Ok, I've narrowed down the problem to the Kouwell IO card that I have plugged into one of the 8-bit ISA ports. I got a DB-9 to DB-25 null modem cable from Amazon today so that I could use the internal DB-25 serial port on the PC instead of the card. I tested it just now, and it worked without issue.

I've already spent a lot of time figuring out how to configure the jumpers and DIP switch on the IO card so that its DB-9 serial port would show up as COM2 and use the appropriate IRQ. Before this issue, the wrong settings on the card were causing the kermit "connect" session to crash after typing two or three characters into the terminal, and sometimes, the kermit shell itself would hang when trying to quit or when trying to change port numbers.

Clearly, if I want to use this card, I still have some quirks to figure out, but I think that will be a puzzle for another day.

Onward, to testing file transfers using the internal serial port! : - )

Do you intend to have the serial port programmed as 7-N-1 on the DOS side? That's a bit unusual

jakethompson1 wrote on 2023-09-25, 20:57:

Do you intend to have the serial port programmed as 7-N-1 on the DOS side? That's a bit unusual

No, that's just the default. That screenshot was from when I decided to not "set term bytesize 8" to see if it would make a difference after having made numerous, various attempts with "set term bytesize 8".