[Gllug] USB <=> RS232 adapter query

John Winters 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 
little rusty.

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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