Attached is an image showing the result of the ISA PC Analyser board. It displays only a double dash -- and no error code
Well, I guess because I have a oscilloscope (2-channel, 100 MHz), do not have a logic analyser, and am not sure what else I can do with my DMM aside from what I've already done. I am by no means an expert and am hoping to learn.
Attached is an image showing the result of the ISA PC Analyser board. It displays only a double dash -- and no error code
OK, that might be a mistake by me. But doesn't one of the unpopulated sockets say 80286? In that case any POST value would be a big surprise to me.
You can find a 8 bit Sigrok compatible LA on ebay starting at around $6, a 16 bit one for less than $20 (even less if you simply use a $8 FX2 dev board with your own wires and cable) and a 400 MHz one for $99 so absolutely no reason not to have one.
Its probably super useful with signals within its data capture range. check sigrok.org and the 'opentechlab' youtube channel (the author of pulseview, the GUI program for sigrok LAs, is great for learning how to use pulseview and LA's generally)
cdev, for someone who has not used a logic analyser before, perhaps you can explain the benefit of a logic analyser over that of an oscillscope, aside from the fact that the logic analyser can view significantly more channels simultaneously? If the advantage is simply one of more simultaneous channels, then it seems this can be performed on a oscilloscope with more switching of the probe. And don't I still need to know what to probe and what is the expected behaviour? Some insight on this would be appreciated.
Thank you for your comments. Could you provide a web address for the $6 logic analyser which you are recommending? e.g. https://www.ebay.ca/itm/CY7C68013A-56-EZ-USB-FX2LP-USB2-0-Develope-Board-Logic-Analyzer-EEPROM-M36/351494353170
The BIOS has 16 address pins and 8 Data in/out pins. Aren't these values going to change rapidly during the boot process on the working board? If so, how would it be possible to pause the scope right at the moment where the output catches the very first event sequence and not the nth state during the boot process?
Could you please explain by way of example how probing all the address pins on the BIOS will let me know if the CPU is starting and how far along it is in the boot process? It seems to me if the dead boards have no codes from the PC analyser card that almost nothing is starting?
mov al, 0D5h ; some post code
out 80h, al ; out it goes to the postcard
Wouldn't this provide some insight as to the high liklihood items for the failure? Like which components must be broken or contain disconnect for this to happen? For example, it doesn't seem like the 206 DMA controller chip needed to be connected for the motherboard to output a PC speaker beep sequence, and that is, in fact, how I troubleshooted one of the now working boards.
Let's say I am able to take a snapshot of the exact moment in the boot process in which the working board turns on, call it item sequence 1 in the boot process. Now if address A0, for example, on the working board has a pulse, and A0 on the dead board does not, how does this information assist in troubleshooting the faulty component?
Concerning probing the chipsets, how would I get the probes onto the pins? In the absence of some sort of clip-on adapter for QFP chipsets, wouldn't I need to solder a wire to the tiny leads on the chipset? And would the same ring true for probing the CPU and other soldered ICs?
If so, then wouldn't it be easier to replace the 206, 481, and 482 chips? Is there not a more systematic and sequential approach to finding the fault?
If I remove the QFP AMD CPU, then probing the PGA-132 socket would be fairly straight-forward and no soldering required. But do you think probing the PGA-132 CPU socket in a power-on state without the CPU installed be of any value? If so, where to start probing and what to look for?
Alternately, as the system has a QFP AMD CPU installed and the board contains a PGA-132 CPU socket, perhaps this combination would be of value to have easy probing access to the CPU's pins. If so, what to probe and what are the expected results at the 0th state in the boot process? Unfortunately, when I went to check if CLK2 on QFP AMD CPU is connected to CLK2 on the PGA, they were not. This is true for these boards. So if CLK2 on PGA doesn't make a direct connection to CLK2 on QFP, then other pins may not be directly connected as well.
