🚀 go-pugleaf

On Sun, 30 Jan 2000 12:46:22 +0000, Mike Tomlinson
<mike@jasper.freeserve.co.uk> wrote:

>In article <A6+rZJAhEik4Ewaw@ghastly.demon.co.uk>, 'Dungeon' Dave
><Dungeon@SpamNot.Here> writes
>
>>Er... I didn't explain myself clearly previously -
>
>Don't worry, neither did I.
>
>> I'm led to believe that
>>the 2-digit code will tell you what component currently plugged into
>>the mobo is preventing it from firing up properly, such as a duff
>>memory chip, processor not seated, VGA card problems etc.
>
>A POST card is not going to tell you 'processor not plugged in
>properly'.  It might indicate, for instance, that data line D0 is not
>responding, but it's a leap of faith to suppose this means the CPU
>hasn't been fitted properly.
>
>Really, the best way to troubleshoot a dead PC is to methodically
>substitute components with known working hardware.

   I don't agree with this.  My POST card has fairly often pointed me
at the problem.  Typical items were 'faulty cache' on older Socket 7
mobos, and that hardy perenial, VGA card not working.

   The other main use is to see if the machine starts at all.  Without
a POST card it is very hard to know if the machine has failed to boot
because of a faulty VGA card or cache or what ever, or just hasn't
started.  The later usualy indicates an incorrect setting of jumpers
for the processor, or possibly a pillar underneath that should have
been removed when the mobo was fitted.

>
>Going off at a tangent (who me?) I have doubts about the value of ISA
>POST cards.  Since in modern PCs the ISA bus is now a sub-bus of PCI and
>hanging off the north bridge, is it really possible for a POST card on
>the ISA bus to access all the relevant hardware to perform checks
>properly?  In the days of all-ISA machines, the POST card had direct
>access to the address and data buses via ISA; today's machines have
>quite some level of indirection between busses and system memory.

   POST cards don't access a bus at all.  The way they work is much
simpler than that.  During POST (Power On Self-Test) the processor
carries out a lot of tests on the hardware.  At each stage it writes a
value to port 80H to indicate where it has got to.
   On all machines port 80H is 'fresh air'.  i.e. there is nothing
there.  All a POST card is a regsiter at that address connected to two
LED-segment displays to show the hex value of the byte writen.  There
is nothing any more clever than that.

   In the early stages of POST, before the VGA card is woken up, a
failure of a test will usualy put the processor into either a HALT or
a tight loop.  At this point the value displayed can be looked up in a
table to get a guess as to what is wrong.

   Regards,
         Harry.