From: William Levak (wlevak_at_sdf.lonestar.org)
Date: 2008-07-08 06:44:04
On Mon, 7 Jul 2008, ruud.baltissen@apg.nl wrote:
> Hallo Ethan,
>
>
>> The docs I just read mentioned one chip inside besides the ROM
>
> The PCB contains two sockets for (EP)ROMS plus a 74LS138 (on a socket as
> well). My wild guess is that this 138 takes care of mapping the EPROMs
> in the the $9000-$AFFF area. (But what if there already ROMs in the,
> normally empty, sockets?)
The EPROM's are mapped to $9000-9FFF. Resistors pull the address lines
from $Fnnn to $9nnn, and the button on top toggles the reset line. On
reset, the 6502 jumps to the reset vector at $FFFC-D, which because of the
resistors becomes $9FFC-D. The first part of the ROM routine checks the
timer and address lines and then loads the rest of the routine to zero
page and jumps to it. At this point a message is displayed to remove the
clip. When it detects that the clip has been removed, it continues with
the rest of the tests in an endless cycle. The PET must be turned off to
terminate the test.
If you have the version with 2 4K EPROMS, there is a switch to select
which EPROM is run. This switch was originally a momentary contact push
button, but I had trouble using two push button switches and the clip all
at the same time, so I replaced it with a toggle switch.
If you have anything at $9000-9FFF, it must be removed.
wlevak@sdf.lonestar.org
SDF Public Access UNIX System - http://sdf.lonestar.org
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