[Coladam] Disk Drive ROM

geoffrey oltmans oltmansg at bellsouth.net
Mon Mar 19 16:09:13 CET 2012


If you look at the schematic for the Coleco drive, the only way to configure the 
MPU is either in Mode 2 or mode 6 (6803 only supports mode 2 and 3). You can 
check the position of the jumper wire at E1,E2,E3 to see what position it's in, 
but I suspect you'll find that it's wired for external ROM use. I have partially 
disassembled a dump ROM and it does contain valid jump vectors at the top of the 
ROM, and the ROM !OE is wired to P47 through an inverter without any additional 
gating. Partial address decoding is used and gives you the 1k RAM in the lower 
32k bank (somewhere, and the upper 32k for the ROM So, I think it's an either/or 
proposition but not both at the same time.

I imagine that Coleco probably intended to use the internal ROM on a 6801 but 
did this as a CYA in case the firmware turned out to be buggy. Run the firmware 
in an external ROM til it's been in there for a little while (and had a little 
field testing by customers), and if they haven't found any problems burn it to a 
6801 after a few production runs and leave off the ROM later.

I could be wrong about all this of course. ;) I have dumped the ROMs out of all 
the 6801 controllers on the stuff I've got (as have Chris Braymen and a few 
others) but I've never tried doing a disk drive 6801 to see what's actually in 
it.





________________________________
From: John Turner <gimechip at live.com>
To: coladam at adamcon.org
Sent: Sun, March 18, 2012 2:18:31 AM
Subject: [Coladam] Disk Drive ROM


Well, I just took apart one of my Coleco Disk drives. Now, this thing was sold 
to me as working, but upon inspection I noticed one of the screws missing from 
the bottom. That's a bad sign I thought. Upon opening the thing up, I notice the 
34 pin cable isn't even plugged in - no way this thing works. Looks like a 
repair project is in order... Anyway, the ROM in this one is actually an EPROM - 
an AMD 2732. I would not have thought that Coleco would have used EPROMs in 
production drives, given they eventually forget their data. Sure, it usually 
takes 20 years or so, but still... perhaps this is a 3rd party ROM? It has a 
sticker labelled:
AD 31 REV.A 9-27-84. I've dumped the ROM to my PC. Bytes 0-7FF are blank (all 
FF's) and bytes at addresses $800-$FFF contain data. This would seem that the 
first 2K of the ROM is blank and the last 2K contains data. Now, since a 6801 
contains an internal 2K of ROM, it makes absolutely no sense to use an internal 
ROM for 2K of storage Unless Coleco used the internal 2K and needed an 
additional 2K. Looking at the 6801 I cannot confirm that it is a 6803 as I 
thought as it is a custom COLECO chip - this leads me to believe that it is in 
fact a mask programmed 6801 made for coleco and must contain 2K of the operating 
program within. Can anyone confirm this? Thanks - John
                          
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