[UK-CONTEST] RS232 1 Radio / 2 computers

[Andy Swiffin]

>>> On 25/07/2006 at 17:35, in message
<8C9A6B7580601F4FBDC0ED4C1D6A9B1D78C0C9 at plextek3.plextek.lan>, "Mark
Marsden"
<mm at plextek.co.uk> wrote:
> Hi Callum
> 
> I asked exactly this question on the N1MM reflector in January this
> year, but I got no replies.
> 
> The RS232 transmit line from the MP can go, by a Y-cable, to both PCs
Rx
> in lines, but only one PC can talk to the MP (at any one time).
> 
> Both instances of N1MM will be polling the MP and will expect
responses
> to their polling messages. I haven't tried it but I would expect
> messages like "Port not found" or something like that.


It's quite a while since I did much with RS232 signaling but I used to
do a lot (X25 and PAD port lines).  Whether you will get anything
sensible with the above depends on exactly how N1mm is talking to the
radio.  I don't know anything about how these 'modern' radios are
controlled through the rs232 port.  The issue will be whether N1mm is
statefuil, in other words does it need to have gone through a series of
events to make sense out of the incoming data or will it just take it
and display it, also is there some handshaking that goes on between N1mm
and the radio as a higher level protocol above the rs232 handshaking. 
Clearly both PCs cannot be allowed to talk to the radio.

At the RS232 level there are two types of handshaking that could take
place.  

Software - the receiver sends a ctrl S to the transmitter to stop it
transmitting while it catches up with the incoming data and then sends
ctrl Q when it wants some more. (I use receiver and transmitter in the
RS232 sense not a Radio sense)

Hardware - There are a number of lines DCD, DTR, DSR which are just to
do with letting the other end know that there's something there.  CTS
and RTS are the hardware handshake lines which carry out the same
function as the software handshake.

Clearly the handshaking can only be carried out by two of the parties -
but it may be that there is actually very little data being transferred
and there will be no handshakes necessary, so, you may get away with
just connecting the TX from the radio to the RX on the other computer. 
These could be be pin 3 on the radio and 2 on the pc assuming they're
DB9s, but check the radio manual because the sense of the two signals
changes depending on whether the port is a DCE or DTE and it could be
pin 2 on both.

I'm assuming the cable will be setup for all the handshaking to go
between the radio and the main PC, you should have nothing to the second
PC, but it may need its hand shake ports looped back to get the port
active (although I think most PCs just don't care), in which case, on
the PC you would connects DTR, DSR and DCD together (1,6 and 4) and RTS
to CTS (7 and 8).

A couple of references I came across that may be useful are below, but
there are dozens more around.

http://www.windmill.co.uk/wiring.html 

http://www.bb-elec.com/bb-elec/literature/tech/faq_rs232_connections_work.pdf


HTH and is accurate ( It's a looooong time since I did much with
rs232!)

73
Andy
gm8oeg