I thought about suggesting full duplex as well, but it involves converting every node of the network. For one or two nodes it's not bad, but it adds up quickly. It's not a hard conversion, but would still take time to do more than a couple of nodes.
Javid ----- Original Message ----- From: "Peter C. Wallace" <[EMAIL PROTECTED]> To: "Enhanced Machine Controller (EMC)" <emc-users@lists.sourceforge.net> Sent: Sunday, February 03, 2008 10:07 AM Subject: Re: [Emc-users] Modbus RS-485 Transceiver > On Sun, 3 Feb 2008, Javid Butler wrote: > >> Date: Sun, 3 Feb 2008 08:27:42 -0600 >> From: Javid Butler <[EMAIL PROTECTED]> >> Reply-To: "Enhanced Machine Controller (EMC)" >> <emc-users@lists.sourceforge.net> >> To: "Enhanced Machine Controller (EMC)" <emc-users@lists.sourceforge.net> >> Subject: Re: [Emc-users] Modbus RS-485 Transceiver >> >> 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 > > > > Even without buffering you still need to wait until the character has been > shifted out of the UARTs transmit shift register before line-turnaround. > You > can do this by polling TBE and TSRE but you would have to poll fast enough > to > meet the minimum response time of the Modbus peripheral. A better way is > to > use a more advanced serial chip (Exar,OxfordSemi,NetMos) that includes > RS-485 > support. There are also some RS-232/RS-485 adapters that use a one-shot to > enable the RS-485 Xmit enable, a little Mickey-Mouse but maybe ok at low > baud > rates. > > As someone else posted, full duplex links avoid the timing problem > altogether. > > > > >> ----- 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 >> > > Peter Wallace > Mesa Electronics > > (\__/) > (='.'=) This is Bunny. Copy and paste bunny into your > (")_(") signature to help him gain world domination. > > > ------------------------------------------------------------------------- > 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. 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