Help me breathe life into this dead? 286 board

[Dhall]

Greetings,

I would like to ask for advise on how to troubleshoot a motherboard to make sure that it is absolutely dead.

I recently purchased a 286 board.

For a couple of images with the memories taken out click here

There is nothing on the board to identify it, so I looked at stason and I found a pretty similar board (only difference is how the ISA slots are arranged?). It is a Peter 286 S?


So what I have already tried:

Fitted it with a PC speaker, an old Trident card, keyboard and a 200 W PSU. Tried with a 16 Mhz Harris and later with a 10 Mhz Intel processor. In both cases:

- I get NO beeps
- I get NO screen
- ALL the lights on the keyboard lit up and stay on, they don't go out.

After this, I've taken out all the on-board memory chips and put in 2 30pin 1 Mb SIMMs. Same result.

Tried it without a video card AND without a cpu. Same result: no beeps, no nothing, keyboard lights stay on.

Could you please tell me what I should try next?

 

[iulianv]

Since the oscillator on the board is 32MHz, you should stick to the 16MHz Harris (do you have another (known to be working) board, to test it on, just to make sure the CPU is OK? same question applies to the RAM used for testing). Also, I see there is an extra JP8 jumper compared to the board you found on stason...

What's puzzling about your photos is the NEC chip in the FPU socket - I don't know of any NEC FPU for 286, so could you please take a clearer photo of it (or maybe post here what's written on it)? It might be a good idea to take it out and test the board again (and maybe play with that JP8, its location makes me think it might have something to do with the FPU)...

 

[Dhall]

Unfortunately I don't have another board to test the Harris in and I have only 1 16Mhz cpu. So if the oscillator is 32 Mhz I cannot use a 10 or 12 Mhz cpu? Are you sure? (I'm still just getting into old hardware that is why I'm asking)
The memory sticks are fine, tested them in a 386dx board.

About the NEC. I cannot take a closer photo, because it would be out of focus range (I have a crappy cell-phone). It (the NEC) reads:

DISA 4236 297
9225M7048

Tried googling it, I found no useful information, only ordering info.

On the Stason schematics it shows its place as an NPU and not a PSU. Maybe it is a network processor?
Regardless, I took it out. It makes no difference. Also put the jumper on the other two pins but to no avail. (even took the jumper off) I will be receiving a pack of 286 cpu-s tomorrow but what concerns me is that the machine behaves the same way with or without a cpu......

Another peculiarity: tested the board with two keyboards. On the first one, all the lights stay on, on the second one, they don't lit up at all.

One thing to note: there is a little corrosion around where the battery was.

 

[iulianv]

The board will run the CPU at half the osscilator speed - you can try to use a 10 or 12 MHz CPU, but it will run (heavily) overclocked (if it will run at all).

I couldn't find much about that NEC part number either, but I'm sure it doesn't belong there. NPU = Numeric Processing Unit, same as FPU = Floating Point Unit (otherwise known as math coprocessor).

Corrosion might render a board bad too - try to identify which traces are corroded and, if you have a multimeter around, test them for continuity.

One other thing you can try is play with the lowest jumper on the right side of the board (the one that selects between 256Kbit and 128Kbit BIOS chips), maybe it has been misplaced and doesn't correspond to the actual BIOS chip type installed...

 

[Dhall]

Thank you so much for telling me to try that again!

I mean "again", because I already played around with that jumper when I got the board, but it didn't work.

This afternoon I pulled out both BIOS chips then put them back. The machine still didn't work but then you suggested the jumper, and trying again...this time it POSTS.

Thank you!

Could you please elaborate on a little about that jumper? 128 k vs 256k. I always thought that that jumper was somehow connected with the cache memory in 386 systems. So it sets the type of bios chip? 128 Kbit vs 256 Kbit? What is the difference? I got it working with the 256 setting btw.


Oh, and you were absolutely right about the NEC chip...I put it back and again, the system would not post. Somebody maybe forced it erroneously into the fpu socket?

 

[per]

You should try to clean up the corrosion from the battery. I've heard that vinegar should work well.

 

[Caluser2000]

30 pin simms are generally used in banks of four.

 

[krebizfan]

30 pin simms are generally used in banks of four.

