One more screwup with the Ace...

From: vassilip_at_dsl.cis.upenn.edu <(vassilip_at_dsl.cis.upenn.edu)>
Date: Mon Dec 16 19:04:23 2002

Removing ICs without a good soldering iron is an exercise in futility.

About 15 years ago I was trying to remove some ROMs from a 8086 machine
(a Japanese machine called Samurai with 2 8 inch floppies, running
MSDOS 1.x - it was not a PC compatible, so it could not run anything
other than its own version of MSDOS).

The machine had a passive backplane with a series of posts on the back
so that you could attach probes to the bus lines. I didn't have a proper
technical manual, but I did have the schematics and I had taken a
dump of the BIOS.

The ROMs in question were on the video controller card and were used to
store the character bitmaps. I wanted to replace the ROMs with static
RAM so that I could change the character set of the machine.

Unfortunately I made a complete mess of removing the two ROMs, but a visual
examination of the board did not reveal any obvious damage. So I soldered
a pair of sockets and mounted the original ROMs to see if the machine
worked. Guess what, the fan worked, but I did not get any disk activity
and the screen remained blank. Time for the funeral.

So I carried that machine to the hardware lab of the University and
asked the help of a friend. He was playing with a new (at the time)
Dolche instrument that could attach to the bus of a microprocessor
and display the code that was running (the actual assembly instructions).

So he was quite excited at the prospect of using his new toy on a real
machine. We plugged the DOLCHE to the Samurai backplane and we found
that the processor was running, but stuck in a tight loop. So we looked
at the ROM listings and found that the machine was waiting for the
vertical blank signal. So we used a probe to connect the output of the
video IC to the appropriate line on the backplane. As soon as the probe
made contact we heard the floppy come to life. The screen was also
working and the machine was trying to boot.

We followed the VBLANK trace and sure enough it disappeared under the
two character set ROMs.

So I had the machine working again, and a war story to tell.

Best Regards

**vp
Received on Mon Dec 16 2002 - 19:04:23 GMT