Kirk- With half duplex EIA-485 timing is very important. Unlike Ethernet, EIA-485 does not support collision detection at the physical layer so collisions between outgoing data and returning data cause data corruption, since the trancievers do not back off and retransmit. After a query the controller has to release the line and allow the responder to talk.
You will probably have to watch flow control. For what you want to do the controller should transmit a query and then immediately tri-state, but converting from EIA-232 it may hold the line high or low, perhaps anticipating the next transmission. You might have to work with the way messages are buffered-perhaps you will only be able to buffer one message at a time. Javid ----- Original Message ----- From: "Kirk Wallace" <[EMAIL PROTECTED]> To: "Enhanced Machine Controller (EMC)" <emc-users@lists.sourceforge.net>; <[EMAIL PROTECTED]> Sent: Sunday, February 03, 2008 4:13 AM Subject: Re: [Emc-users] Modbus RS-485 Transceiver > On Sun, 2008-02-03 at 18:12 +1100, Peter Homann wrote: >> Hi Kirk, >> >> A couple of things. >> >> The ModIO I sent you has a RS232-485 converter built in. With this you >> can run >> a serial cable from the ModIO to the PC, then run additional Modbus >> devices >> from the RS485 terminals on the ModIO. It saves from needing an external >> converter. >> >> When using RS485 I recommend using full duplex rather than 1/2 duplex. >> Some >> times there are issues with buffers in the PC and timing for switching >> the >> transmission direction >> >> Cheers, >> >> Peter. > > I'm looking forward to getting my hands on it. > > Since I don't know what I am doing yet, I was thinking that I could try > to address the timing issues. Is it that the serial port uses internal > buffers, so you don't know exactly when data will be sent and due to the > query and response nature of Modbus, the timing is important? I seem to > recall that some parallel ports had the same problem, but the driver was > broken, which disabled the buffer, so ports with fifo's were okay to use > with EMC. I wonder if the buffers on the serial port could be disabled > too. It would be nice to be able to use Modbus in its common forms. But > since you have covered this territory, you could probably talk some > sense into me. > > -- > Kirk Wallace (California, USA > http://www.wallacecompany.com/machine_shop/ > Hardinge HNC lathe, > Bridgeport mill conversion, doing XY now, > Zubal lathe conversion pending) > > > ------------------------------------------------------------------------- > This SF.net email is sponsored by: Microsoft > Defy all challenges. Microsoft(R) Visual Studio 2008. > http://clk.atdmt.com/MRT/go/vse0120000070mrt/direct/01/ > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users > ------------------------------------------------------------------------- This SF.net email is sponsored by: Microsoft Defy all challenges. Microsoft(R) Visual Studio 2008. http://clk.atdmt.com/MRT/go/vse0120000070mrt/direct/01/ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
