go down the A8 line to check if there's a disconnect somewhere.

Reply 6 of 14, by snufkin

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Have you got photos of your clone board? Plasma's link says IBM had two versions of the motherboard, with one only handling up to 256kB and the other up to 640kB, with a possible mod to convert the 256kB to 640kB by adding a jumper to E2.

Reply 7 of 14, by T-Squared

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This is the clone system I have, a Sanyo MBC-775.

20220115_175049.jpg
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This is the processor/memory/BIOS/Parallel/Turbo board.
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I was able to replace the existing memory with sockets, and instead of the original v2.33 semi-IBM-compatible BIOS, I'm using one of the Yangtech Turbo XT BIOSes from minuszerodegrees. (Although I still have the original BIOS chip)

There are what look like 4 or 5 jumpers, and one of them I've been able to identify as a grounding jumper (5160 Equivalent E1)

Reply 8 of 14, by snufkin

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Have you traced where the equivalent of U84 is? On the 5160 it's a 74LS158, like your U47 and U46. Pin 4 of that on the 5160 connects to A8 of the 256kb chips. The other chip in the mod is U44, given on the 5160 schematic as a 24510, but on at least one photo is a N82S129 bipolar PROM. Can't see it on yours, but it's probably socketed (what's U45?). On the 5160, pin 1 of that needs to be grounded, which that board does through E2. This assumes the equivalent on your board is programmed the same way as the IBM one.

Reply 9 of 14, by T-Squared

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I already looked at this board as thoroughly as I can without burning my mind out. 😜

When I first got it, this system was actually missing several ICs needed for what I assume was extra memory decoding. I was able to get them through Mouser. (An S280 parity Generator, an extra 158, and a 139)

Also, this system doesn't use a PROM. It uses the equivalent in 244, 245, and 373 logic chips (among others), which is why the board is so heavily populated. It'll be more difficult to put this together.

Reply 12 of 14, by Horun

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T-Squared wrote on 2022-01-15, 23:58:
This is the clone system I have, a Sanyo MBC-775. […]
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This is the clone system I have, a Sanyo MBC-775.

20220115_175049.jpg

I was able to replace the existing memory with sockets, and instead of the original v2.33 semi-IBM-compatible BIOS, I'm using one of the Yangtech Turbo XT BIOSes from minuszerodegrees. (Although I still have the original BIOS chip)

There are what look like 4 or 5 jumpers, and one of them I've been able to identify as a grounding jumper (5160 Equivalent E1)

Thanks for the picture ! I like to look over old boards. Unfortunately have no helpful information.

Hate posting a reply and then have to edit it because it made no sense 😁 First computer was an IBM 3270 workstation with CGA monitor.

Reply 13 of 14, by T-Squared

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All right, good news and bad news.

Good News: I was able to get that Turbo XT BIOS working.
Bad News: The memory problem still isn't fixed, and it's not the fault of the BIOS (i.e. It's not a BIOS mixup, like one that can only count up 256kbytes of memory.)

Reply 14 of 14, by T-Squared

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Okay, looking at everything else, the lines and logic seem connected. (I didn't have to re-map everything.)

From this point forward, everything now seems to involve the 244 and 373 logic equivalent of that 24S10 PROM used in the 5160, because the connections from A16-A19 on the CPU side, and the connections from A8 on the memory side all meet up there. Strangely, the connection to the 24S10-equivalent logic also seems to go off of the CPU/MEM board and onto the video/ISA/Disk controller board. (There are two boards, the CPU/MEM board is at a right angle to the vid/ISA/disk board; apparently they didn't have enough room for the extra logic chips.)