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MODBUS master / ASCII slave

Created by: sunn7 at: 8/13/2010 4:02 PM (4 Replies)

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8/13/2010 4:02 PM	Rate (0)
sunn7 Posts: 152 Rating: (2)	hi all I am using MODBUS card, 6ES7 341 1CH02-0AE0 i have read some threads about the modbus but still, i have some basic questions.. 1) what is the difference between MODBUS MASTER and MODBUS SLAVE configuration, which configuration I have to follow 2) there are 3 PQM meters connected to MODBUS card, customer gave same HEX address for 3 meters !!! then how to read differniate between information form each meter. do I have to specify/ define address for each PQM meter. Then how this addresscommunicated to those Meters any help , greatful sun
sunn7 Posts: 152 Rating: (2)	Last edited by: sun7 at: 8/13/2010 4:15 PM Last edited by: sun7 at: 8/13/2010 4:14 PM Last edited by: sun7 at: 8/13/2010 4:14 PM Last edited by: sun7 at: 8/13/2010 4:04 PM
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8/13/2010 5:00 PM	Rate (1)
dchartier Grandmaster-Expert Joined: 9/27/2006 Last visit: 3/23/2025 Posts: 12344 Rating: (2707)	sun7 1) what is the difference between MODBUS MASTER and MODBUS SLAVE configuration, which configuration I have to follow Hello sun; Modbus uses a master/slave communication protocol, where the master polls (requests data from/writes data to) the slaves. There can only be one master on a Modbus network; there can be up to 32slaves on a RTU bus segment; more are possible with a repeater. (Note: question 2 indicates you already have an idea that your PQM modules will act as slaves and your S7 CP 341 will act as master on this bus). In the master you must create (program or configure) Modbus "jobs", requests to the slaves, using Modbus functions; you must also take care of diagnostics and errors. The various Modbus functions are well explained in the Modbus RTU Master manual, along with the different DB configurations needed for every Modbus job. In the slaves all you have to configure is the data area that will be requested or written to by the master. The Modbus RTU slave manual explainsthe S7 memory (DB) arrangements necessary to translate Modbus addressing to S7 addresses. 2) there are 3 PQM meters connected to MODBUS card, customer gave same HEX address for 3 meters !!! then how to read differniate between information form each meter. do I have to specify/ define address for each PQM meter. Then how this addresscommunicated to those Meters Your Modbus slaves must each have a distinct and unique Modbus address, or there is no way the master can differenciate between the slave units, or even use the bus. Ask theGE techs (or whoever the vendor is in your location)to change the addresses, or do it yourself, I believe you can change the Modbus address with the front buttons inside the "Communications" menu (long time ago, bear with me if my memories are blurry). But it must be done prior to starting the bus, or all you will get is error messages. Hope this helps, Daniel Chartier
	Last edited by: dchartier at: 8/13/2010 9:04 PM
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8/13/2010 5:42 PM	Rate (0)
sunn7 Posts: 152 Rating: (2)	Hello dchartier thanks for info. it cleared manythings to me. what could be the master in case the modbus card is Slave???? the customer says modbus card is always configured as slave, what he means by that
sunn7 Posts: 152 Rating: (2)	Last edited by: sun7 at: 8/13/2010 5:54 PM
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8/13/2010 6:04 PM	Rate (1)
dchartier Grandmaster-Expert Joined: 9/27/2006 Last visit: 3/23/2025 Posts: 12344 Rating: (2707)	Hello sun; Here is a quote from GE concerning the communications setup for the PQM meters: The PQM is always a Modbus slave; it cannot be programmed as a Modbus master. See http://www.gepower.com/prod_serv/products/metering/en/downloads/power_quality/pqm_ch_7.pdf Andthis matchesmy experience that our PLCs were always setup as masters when connecting to PQM monitors. So in your case you must set the PLC CP 341 as a Modbus RTUmaster on the bus. Hope this helps, Daniel Chartier

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8/13/2010 6:10 PM	Rate (0)
sunn7 Posts: 152 Rating: (2)	hello dchartier many thanks for info. now i am sure he is mistaken, thanks cheers sun
sunn7 Posts: 152 Rating: (2)
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