adccommunitymod (AutomationDirect) asked a question.
Modbus Comm. without scanning
Created Date: April 19,2007
Created By: markvigl
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I'm using a DL05 for data acquisition as a Modbus slave. I 've noticed the port will still answer polls even with the unit is in stop mode. Data in the registers is, of course, not being updated. This to me opens up the possibility the unit could stop scanning while still giving the appearance of active operation. Any thoughts on how to stop slave Modbus communications when the program isn't running? Thanks in advance.
Created Date: April 19,2007
Created by: Steve Bailey
I'm not sure that I understand your issue. Why would you want the slave to stop communicating?
You could include a register that indicates whether or not the slave PLC is running. A simple way is to increment the value in a V memory location every scan. Include that address within the range of data requested by the master. When the master detects successive identical values, it can infer that the slave PLC has stopped running.
Created Date: April 19,2007
Created by: markvigl
Steve, thanks for your reply.
That solution was discussed. However, these units arn't exclusively used on the network. Changing the host software just for these units isn't my first choice.
Created Date: April 19,2007
Created by: Steve Bailey
Your latest response implied that the DL05 is on an RTU network. That implies RS485 communications which implies that you have an RS485/RS232 converter at the PLC, since the PLC's serial ports are RS232. Wire up a PLC output to control the power to the converter. Add a rung of ladder to turn on the output whenever the PLC is running. When the PLC stops, the output goes off, killing power to the converter and the slave node stops responding to the master.
Created Date: April 19,2007
Created by: bcarlton
Are there any of the data registers whose static state would indicate the suspension of updates? In other words, could the PLC being in stop mode be determined just from the nature of the data currently being received?
Created Date: April 19,2007
Created by: markvigl
Steve, again thanks for your reply.
Yes, it is an rtu network. Units are all remote, communicating using FSK modems over leased analog data lines, RS-232 to the modem, +5vdc being provided from the DL05 port to power the modem. I like your idea about routing modem power thru an output.
Created Date: April 19,2007
Created by: markvigl
I'm using a DL05 for data acquisition as a Modbus slave. I 've noticed the port will still answer polls even with the unit is in stop mode. Data in the registers is, of course, not being updated. This to me opens up the possibility the unit could stop scanning while still giving the appearance of active operation. Any thoughts on how to stop slave Modbus communications when the program isn't running?
Thanks in advance.