It's got to be a null modem adapter then..?
baud rate is 115200, data 8, stop 8, parity none, fcontrol none (probably hw none, sf none)
Are you by any chance running windows 10? I saw this last month.. i laughed.. because this stuff should just work. i plugged into my linux notebook.. minicom /dev/ttyUSB and it worked.. it was for a 10gige device, but same cisco-style cable.. and same typical prolific driver.. honest.. i dont know what to say.. I'm not sure a null modem is required, do you? its like a gender changer looking adapter but its labled null-modem..
hmm.. im sorry.. its a windows driver thing it looks.. one would think they have a testing division. this is one of the most widely supported devices in the world.. I bet windows 10 broke a boat load of GPS support with whatever "features" they added.. *sigh*
Cheers,
