--- In "gmaglathlin" <> wrote:
>
> I am endeavoring to use the RS-485 driver on a TS-752 driven by a TS-7550
> board. I successfully open /dev/pts/1 and can write data to the port which
> is reflected back to me, exactly as it should on a two wire RS-485 interface
> with the transmitter enabled. My problem is I can not disable the
> transmitter (ie place it in the high impedance state). Using setdiopin 27,0
> or setdiopin 27,1 makes no difference. The data is reflected (ie the
> transmitter is enabled) in either case. This is the dio that is supposed to
> control the tx enable of the driver. I have tried this with versions 4, 5
> and 6 of the FPGA bitstream. Anyone had this experience or ideas or know the
> obvious thing I am missing.
> Thanks in advance,
> Greg
>
I think you mean you can't disable the receiver, if you always get what you
send. I had a long mail from Kris at technologic.
We used to use external RS232/485 converters, and with those you don't get what
you send as the receiver is disabled while you send. On the Ts752, and on our
own board using very similar circuitry, the receiver is not disabled. Look at
the schematic; you will see that the RXEN is tied low.
Kris said this is deliberate so that you can check that what you sent was
actually sent. If for example there is a low impedance or short on the D+/D-
lines, you can detect this as won't receive what you send. You can implement
collision detection this way, although it's going to get complicated in real
life.
Although it is possible to work around this in software, it's a real nuisance
for us as our programs that talk to external devices are sometimes connected
using RS232 and sometimes with RS485. Now the software has to cope with the
possibility that sometimes it receives its output and sometimes not.
The sensible way to deal with this is an echo-test routine as the start of the
program - send an invalid message, see if you receive it, set a flag
accordingly.
By the way, the ISO485 board (for the TS2750 series) is really horrible as you
have to set RTS low otherwise you will never receive anything.
Martin
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