486 SIS496 failure to RAM test, hangs, showing 2x the installed RAM in the BIOS - Page 1
Forum section: electronics / general

author: gnif
author_rank: Unknown
timestamp: March 06, 2021, 02:34:17 pm
content: Hi All,Recently I built a Retro 486 Socket 3 SiS496 machine, however, I made a mistake and damaged it, blowing up the southbridge (dropped a screw into the open case while playing around with jumpers). This motherboard has now become a parts board unless I can obtain another 85C497 for a decent price.Because I wanted to continue to play with this particular chipset I purchased another "tested" board from eBay with the same chipset, the KM-S4-1 and this is where this problems started.At this point I know that all my hardware is good and compatible, FP RAM, CPU, etc. However the BIOS won't proceed past the RAM test at power on, it gets to 640K on the memory test and hangs. If I enter the bios before the test starts I can see the BIOS is reporting that I have 2x the memory installed than I actually have. I have verified there are no shorts with the SIMM presence detect pins. Turns out these boards do not use them anyway as the SiS496 datasheet (attached) reports that memory is probed for size by writing and reading test patterns and it seems these pins are just grounded out on the SIMM slot.Further to this, if I swap out the BIOS from my damaged board (AMIBIOS) into this one (originally AWARD BIOS), I get resource conflict warnings, however, the system now detects the memory correctly and boots. This (I think) confirms it's not a hardware fault, but likely a bad BIOS, however, I do not at present have a spare EEPROM to flash or the equipment to erase and program the UV erasable EPROMS these two boards came with.I have ordered a programmer and eraser (I have wanted these for a long time now anyway), however any other ideas from anyone here on what else might be the cause?

author: CJay
author_rank: Super Contributor
timestamp: March 06, 2021, 03:08:10 pm
content: Can you boot to a RAM diagnostic with the 'working' BIOS and if so what does it tell you?I wonder if you have an address line fault?

author: gnif
author_rank: Unknown
timestamp: March 06, 2021, 03:16:14 pm
content: Quote from: CJay on March 06, 2021, 03:08:10 pmUnfortunately in my efforts to track down the fault, I damaged the working BIOS I had. I do have a fast enough DSO to check for such things if you could point me in the right direction?

author: DrG
author_rank: Super Contributor
timestamp: March 06, 2021, 04:54:07 pm
content: Is this too simple of a possibility?Using only two of 4 SIMM slots, the order may be important? Rather than 1 and 2, try 2 and 4 or 1 and 3.That MB (KMS-41-1 and award bios) says "The board supports up to 128MB of onboard DRAM memory. Bestof all, it automatically detects installed memory modules, so it is not necessary to configure memory with jumper switches.Indeed, I see no switch pertaining to SIMMs.The AWARD BIOS might have something, somewhere, but it gets pretty entangled.

author: gnif
author_rank: Unknown
timestamp: March 07, 2021, 02:20:17 am
content: Unfortunately no, the order makes no difference, even a single module in any slot reports 2x the capacity of the module.I checked all the address lines by using the math function of the DSO to allow me to difference them from others, all appear to be functioning correctly, along with the RAS and CAS lines.

author: gnif
author_rank: Unknown
timestamp: March 07, 2021, 02:51:19 am
content: I just discovered if I mask off A10, RAS1 & RAS3 to make this 16MB DIMM appear as a 4MB DIMM the machine posts properly. It seems it's got an issue addressing larger DIMMS, which is strange as the chipset clearly should be able to address the larger DIMMs.

author: amyk
author_rank: Super Contributor
timestamp: March 07, 2021, 02:54:37 am
content: The symptoms do sound like BIOS corruption. Boards of this era don't really use SPD but determine the amount of installed memory via probing. If something in the BIOS related to the memory test code was very subtly corrupted it could result in that 2x you saw.Once you have the means to read and write EPROM you can check. This link contains an attachment which might be the appropriate BIOS for your board:https://www.vogons.org/viewtopic.php?t=43679

author: gnif
author_rank: Unknown
timestamp: March 07, 2021, 03:39:35 am
content: Well now I have it posted I can read the EPROM and have made a dump, I also hot-swapped it for an AM29F010 from my junk bin and tried to flash it, unfortunately, either the EEPROM is damaged or it has page protection enabled as it can only write 1/2 the chip.THanks for the link though, I actually have the exact BIOS for this motherboard so I can compare, I will update this shortly.

