Description
The RC2014 Orton 3C was never designed with I/O in mind, other than serial via the /INT and A15 lines. The fact that it can work with regular I/O modules is fantastic, although there are some side effects. The Orton 3C-Plus Output shim gets over these issues.
If you are using your Orton 3C in its pure configuration of Front Panel and CPU/RAM Module, you do not need this shim.
If you are using your Orton 3C with a backplane and a Front Pane I/O Module then this may be useful. Starting your code at 0x0001 with a NOP at 0x0000, however, will work just as well.
However, if you have different I/O modules, or write large programs, this shim will make life easier for you.
The Orton 3C does not use the /MREQ line to decode memory writes. Therefor, a port write looks the same as a memory write to the RAM. Memory corruption when using I/O devices is generally a bad thing. In most real world situations for the 3C this isn’t a big issue though. But if you use various I/O devices with port addresses that overlap your code, then it can be bad.
This shim sits between the Orton 3C Front Panel and the backplane holding the CPU/RAM module. It intercepts the Write Enable signal from the Front Panel and only passes it through if it is a genuine memory write.
