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    Technology/z80/CPM/KERMIT, (sdf.org), 11/28/2018
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I had previously mentioned that I had my z80 going, but that 
I didn't have a way to backup my files. Now that I think 
about it, I believe that I partially solved this problem 
before, but since I spent the time to solve it again, I 
figured I'd post about it.

The Grant Searle z80 CP/M SBC has two serial ports. One is 
built out with a max232 chip, so you can just plug in a 
normal DB9 serial connector and hook it up to your pc (or 
other hardware that does the RS232 thing.) The other serial 
port on Grant's design is a TTL connection, which lets you 
connect easily to modern hobby stuff, to Grant's terminal 
design, etc. 

In CP/M, thanks to Grant's BIOS, the IOBYTE is handled such 
that you can select the serial port you want to use for your 
terminal at boot time by pressing space, and it will be 
assigned to the CON: device automagically. The TTL serial 
port is assigned to the logical CRT: device, while the RS232 
is assigned to the TTY: device. If you press space on the 
CRT:, then it's set as the CON:, likewise if you press space 
on the TTY:... this happens at the monitor level, before you 
boot into BASIC or CP/M.

In theory, you can use the device that isn't assigned to the 
CON: for other things, via other CP/M logical devices. Also, 
you can change device logical assignments with STAT or by 
poking the iobyte (3). It's a nice setup. Except, it doesn't 
work the same way in both CON: setups. I had to learn this 
the hard way (by fiddling. It's OK, it's also the fun way.)

Because I have a serial terminal with a DB9 port right on 
it, I was using the RS232 connection for my main terminal, 
and the TTL as my second serial port. It would *kind of* 
work, sometimes. At first I thought it was because I had the 
wrong software in CP/M. Then I thought I had the wrong 
parameters on the other side of the equation (minicom on a 
modern computer with a USB->TTL cable.) I went through all 
kinds of troubleshooting, getting very mixed results.

Finally, I figured it was my USB->TTL cables (both of them. 
they were cheap, after all.) So, I hooked up the second TTL 
port to a full max232 setup, and promptly got the same poor 
and inconsistent results.

Just for kicks, after all else failed, I decided to try my 
terminal on the second port (now that it had RS232 connected 
up), and the first port as my secondary... that is, the 
original TTL on Grant's design was now my primary, the RS232 
on Grant's design was now my secondary. This is, I think, 
how Grant intended things to be anyway. And, bingo, 
everything works consistently. I don't exactly know why, but 
this setup works right, the other does not.

Today I went back and tried out KERMIT again, and was able 
to transfer a binary file properly without a hitch. Thanks 
KERMIT.

So, I have a functional backup/transfer solution, and I 
didn't have to write it myself. Now, I can use the computer 
for whatever I want. Within reason.