author: gnif
author_rank: Unknown
timestamp: March 07, 2021, 03:47:58 am
content: no dice:Code:$ md5sum BACKUP.BIN KM-S4-1.AWA6bf31cd246d0d4cae00580679d093920 BACKUP.BIN6bf31cd246d0d4cae00580679d093920 KM-S4-1.AWA

author: CJay
author_rank: Super Contributor
timestamp: March 07, 2021, 08:13:35 am
content: Ugh, unpleasant memories (literally and mentally), it was all still a bit wild west back then, the dodgy fake cache chips were doing the rounds and manufacturers would often be 'optimistic' about the capabilities of motherboards with manuals claiming things the chipset was capable of if implemented properly but not actually routing the signals or even including the necessary hardware on the boards.Even back in those days there were BIOS checksum routines so you'd have to be *really* unlucky for it to be so subtly corrupted yet still able to POST and boot, it's far more likely that the SIMMs you have aren't properly supported by the board.Before I started messing about with BIOS reprogramming I'd try and locate some different SIMMs.

author: m k
author_rank: Super Contributor
timestamp: March 07, 2021, 08:43:51 am
content: I remember some bank, chip count, comb issues but can't remember what sizes were in question.

author: gnif
author_rank: Unknown
timestamp: March 07, 2021, 09:05:18 am
content: I am not convinced it's the RAM, as stated in my OP, it actually works fine if I use the AMIBIOS from another motherboard. Also by masking of the extra RAS lines and A10 I can use the RAM just fine as 4MB SIMMs.I did however notice this at POST when it's probing the ram... looks nasty!This is coming from pin 126 on the 85C496 where it's being used as MA11 (memory A11). I replaced the 74F244 that buffers this to be sure it wasn't a bad input loading it down also. MA10 does not exhibit this same behavior.

author: m k
author_rank: Super Contributor
timestamp: March 07, 2021, 09:58:40 am
content: Not RAM but use of it.Back in the day and era some supported and some didn't, also with same chip sets but still different boards, like is your situation with those ROMS.But anyway, that picture is not nice.

author: CJay
author_rank: Super Contributor
timestamp: March 07, 2021, 10:19:00 am
content: Quote from: gnif on March 07, 2021, 09:05:18 amAre the two motherboards identical or do they just use the same chipset?I don't think the RAM and/or board* are necessarily faulty, I just think the RAM isn't correctly supported by the board and/or BIOS you have, that you can swap BIOS chips and have it POST would also point to that as well.I think perhaps you need to slow down a little and verify the basics, you need to know if the original BIOS chip on the board you're working on is good or bad.Can you lash up an Arduino or something to read the chip?As for A11, I'd expect every transition to be bad if it were a fault not just some of them so it mioght be a red herring but if you want to dig down then you could do worse than power it all down and take a digital multimeter on Ohms or even diode to the address lines on the SIMM slots and make some comparisons.

author: gnif
author_rank: Unknown
timestamp: March 07, 2021, 10:29:57 am
content: Seeing as I am using FPM memory which is considered the compatible RAM for this chipset (it supports EDO too), I have tested two sets of RAM, different brands, etc, same results, it's highly unlikely it's a ram compatibility issue.The datasheet for the 85C496 is complete including pinouts and register definitions, there are only a few configuration options available to the BIOS for the RAM which are timing and drive strength, which is all exposed in the BIOS. As such, there is no way two of these boards with the same DRAM controller could be more/less compatible then the next.Quote from: CJay on March 07, 2021, 10:19:00 amQuote from: gnif on March 07, 2021, 03:47:58 amAlready done.Quote from: CJay on March 07, 2021, 10:19:00 amExcept this is during the most power-intensive phase of power up.... all devices are reset-reinitializing, etc. Perhaps a decoupling issue?

author: m k
author_rank: Super Contributor
timestamp: March 07, 2021, 11:10:43 am
content: I tried to understand what is in the picture.Is it accepted that derivation is seen mostly every time?If so then pulse is dropping normally when there is a peak down.Are there then current limiting someting during first pulses?

author: gnif
author_rank: Unknown
timestamp: March 07, 2021, 12:37:32 pm
content: Every time yes.This motherboard is fairly primitive, the 5V rail is coming directly from the AT power supply, with no onboard current limiting.

