Re: RS485 Communications

In article <[EMAIL PROTECTED]>, 
[EMAIL PROTECTED] says...
> 
> "Richard Skinner" <[EMAIL PROTECTED]> wrote in message
> news:[EMAIL PROTECTED]
> > I have a PC104 SBC that has onboard RS485 ports.  I previously wrote
> > standard RS232 comm port routines to talk to devices on the rs485 network
> by
> > using a 232 > 485 convertor and everything worked great.  Now, however, I
> > want to use the onboard 485 ports.  I have two questions....
> >
> > 1, my port is a 4 wire, my network is a 2 wire.  This is how I converted.
> > +TX & +RX Tied together,  -TX & - RX Tied together.    The -TX & -RX hook
> to
> > A on the Network, and the +TX & + RX hook to B on the network.  Is this
> > correct?
> >
> >
> > 2.  In my standard Rs232 Routines, I modified to simply Raise RTS, wait 10
> > ms, then then the string, wait 10ms, lower RTS.
> >      I don't do anything special at all waiting on chars to arrive.
> >

This is why the chief engineer had someone invent oscilloscopes!

1.  10mSec seems like a long delay after transmission.  What is the
baud rate?   You should normally delay only about one character time
after the end of a transmission to switch to receive.   This is
especially true if your device is a system master.  At faster baud
rates,  some devices my have completed their response in 10mSec.
2.  Get a scope and make sure your data is actually being transmitted.
Then see if you can find any responses.
> > Problem is, none of this works.  Any pointers?  I assume I can use the
> > standard Serial Port Routines if I turn off  All flowcontrol and use the
> RTS
> > line right?
> >
> >
> > I find tons of info on the difference in the specs, but not much about
> > writing to the ports.
> >
> > Richard
> 
> At least there has to be a jumper / control bit determining which of the
> incoming data buffers is connected to the UART receiver: you cannot feed it
> from RS-232 and RS-485 buffers at the same time. The feature is not standard
> on a PC serial port, it could be using the extra output bits present on
> 8250 -series interface chips. Go after the hardware documentation / vendor
> for details.
> 

Then verify the settings with the oscilloscope!  ;-)


Mark Borgerson