Multiple serial consoles via null modem cable

Olivier Gautherot olivier at gautherot.net
Tue Jan 12 16:32:22 UTC 2010


Hi Marin,

I'd like to ask you about the following - is it possible to have multiple
> serial consoles coming from a single host?
>

In theory, no :-) RS232 is a point-to-point connection. If you want to feed
several consoles through the same serial port, you will need some kind of
switch to route the traffic to the corresponding port (whether the cable
needs to be null modem or not wil depend on the switch itself).


What I am talking about is connecting multiple machines using a null modem
> cable - I know it is possible only to connect two machines and they need to
> be connected on sio0 (COM1).
>
> I'm thinking about the following situation - 1 system acting like a host
> with a serial port hub, each port of the hub is connected to a different
> machine on sio0, using null modem cables.
>
> This would make the first machine something like a cheaper kind of a
> terminal concentrator :)
>

The options that come up to my mind are:
- a mechanical switch but it would not allow seamless routing (requires
operator action to change channel)
- a software switch using ESCAPE codes to change channel through an ad-hoc
device driver - but there, make sure you don't miss bytes or the traffic may
end up on the wrong terminal
- if you fancy a home design, you could use the parallel port to send an
address word but you will have to design your own seial port emulation on
top of the standard sio0 to synchronize the signals... I would not bet too
much on this one ;-)

Also, evaluate if you need priority management: you may get to a situation
where you need to send urgently characters to a terminal while another one
has a long queue pending. In this case, you may be better off with
Ethernet-to-Serial or USB-to-Serial converters.


Hope it helps
Cheers
--
Olivier Gautherot
olivier at gautherot.net
Cel:+56 98 730 9361
www.gautherot.net
http://www.linkedin.com/in/ogautherot


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