author: m k
author_rank: Super Contributor
timestamp: March 07, 2021, 03:51:40 pm
content: So first time rising fails 3 times and then 2 times twice.Then there are 2 shorter pulses and 1 longer that are fine.Then 3 failed ones, must be even that peak down is missing.Some kind of an access problem is a first thought but what are left after RAS, CAS and address lines.Other chip select?

author: DrG
author_rank: Super Contributor
timestamp: March 07, 2021, 10:18:43 pm
content: @gnif I recognize your frustration, admire your perseverance and wish you well with the project. I also have memory flashbacks about those old MBs.Not that it is going to be helpful (largely because the other BIOS recognized the memory correctly), but I thought you might get a kick out of this thread...https://www.vogons.org/viewtopic.php?t=62753where he had a memory mismatch error problem....let me save you having to go through every message and all the things he tried until he found this:It's like Roseanne Roseannadanna used to say "It's always something..." You just haven't had your Roseanne Roseannadanna moment, yet.

author: james_s
author_rank: Super Contributor
timestamp: March 07, 2021, 11:05:10 pm
content: I remember having lots of weird problems like this back in the era these boards came from. Lots of stuff that theoretically should work together didn't, things like RAM, hard drives and various expansion boards that would work in one system but not in another, various voodoo involving address ranges and IRQs. We've really been spoiled over the last 15-20 years or so, all of this stuff has gotten dramatically better. Don't assume anything works the way it does with a modern PC. I don't think any of the 486 boards I owned even supported flashing their own BIOS, if you wanted a BIOS upgrade you purchased a physical ROM chip and swapped it in.

author: gnif
author_rank: Unknown
timestamp: March 08, 2021, 02:16:38 am
content: Thanks guys. Yeah, one of the first things I tried was to pull the CPU ZIF socket apart to check for bent pins...I grew up with this era of computers and built them, I remember all the odd/strange issues also. As frustrating as this is, to be honest, I am actually having a lot of fun as in the past I would have just looked at the motherboard not knowing what to do, and with modern motherboards, it's still like that now they are so integrated/complex.Anyway, last night I noted that some of the pins on the northbridge had cracked joints, I could move them if I poked at them, so I attempted to reflow the solder with hot air after applying a liberal amount of flux to the chip's legs. This was a mistake, the PCB has warped over the years and once the solder started to flow more pins came away. Also the MA11 line was one of the pins coming off the PCB, which would explain the intermittent behavior.I have decided to swap the southbridge chip over to the other motherboard I have that I damaged as this chip has fewer pins and it's within my ability to move it.

author: gnif
author_rank: Unknown
timestamp: March 08, 2021, 04:45:31 am
content: SUCCESS!!!I transplanted the chip, verified all 200 odd pins were soldered properly, put in the only working BIOS I have (the one also from the other board), it booted and detects ALL the ram correctly!This confirms it was a faulty DRAM controller in the northbridge!

author: james_s
author_rank: Super Contributor
timestamp: March 08, 2021, 10:12:58 pm
content: Wow nice job, it's always great when something like this comes together and works.

author: DrG
author_rank: Super Contributor
timestamp: March 08, 2021, 11:05:30 pm
content: It really does feel good to solve it eh? Good deal!

author: gnif
author_rank: Unknown
timestamp: March 08, 2021, 11:14:21 pm
content: Indeed, swapping the chip was the last resort thing as I only have basic tools and an el-cheapo hot air station. Had to place it twice as getting it aligned was quite difficult.Now I just need a UV Eraser and EPROM programmer so I can put the right BIOS back on this board to fix the UARTs (Different SuperIO chip).For now, though, I have a PLCC through-hole socket and a ton of saved EEPROMs that I should be able to make an interposer for.

author: DrG
author_rank: Super Contributor
timestamp: March 08, 2021, 11:31:07 pm
content: You're giving me more flashbacks....I dug up my old EPROM eraser - got to be 30+ years old...I used to use it on those windowed PIC parts as I recall. I lightened the area with the bulb for visibility - no idea if it still works.

author: james_s
author_rank: Super Contributor
timestamp: March 09, 2021, 02:14:29 am
content: It probably does, not much in there except a UV germicidal tube, a ballast and a starter. The one I have is a different brand but the same idea, I still use it occasionally.