To directly answer your questions - the faulty boards do not beep at all; the keyboard LED does not flash; the POST card does run through the codes in sequence on the working boards.
Is there a systematic approach to probing rather than random probing of the boards?
I do not know what to look for, when it should occur, and on which pins. Do you know? It seems like we would want to know the very first course of action the motherboard takes upon power up, e.g. like initialise the keyboard controller, and if so, which pins should look like what on the scope?
In a power-off state, I have confirmed that Vcc and Vss are reaching the chipsets and the CPU. What I found a little odd was that, according to the AMD datasheet, pin 34 = Vss and pin 35 = Vcc. For this set of pins only, the motherboard is wired such that pin 34 = Vcc and pin 35 = Vss. I figure it might be a typo because the working boards are wired the same as the dead ones.
I suppose could measure the supply current. What information will this provide for the troubleshooting process? I figure the dead board will have a lower current draw than the working one. The easiest for me would be to plug-in a kilo-watt meter, albiet with less significant digits then splicing in the DMM. EDIT: working board runs with 0.43 A ac and dead board runs with 0.36 A ac, as measured with a Kill-A-Watt.
ISA postcode boards work by mapping Port 80h. Bios writes codes like this:Code: [Select]mov al, 0D5h ; some post code
out 80h, al ; out it goes to the postcard
no post codes can mean not executing bios at all, but it can also mean executing bios and hanging before having opportunity to output first post code, or something wrong with ISA address decoders, or something between
Do you have a link to provide for such a circuit emulator?
I do not really intend to spend much money on this.
ISA Post cards with various EPROM sets - could you provide a weblink?
Why do you say these UMC boards were crap? These particular boards were made by MSI, which has a decent reputation and still exists today.
OK, so you are recommending that I probe RESET on the AMD386DX and probe OE on the BIOS socket? I can certainly do this. Is it helpful to probe any of these without the CPU's presence? I was thinking to remove the AMD 386DX on one of the dead boards, and keep one on another.
OK, on the EPROM, I will probe addresses A0-A15, Data I/O's Q0-Q7, and OE. Let the power be on 20 seconds or so to view changes? Or is a 5s timeframe good enough? Also, is using the native MSI AMI BIOS preferred, or is MR BIOS OK?
When looking at the EPROM's address and data pins, should each of them at least have one incident of a low-to-high or high-to-low swing? From your wording, it sounds each of them should have at least one transition.
OK, I will apply pressure to the 4 QFP chips when cycling the power. Which pins in particular am I probing while doing this? chip select, write enable, and output enable inputs?
For running the continuity meter from GND to X pins on the ISA slots, I assume with power off?
I did check all the BIOS chips in working boards, they all appear to be of the same revision. But I generally use the MR BIOS for um480 series. I've used it for years in another UMC 480 series board and it seems to work in this MSI um480 boards. The board does not have a parallel port built onto the motherboard - it is on an ISA I/O card. How can I read the post codes from the parallel port?
Thanks for the weblink. It does contain a user manual for the UMC 481B / 482A chipset, although my board has the 481A revision marking. Unfortunately, that manual makes no mention of the 386, only the 486. Perhaps that is where the difference from B to A is.
why hell bent on removing the QFP AMD - you are observant! And the reason is because, at the end of the day, I plan to remove it anyway, or at least float it, so that I can use the PGA-132 with different CPUs, e.g. TI 486SXL, DRx2, IBM DLC3, etc. If I don't want the CPU on there, and plan to remove it, doing it sooner would allow for a quick test if the CPU is bad.
If you 'scope the parallel port data pins pre VGA init on an IBM PS/2 ISA machine you'll see some very interesting activity which can be decoded with a couple of HP hex LED displays
Quickly, though, I do have an extra PGA-132 which I could sandwich in there and solder a 30 AWG wire to. The kynar jacket on the wire is pretty thin and a lot of wires will be bundled up, so not sure if there will be a capacitative coupling issue between adjacent wires.
