Concept XXMIT for poll-response protocol

J

Thread Starter

John Fuhrman

We are working on a poll-response protocol using Concept 2.5 xxmit with a Modicon Quantum PLC. Polling (transmitt) looks great and the slave device responds (seen on protocol analyzer); however, we never get any data into the msg_in array. The oscilliscope shows that the slave response starts 3ms after the poll ends. When we reply manually to the poll using a laptop, the msg_in array is populated correctly and the bytes received counter shows the correct response length. Therefore, we've concluded that we have a timing problem (xxmit isn't ready to receive quick enough). Are best guess is to use command word 200h for the send, then change it to 480h as soon as the send is done without cycling the "start" input. Anybody know how to play with this better? Has anybody used xxmit for a poll-response protocol? If not, can anybody reccomend an industrial serial data buffer (24vdc or port powered, din rail mount) like the one they should have in the [email protected]%$& PLC?
 
We have a similar problem communicating to a GE Modbus Concentrator via an RS232/485 converter. The RS485 is half duplex. We have confirmed Tx but we get no response, e.g. the XXMIT gives fault code 116.
Any suggestions?

(Message originally posted June 13, 2001)
> We are working on a poll-response protocol using Concept 2.5 xxmit with a Modicon Quantum PLC. Polling (transmitt) looks great and the slave device responds (seen on protocol analyzer); however, we never get any data into the msg_in array. The oscilliscope shows that the slave response starts 3ms after the poll ends. When we reply manually to the poll using a laptop, the msg_in array is populated correctly and the bytes received counter shows the correct response length. Therefore, we've concluded that we have a timing problem (xxmit isn't ready to receive quick enough). Are best guess is to use command word 200h for the send, then change it to 480h as soon as the send is done without cycling the "start" input. Anybody know how to play with this better? Has anybody used xxmit for a poll-response protocol? If not, can anybody reccomend an industrial serial data buffer (24vdc or port powered, din rail mount) like the one they should have in the [email protected]%$& PLC?<
 
L

Lynn August Linse

One idea - can you try an external RS-232 to RS-485 converter which does NOT require RTS/CTS? And make sure the Quantum thinks that there is RS-232
only there? "www.robustdc.com":http://www.robustdc.com sells such a unit, but I'm sure others do also.

Then the Quantum *MAY* be allowing a closer to full-duplex behavior such that the rapid response is not lost. It may not be that the "xxmit isn't
ready to receive quick enough" - it may be that XMIT is still in the RTS/CTS timing loop to control the RS-485 duplex when the response starts.

Best regards

Lynn August Linse, alist (at) linse.org
26741 Portola Pkwy, Suite 1E #242
Foothill Ranch CA 92610-1743 USA
 
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