Second Opinion on Unknown PET option roms

From: William Levak <wlevak_at_SDF.ORG>
Date: Sun, 12 Sep 2021 04:21:21 +0000 (UTC)
Message-ID: <Pine.NEB.4.64.2109120358320.2539_at_sdf.org>
I have checked my notes again and the 6502.org site.

The ROM in question is 901447-30. This is a 2K ROM for the test fixture 
for PETs without CRTC.

The reset button on the test fixture toggles the reset line on the 6502.
Normally, this would cause the 6502 to fetch the address at $FFFC-D and 
jump to it.  Since the test fixture pulls A13 and A14 low, this causes the 
fetch address to be $9FFC-D. Therefore the ROM address MUST be $9800-9FFF.

NOTE: If there is a ROM already in the PET at this address, it must be 
removed or it will conflict with the diagnostic ROM.

The test fixture can be used on the original PETs with a slight 
modification to the system board. Since the original PETs did not decode 
addresses in the $9000 block, this must be enabled. Cut the trace between 
pins 4 and 5 of the 74LS21 at G4, and solder a wire from pin 4 of G4 to 
pin 10 of the 71154 at G2.

The test fixture for PETs with a CRTC contains 2 4K ROMs, one for 40 
column machines, and one for 80 column machines. A switch selects which 
ROM to run.

I thought I had uploaded all 3 of these ROMs to zimmers.net, but I cannot 
find them there. I can upload them if they are not already there.

wlevak_at_sdf.org
SDF Public Access UNIX System - http://sdf.org
Received on 2021-09-12 07:00:03

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