| Posted By
DeadTED
on 2013-02-19
08:11:11
|  Using 27c256 to replace 27128
As the topic suggests, how do I use a 27c256 to replace a 27128, it would be nice if I could stack two rom images per chip giving me a total of 4 images on two chips, but would also be happy if I could get a one for one swap. Looking at putting script plus and calc plus in the plus 4 to replace the inbuilt software.
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Posted By
crock
on 2013-02-19
11:22:44
 | Re: Using 27c256 to replace 27128
Very easily. The only difference between the two is pin 27, which is the program enable (/PGM) on a 27128 and A14 on a 27256. On the C16 and plus/4 boards pin 27 of the roms is tied to +5v, as you would expect.
So, to do a 1 for 1 swap, you don't need to do anything. Just make sure you put the image in the upper 16k of the 32k eprom.
If you want to be able to switch between 2 images, you need to have a method of switching pin 27 between ground and +5v. Unless you want to avoid taking a sharp instrument to the board and cutting the trace, I think the best way of doing this is to put the eprom in the socket with pin27 bent out of the socket.
Using a SPDT (single pole, double throw, aka "changeover") switch you need to wire it so the switch directs pin27 to either ground or +5v. If you have a SPST (single pole, single throw, aka "on-off") switch then you need to put a pull-up resistor (about 1k) between pin 27 and +5v and then wire the switch between pin27 and ground. When the switch is "on", the lower 16k will be selected and "off", the upper 16k.
cheers, Rob
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Posted By
DeadTED
on 2013-02-19
19:06:34
| Re: Using 27c256 to replace 27128
Rob, thanks heaps for a detailed response, I have a couple more questions.
1. Assuming I put script plus and calc plus on the two EPROMs, then the switch would need to switch both roms, with the machine off, right?
2. Is there any way, I can get the f1 and f2 keys to invoke the roms,like when you plug a cartridge into the cartridge port? That way it would be a soft rather than hard switch.
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Posted By
crock
on 2013-02-20
17:34:51
| Re: Using 27c256 to replace 27128
1. Right - just wire the switch to both eproms in parallel.
2. This is a good point. As calc+ and scipt+ are cartridges, the code in them may make assumptions on which ROM bank they are running in an thus *may* not work as an eprom plug-in replacement for the 3+1 software. On reset the kernal cycles through the banks, probing for whatever is present and handing over control if a cartridge is detected. I don't know if it's the kernal or code in the option ROM's which enable the 'F1' to activate the option software.
The banking logic allows for 4 different ROM sets. Usually this consists of the system bank which is normally the basic & kernal, the 'option' bank which contains the 3+1 software and two cartridge banks, C1 & C2. I'm not aware of any +4 cartridges that make use of the C2 bank on the cartridge so in theory you could modify the logic to have C2 select the upper 16k of 27256 option ROM's , but that would require some hardware modification of your +4 as well as the software.
In short, I don't think you could do it through software alone.
Rob
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