if only rxd/txd/0v was required to interface with a certain preloaded mcu, is it plausible that that would work through mac's rs422-type serial as well?
I know I'll still have to see about actually being able to write something onto to it that way, but so far the pdf seem to suggest that its being programmed textually through the os's com port hence why I'm perhaps wondering if I could try get away with this 'kludge-ish'
MCUs usually use some kind of "UART" which tends to be 5v, 3.3v or even lower with logic states represented by 0v and +ve.
The Mac uses a standard called RS422, although it can be bodged to RS232 through a non-compliant one simple trick that doesn't work in all instances.
At the very least, you'll need a UART to RS232 adapter and then a cable to adapt to RS422...
At that point there is about a 25% chance it will work, because I find there is some quirk with one of the most common UART to RS232 chips made by FTDI that means they don't work with old macs.
For background, RS232 is technically something like a signal of -3 to -15v for logic 1, and +3 to 15v for logic 0. This means the recieving hardware should react to -3 and +3v at a minimum, but also cope with -15 and +15v. While on the other hand RS422 uses what are called differential pairs, where the signal is represented by the gap in voltage between two lines (so you have +RX and -RX, as well as +TX and -TX). RS422 voltages are something like -6 and +6v a the same time on the two wires, where the polarity of signal defines if it is a logic 1 or 0....
But anyway. You can do it but it is faff. You can do it properly (possibly speak to
@cheesestraws who made his AirTalk do things properly) or via RS232 which is technically easier, but be aware that sometimes Macs don't like certain adapter chips and there are about 4 ways to wire an RS232 to RS422 cable.
Apple's trick to make RS232 mostly work was to clamp one side of the pair to a fixed voltage which forces it to behave referenced to that voltage and more like RS232... But not perfectly to spec.