For $6, I might just grab that LA board. Actually, if there is a slightly better one (e.g. faster & more ports) for under $25, any recommendations?
why hell bent on removing the QFP AMD - you are observant! And the reason is because, at the end of the day, I plan to remove it anyway, or at least float it, so that I can use the PGA-132 with different CPUs, e.g. TI 486SXL, DRx2, IBM DLC3, etc. If I don't want the CPU on there, and plan to remove it, doing it sooner would allow for a quick test if the CPU is bad.
Track down and try to follow POWERGOOD as well.
On power up and release from reset the CPU should start to fetch code from the EPROM almost immediately (after a few microseconds at most), it gets complicated on some boards because very early on they 'shadow' copy the EPROM into RAM and run from there but you should at least see the activity while that's happening and possibly beyond that.
For running the continuity meter from GND to X pins on the ISA slots, I assume with power off?
Yes, definitely, you're looking for shorts to ground or 5V where there shouldn't be any, comparing readings from a good board with a bad board can also be useful.
A bit more about the free run harness.
I have done is setup the test bed. I'll attach a photo.
I have the UMC 206/481/482 chipset (though I suspect the 206 datasheet I have might not be correct)
I also ran the two systems with the probe on the CPU's RESET pin. On the working board, at hard reset, the RESET pin goes to Vcc, then remains at GND. On the defective board, the RESET pin stays at Vcc. What can we deduce from this information?
The processor isn't being released from reset
You probably don't need to remove that QFP chip to use the PGA socket, at least on the boards I remember the on-board CPU was disabled either by jumper or automatically.
Im guessing you already tested using socketed cpu on one of the working boards to confirm its fully operational on this board model, and experimented with 50MHz(or even 25MHz) clock gen chip?
That looks to be the same datasheet for the UMC Um82C206 that I have. The pin count is wrong for what I have
for what I have and I am measuring GND (Vss) pins for the 206 in completely different locations as specified on that datasheet.
I'm getting GND around pins 71, 55, and 14. I also do not measure a Vcc anywhere on the 206, even on the working motherboard. Does the Vcc from the PSU head to a resistor greater than 50 ohm, in series, before heading to the 206? Or does Vcc from PSU head to one of these IC's, perhaps to change the output impedance?
From what I can tell by DMM probing, the power good (PG) pin from the power supply heads only to GD4069UB (HEX Inverter), the output of which heads to one of the pins on the UMC 206 (6th in from the top left), or around pin 69 on the wrong datasheet
Nice 386 resources; wish I had time to really get into them. These days, I hardly have time to shop for groceries.
The RESET pin from the CPU goes to pin 5 (PESCPU) on the 482 chip. I couldn't find any description for what PESCPU stands for. Under which conditions does the 482 change PESCPU's output from high to low? Faulty 482, or could it be something else? Should I cut the trace and force RESET to GND?
One other item of note is that on this particular dead motherboard, I do get the three keyboard lights illuminating at power on. They go on for less than a second, then all go out. If connected to the KVM, they go on, out, on, then out again.
Without CPU RESET pin going low, what is left to probe? I assume I can disregard the previous probing recommendations at this point. Is the evidence pointing towards the 482, or more likely some other component which feeds into the 482 is faulty, and is making the 482 not want to set RESET to low?
RaszQuoteIm guessing you already tested using socketed cpu on one of the working boards to confirm its fully operational on this board model, and experimented with 50MHz(or even 25MHz) clock gen chip?I did not test my PGA AMD 386DX CPU in one of the working boards because there is no float jumper for the soldered-on QFP AMD 386DX. If inserting the PGA CPU shows a working motherboard, I would not know if it is the QFP or PGA CPU which is working. I know the PGA AMD 386DX works in other boards (not this MSI board).
I have checked the ISA, KBC, and BIOS to ensure that no pins, other than those indicated on the datasheet, are connected to the corresponding Vcc or GND.