I know that the UART on the terminal works, because I made serial loopback plugs out of a DB9 to DB25 converter. If I plug the DB25 plug into the back of the terminal, I get on-screen indications that the CTS and DSR signals are being detected, and when I type on the keyboard, it gets echoed back. So the terminal can talk to itself over its DB25 port.

On the PC side, I plugged the DB9 loopback plug (made from the DB9 end of the same converter) into two different USB-to-serial converters, and eventually pulled a PC out of a closet that had a real serial port. The DB9 loopback plug confirmed that the PC's end of the serial connection was capable of talking to Minicom on Linux on all 3 serial devices.

But, if I connect the PC to the terminal, the CTS and DSR indicators on the terminal turn off, and no data can be transmitted in either direction.

I heard somewhere that modern implementations of RS-232 completely ignore all pins except for 2 (RXD), 3 (TXD), and 5 (GND), possibly lacking the circuitry for the other pins. So, presuming that this was the cause of the problem, I disassembled another DB9 to DB25 converter and rewired it so that DTR and DSR and RTS and CTS were soldered together as in the loopback plugs, with the data and ground pins going all the way through. This way, the terminal would see its own signals as if they were coming from the PC, and since the USB-to-serial connector doesn't care either way (or so I presume), it wouldn't matter. The result was a loopback effect when the cable was plugged in on both ends, regardless of whether the terminal or the PC were on or off. Both ends got the loopback effect. If I unplugged either end of the cable, the loopback effect disappeared.

It was after doing all this that I realized that I had the PC with the real RS-232 port on it. But I got the same results with this PC as with the USB-to-serial devices.

The terminal has all kinds of serial-related settings, not all of which I understand. The most confusing settings are on the "modem control menu", and include "duplex mode", the options of which are FDXA, FDXB, FDXC, HDXA, and HDXB. Research indicates that "FDX" and "HDX" mean full and half duplex, but I don't know what the A, B, or C are for. On the PC side, Minicom has no duplex settings at all. I tried all of those modes, and none of them worked.

The other thing I didn't understand was the "turnaround/disconnect character", which seems to indicate a character for disconnecting, and another for "turnaround". The disconnect character can be disabled, but the only thing that can be disabled related to "turnaround" is auto-turnaround (but the character can be set to something unlikely to be sent to the terminal unless I accidentally dump a binary file). There's also an "initial direction", either "receive" or "transmit." Neither setting allows communication to take place, and I presume that this setting only pertains to half-duplex mode anyway.

There are the usual serial settings as well, such as bits per second, data/parity/stop bits, and flow control (in addition to the usual XON/XOFF, the terminal supports something called "DTR BUSY" which may or may not be the same thing that "hardware flow control" does in Minicom. It doesn't matter what any of these are set to, or if they match the settings in Minicom: No data passes though regardless. I also tried different baud rates, going all the way down to 300 BPS in case it was a noise problem.

I had just one moment when I thought I was nearing success: I jammed some pieces of wire into the two data pins and the ground pin of a DB9 gender changer, connected the other end of the wires to the USB-serial adapter, and then plugged this jury-rigged setup into the terminal. On the terminal end, I had the jury-rigged DB9-to-DB25 converter I described above. When I typed into Minicom, garbage appeared on the terminal.

So I took yet another DB9 to DB25 converter, and replicated the wiring implied by the setup I just described: On the DB9 end of the converter, only pins 2, 3, and 5 went through to the DB25 side, and on the DB25 side, DTR/DSR and RTS/CTS were soldered together. And it didn't work. No data went through.

I also took a voltmeter to the rig. I determined that there are 10V on the RXD and TXD pins when no data is being transmitted.

I'm at the point where I'm going to have to buy an oscilloscope and learn some actual electrical engineering. What else should I check? And how difficult will it be if I have to build some kind of a converter from scratch? What equipment would I need? How likely is it that somebody has already built such a device and I can just go buy it somewhere?