Only for 386 or 486 systems. 30 pin simms tend to be paired in 286 systems like the one in question; I think there were even some 8088 based systems that had 30 pin simms where the bank was a single simm.

 

[iulianv]

On 16bit data bus CPUs (like 286 and 386SX) just a pair of 8bit 30pin SIMMs will indeed work (no absolute need for two pairs).

A lot of chips are 40pin and will fit into a 287 FPU socket - maybe who had the board before didn't know much about old hardware and was just glad he (or she) found a place for that chip . I would hold on to it, at least until proper identification - it's always a nice challenge to id an old chip...

128K vs. 256K is about BIOS chip size (if you start peeling the label off one of yours you'll probably find a part-number containing 27C256 - but don't do that, it might not stick back and you'll have to cover the UV windows of the chip with something else than the original label, which won't look good ) - if you set the jumper for 128 instead of 256 I'm guessing the board will only load half of BIOS, which is not enough for it to POST. There is no cache on 286 boards (at least I haven't heard of any so far).

 

[Dhall]

You should try to clean up the corrosion from the battery. I've heard that vinegar should work well.

Thanks, I will definitely try it.

I would hold on to it

I'm holding on to it alright, I'll let you know if I managed to identify it. (if you are interested that is)

Thanks for the replies guys. After 18 years, I'm a 286 owner again!

One last question: if you click here you can see a photo of the on-board memory I pried out earlier and the sockets. My question is: why are there two types of memories and what is the difference? Or rather why are there different sockets for the on-board memory? (Also, am I right in calling them on-board memories or is there a more precise term?)

 

[iulianv]

The smaller ones are the parity chips (the equivalent of the "different" single chips on the 30pin SIMMs in the first photo), and are also smaller in capacity (256kbits/32kbytes for the DIP-16 chips vs. 1mbit/128kbytes for DIP-20). I just call them DIP memory (Dual Inline Package) and I guess I could refer to them as on-board memory if they were soldered instead of socketed .

On some 286 boards there is a jumper for enabling or disabling RAM parity checking (in the latter case populating the parity sockets is not mandatory; also, most 30pin SIMMs I've seen have the parity chip, but 30pin non-parity SIMMs do exist too); I'm not sure if parity chips are mandatory or optional on boards missing that jumper...

I just found that NEC chip mentioned here (it seems to be used as a keyboard controller, just like the labeled chip next to the RAM sockets on your board): http://www.linuxmisc.com/19-linux/26912db35949aacb.htm

 

[Dhall]

Thank you for the explanation guys. Is everything okay with this thread? I mean I keep getting emails about new comments here but I can't see them.

Iulianv's message is here, but I got emails today that krebizfan and modem7 has also replied to the thread but I can't see their comments....

 

[krebizfan]

I responded then noticed iulianv had already wrote an equivalent message. I deleted mine. Sorry about that.

 

[Dhall]

I responded then noticed iulianv had already wrote an equivalent message. I deleted mine. Sorry about that.

I see, next time please just leave it there. As I'm quite new to this hardware any little piece of information is very helpful. Read you comment in the mail, thanks anyway.

 

[Dhall]

Ah so the smaller DIPs are parity chips. Aha..so every pair of DIP 20 needs 1 DIP-16 to work? That is why there were 8 DIP-20s and 4 DIP16s?

Using only DIP 20 in such a board is out of the question? Because it certainly accepted SIMMs with no parity chips (3 same chips on the stick)

Oh, and a very good question. If I want to put them back and use DIP memory instead of SIMMs...how do I know if I'm putting the DIP chip correctly into the socket and not "upside down" ?

 

[Stone]

Using only DIP 20 in such a board is out of the question? Because it certainly accepted SIMMs with no parity chips (3 same chips on the stick)

Nope! 3 chip SIMMS are with parity. Two 4-bit chips and one 1-bit chip... for parity. Look closer. The two chip SIMMS are without parity.

...how do I know if I'm putting the DIP chip correctly into the socket and not "upside down" ?

Both the chips and the sockets are keyed. You just have to look at them to notice it.

 

[Dhall]

Nope! 3 chip SIMMS are with parity. Two 4-bit chips and one 1-bit chip... for parity. Look closer. The two chip SIMMS are without parity.

Thanks for pointing that out...indeed, even if they look the same, the third chip is different...how it never occurred to me?