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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

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.

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

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.