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Retrocomputing

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    I would bridge pins to simulate hardware flow control as well, just in case you haven't switched it off! See ni.com/tutorial/3450/en for the DB9 and DB25 pin connections. Also, can you transmit from the raspberry pi to the terminal? You didn't mention trying that in your post. It all else fails, even a cheap multimeter should "flick" the pointer if you are actually transmitting data. A breakout box is a better diagnostic tool - you can get "cheap and cheerful" ones ($25) from Amazon, Ebay, etc. Commented Sep 6, 2018 at 0:29
  • good thoughts - I will wire the hardware control loopback and see what I get. This is when you kick yourself for ever throwing anything out as I used to have a breakout box and all that gear. Commented Sep 6, 2018 at 1:57
  • I was going to suggest voltage levels on the USB-RS232 converter, but if even a loopback doesn't work then that's not it. You need one of these I probably have one around here somewhere...used to use them quite a bit a "few" years ago. Commented Sep 6, 2018 at 3:31
  • Another basic test is to measure voltages and see how well they are in the RS232 range. Some USB dongles are very close to the 3V "undefined" level, which can cause trouble with vintage equipment. Commented Sep 6, 2018 at 5:42
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    @dirkt USB is out of the equation for now - trying to get characters echoed back to the terminal via a simple loopback. If I get past that hurdle will try USB to a pi again. Commented Sep 6, 2018 at 5:55