ISA Board Problem with PII Mother boards and later

[Ken J]

I'm at the end of my sanity.

I have an old Custom ISA board that is used in about 100 systems we have. It was originally designed to work on the 80286 system. It is only an 8 bit board mapped as 1k of memory at C800. The interface chip is an IDT7130 1K X 8 Dual-port Static Sram with support IC's and Interrupt 3 output (com 2). 19 address lines threw 2 74ls245 buffers and 1 74ls688 comparator. BALE for access and SMEMW / SMEMR for write/read data. 8 Bi dir data lines.

Up until the PII it worked on every thing. With the advant of the PII some boards would work, some would not even boot, some would boot but no access to this ISA board. AS it is getting harder and harder to source old 486 and PII boards that work, I've been trying to find the problem of why some boards work and some don't. The software only runs Dos 5.00 and it boots from the CD, no HD in the unit.

I have a PII board PC-chips M741 with the Xcel 2000 chipset. This board will 1 out of 40 reboots see the ISA board properly. I have checked every single Timing signal I can think of. I've poured over the schematics of the board and timing waveforms of the IDT chip and the ISA port. If I go into Dos "Debug" I can see the board every time. I can use debug to write to the board. I did eliminate one PAL chip that was needed only when used in a 486 or older, that controlled a set of 27c256 boot chips and routed the BALE directly to the 74LS688. In my test unit this work fine. With the M741 it does not. The only thing I can think of is that their is somehow a incompatibility between how the interrupts are being handled. I have no support from head office as of course they want us to sell the customer a new unit. I don't even think that they have programers that know what is going on in these old dos units as all the new stuff is usb and XP.

So I thought that someone here may know what difference came about with the advent of the PII boards

and maybe I can find a possible work around for it. Another thing, it seems the old AMI Bios has the least trouble and the AWARD bios the most trouble (they usually will not startup at all)

Ken

PS. I have tried every single Bios setting you can think of. Taken battery out for full reset, etc etc etc.

Insanity is an idle thought away.

[jcarle]

You're dealing with a change in architecture. Many things changed with the advant of the 386.

The 286 has 16-bit processor registers, whilst the 386 (and on) has 32-bit registers.

Protected mode changed with with 386 (and on) as well.

Something that may also be affecting your board interface since it directly maps to memory could be the A20 line.

The A20 or address line 20 is a well known address line in x86 bases systems because it is initially disabled when the system starts. Disabling the A20 line causes the first 64-KiB of memory above 1-MiB to address the first 64-KiB of memory. This is done to maintain backwards compatibility with real mode x86 programs which used this trick to conveniently wrap around the top of the address space within the constraints of real mode.

Also noteworthy...

The Intel 80486 introduced a A20M# pin that indicated when low that all external memory bus access should be zero. The introduction of this pin reduced the components required to implement the Gate A20 circuitry on a motherboard. No changes for software manipulation of the Gate A20 were made.

Full Article on Wikipedia.

[LLXX]

ROM shadowing may be causing a conflict with the card. Look in the BIOS setup for "ROM shadowing" option... disable it for segment c800.

[nmX.Memnoch]

You've probably already done this but I'll throw it out there anyway...

Be sure you're disabling the motherboard's COM2 port. There should be an option in the BIOS. Being that the boards are that old though, it may be a jumper setting.