Re: RS485 Communications

Yes, this was an Advantech board.  I fixed it by doing the following:
1st - fixed the IRQ Vector problem  - I was using the wrong value for the
non-standard port.
2nd - Using standard RS232 Comm Routines however I turned off all the port
options (required for modems)
3rd - I simply switch the RTS before sending data, then switch it back
immediately after the send buffer is clear.

I'm communicating okay at 38,400 using Modbus RTU (4 wire to 2 wire setup).
I never used 485 before beside with an 232 to 485 adapter so I wasn't sure
what to expect.

Thanks,
Richard



"Thierry C. (NOOS)" <[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.
>
> Although I don't know much about RS85 and I don't know the pinout of the
> onboard port in RS485 mode, I've had the same problems with ADVANTECH
> hardware, trying to talk to industrial I/O ADAM modules from a SBC
> motherboard (in the "embedded" product line), by switching the direction
> using RTS too (and the RTS toggle mode that sets automatically RTS during
a
> write operation).
> By calling their technical support they explained me that the RS485 ports
on
> their motherboards don't fully support RS485, especially the automatic
> detection of receiving data, a functionality required to communicate with
> most RS485-compliant devices.
> Then I guess that RTS is unable to switch early enough the line in receive
> mode, causing incoming data loss.
> The only solution we found was either to buy an extension card with real
> RS485 ports, or to use a RS232<-->RS485 adapter like you did.
>
> > 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
> >
>
> I hope it did help you.
>
> Thierry C.
>
>