[Gllug] USB <=> RS232 adapter query
john at sinodun.org.uk
Sun Mar 4 17:06:51 UTC 2012
I've been playing today with a cheap USB <=> RS232 adapter, and have
completely failed to get it to work.
I'm trying to connect the RS232 port on my PV inverter to a Sheevaplug
for data-logging purposes.
If I connect it by way of a null-modem cable to a real COM port on a PC
then all is hunky. I switch off flow control (because the connection on
the inverter is just a 3 wire one) and I can send queries and get back
responses. However, I don't want to leave a fully fledged PC switched
on next to it all the time. I'd much rather use a 3W Sheevaplug to do
the same job, but the Sheevaplug doesn't have an RS232 port.
No problem - just use a USB <=> RS232 adapter, except I am quite unable
to get any communication to happen through it.
I used to do quite a lot of fiddling with serial ports back in the
Prestel days, and I though I knew what I was doing, but it's all a
My recollection is that +ve is space and -ve is mark on data lines, and
then it's the other way around on control lines (+ve is yes, and -ve is
no). However, both the real COM port and the USB one seem to put -ve on
RTS to indicate readiness. Huh?
Given a real COM port, I can switch off RTS/CTS flow control, loop pin 2
back to pin 3 and then anything I type into Minicom comes straight back
to me. If I switch on RTS/CTS flow control, I also need to loop RTS
back to CTS (pins 4 and 5) to re-establish the flow of data. That's all
fine and just what I would expect.
However, with the USB COM port, I can't get any data to flow at all,
even if I switch off hardware flow control. (Perhaps it is ignoring the
instruction to ignore hardware flow control.) It puts a negative
voltage on pin 4 (RTS) and a positive one on pin 20 (DTR), and that
seems to be about it.
Perhaps I've got a faulty USB <=> RS232 adapter. It was very cheap.
The manual which came with it is a joke.
Can anyone recommend a USB <=> RS232 which is known to work with Linux?
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