Coco 1 and Coco 2 (Dragon hybrid!)

A forum for members to offer for sale any Dragon related items to other members. DO NOT POST LINKS TO AUCTIONS. I will use this forum in the future to offer all my spare games to members of this site before they go on e-bay.
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fridgemagnet
Posts: 89
Joined: Mon Nov 24, 2014 8:33 pm

Coco 1 and Coco 2 (Dragon hybrid!)

Post by fridgemagnet »

Hi all,

Starting to get to the interesting stuff now ie. the machines. I have a Coco 1 and Coco 2 looking for a good home, again all I'm asking for here is enough to cover postage (especially as they are both in an "interesting" configuration).

Both of these underwent my "Coco to Dragon" conversions - see http://www.onastick.clara.co.uk/cococnv.htm which effectively replaced the original Tandy ROMS with Dragon versions & had adaptor boards fitted such that the keyboard wiring differences were mitigated. As such, they can (bar the exception of the printer port) work as a basic Dragon. My intent was to try and put them back to their original fit before posting them on here however that didn't quite go to plan...

On the original Coco 1, the conversion was never that reliable because of the messy wiring needed to get the EEPROMS wired into the PROM sockets. So I've removed this, the keyboard mod and replaced the original BASIC & Ext BASIC PROMs. This now fires up nicely as an original CoCo. However it's only reporting a 16K memory fit whereas I'd fitted it (& jumpered it) for 64K DRAMs. I'm not entirely sure why this should be the case, I don't remember there being anything else necessary for it to detect the memory fit so it's possible one of the chips has died. The one oddity which *might* be related is that I'd put a flying lead between one of the 4164 DRAMs (pin 3, the write input) and a 74LS02 NOR gate (pin 2, another input). Not entirely sure why I did this, I'm assuming it was something to do with the Dragon conversion although I didn't mention it in my original article. Having snipped it though, doesn't make any difference to the memory being reported. The other features to note about this are that the power switch doesn't work (is permanently on) and I fitted a 5-pin DIN socket on the rear which was designed to feed to a composite monitor (like the Dragon's monitor output) - that is no longer connected.

On the CoCo 2, these used EEPROMS rather than PROMS for their BASIC ROMS, so swapping over to a Dragon ROM was much easier and also much more reliable. Unfortunately I can't find the original CoCo ROM for this, so I've left it in it's "hybrid Dragon state". Seems to fire up ok.

Cheers,

Jon.
prime
Posts: 266
Joined: Fri Apr 10, 2009 1:40 am

Re: Coco 1 and Coco 2 (Dragon hybrid!)

Post by prime »

fridgemagnet wrote:Hi all,
On the original Coco 1, the conversion was never that reliable because of the messy wiring needed to get the EEPROMS wired into the PROM sockets. So I've removed this, the keyboard mod and replaced the original BASIC & Ext BASIC PROMs. This now fires up nicely as an original CoCo. However it's only reporting a 16K memory fit whereas I'd fitted it (& jumpered it) for 64K DRAMs. I'm not entirely sure why this should be the case, I don't remember there being anything else necessary for it to detect the memory fit so it's possible one of the chips has died.
Well if it's firing up to basic with 16K available then the ram is probably ok, as the chips are only one bit wide so each data line goes to one of the 8 4164s. So if one of the 4164s had died then it would take out one of the data lines which would prevent the machine from booting.

It could possibly be that the ROM in the CoCo 1 only 'knows' about the 4K and 16K configurations, these are usually set by one of the PIA inputs IIRC, so it could be that the SAM is only being initialized expecting 16K chips and so is only able to see 16K of memory, as with 16K chips it would think it has 1 or two banks of 16K chips and so would only be activating the /CAS for the RAM in the 0..16K area.

Cheers.

Phill.
fridgemagnet
Posts: 89
Joined: Mon Nov 24, 2014 8:33 pm

Re: Coco 1 and Coco 2 (Dragon hybrid!)

Post by fridgemagnet »

prime wrote: Well if it's firing up to basic with 16K available then the ram is probably ok, as the chips are only one bit wide so each data line goes to one of the 8 4164s. So if one of the 4164s had died then it would take out one of the data lines which would prevent the machine from booting.

It could possibly be that the ROM in the CoCo 1 only 'knows' about the 4K and 16K configurations, these are usually set by one of the PIA inputs IIRC, so it could be that the SAM is only being initialized expecting 16K chips and so is only able to see 16K of memory, as with 16K chips it would think it has 1 or two banks of 16K chips and so would only be activating the /CAS for the RAM in the 0..16K area.

Cheers.

Phill.
yes, after I wrote that original post I thought about it some more and came to very much the same conclusions as you Phill. I know that bit #2 on the PIA at $ff22 is used for something similar on the Dragon, so on D32 detects which type of RAM is fitted such that it can configure the SAM correctly. From the limited info I can find, that same bit appears to be a 4K/16K selection bit on the CoCo (mine is set to 16K so that tallies) but there doesn't seem to be a 32K equivalent. Given the different flavours of CoCo around then (4,16 & 32K) maybe there were different ROMs around as well. Seems odd though given that the hardware there can clearly support the 64K fit yet the stock ROM doesn't. What I probably need to do is find a decent CoCo forum out there and ask.

Rgs,

Jon.
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tormod
Posts: 416
Joined: Sat Apr 27, 2013 12:06 pm
Location: Switzerland
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Re: Coco 1 and Coco 2 (Dragon hybrid!)

Post by tormod »

The place to ask for anything CoCo would be https://pairlist5.pair.net/mailman/listinfo/coco
fridgemagnet
Posts: 89
Joined: Mon Nov 24, 2014 8:33 pm

Re: Coco 1 and Coco 2 (Dragon hybrid!)

Post by fridgemagnet »

Thanks for that Tormod.

Having done just that, came across this thread (started by Phill I note some 8 years ago):

http://coco.maltedmedia.narkive.com/2DM ... ifferences

which, sifting through suggests that the Color BASIC 1.0 ROMS would only support the 4/16K configurations (I have 1.0 BASIC, 1.1 Ext Basic in mine). There was a suggestion that you could POKE the SAM chip into the 64K mode - something I'd already considered but discounted because as the mailing list suggests, it needs to be done very early in the reset cycle before the RAM is used.
fridgemagnet
Posts: 89
Joined: Mon Nov 24, 2014 8:33 pm

Re: Coco 1 and Coco 2 (Dragon hybrid!)

Post by fridgemagnet »

Quick update on these items, thanks in part to some helpful folk on the CoCo list both these machines now have a working 64K memory fit and have the original CoCo Extended BASIC ROMs fitted. So if anyone is interested in either/both of these let me know.

Rgs,

Jon.
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