(me too)! Re: [Soekris] rs485 instead of rs232

[Henry Spencer]

On Tue, 13 Jul 2004, Chuck Yerkes wrote:
> The question is how does BSD see the devices.  Or more specifically,
> how do I get into from a certain device from a BSD commandline or
> perl (best) or C (less best, but passable)?

Unfortunately, the answer is that in general it doesn't see them.  It sees
a serial line, on which you can send and receive bytes, and maybe enable
and disable transmission if you can figure out exactly how.

Which bytes you have to send/receive to talk to a particular device is
your problem.  There's no way BSD, or any system, can deal with all that
for you, because RS-485 devices vary enormously in how you talk to them,
and there is no standard. 

RS-485 just defines (roughly speaking) which voltage means 0 and which
voltage means 1.  It doesn't define a standard packet format that all
devices have to implement, or an addressing scheme that all devices have
to recognize and respect, or command codes that all devices have to
understand and respond to.  It's not plug-and-play like Ethernet often is
now; it's one small part of a Do It Yourself toolkit. 

                                                          Henry Spencer
                                                       henry at spsystems.net