Modbus Ascii Serial

[Muhammad Daniyal]

Hi Dear, I am working on a project the communication system topology is mentioned below. i am using the Delta product, if anybody has worked on this product, kindly let me guide through the communication scenario. there is a network of Modbus Ascii with below mentioned devices. PLC: DVP-EH (Station Address bydefault 01) HMI: DOP-AE (Address 02) PC: (RS232 to RS485 converter) Kepware OPC server installed driver of "MODBUS ASCII SERIAL" for SCADA application. *-The question is that how can we configure the 1 Master (PC) and both other devices on Slave like (HMI) and (PLC). we have successfully communicate the device with (PC to PLC) and (HMI to PLC) on Modbus Ascii individually. but when all of three devices wheres come into the network it may display the error message on both HMI and PC time by time. if you have any query regarding above, kindly let me inform for further. Regards. Edited March 17, 2009 by Muhammad Daniyal

[Steve Bailey]

It has been my experience that HMI panels can only be masters on serial communications networks, not slaves. There can only be a single master on a modbus network.

[Tech Fred]

Kepservers Modbus ASCII driver is a Master only and probably the HMI's are both masters as well. Modbus serial protocols only allow for a sinlgle master to be on the network. Are the HMIs limited to Modbus ASCII or can the also do Modbus RTU? if they can do Modbus RTU then you could create e Modbus Seral slave devices and have the HMI's connect to them. Then use a product like our LinkMaster to bridge data between the ACSII and RTU devices. Contact our Tech support for more detail on how that can be done. Fred Loveless Kepware Technologies

[Muhammad Daniyal]

Dear Fred, In HMI case there are a bit difference in the PLC device address for Modbus Ascii Slave. in kepware we have created the tag addresses according to the PLC devices, but HMI does not support thoes kind of addresses in Modbus Ascii Slave. HMI Addresses: 40000 to 50000 for word 1 to 2048 for Descrete Input Kepware Addresses scheme varies according to the PLC devices. how could we manage these addresses in HMI?

[Muhammad Daniyal]

Dear Fred Could you tell me what station address will be reserved for Modbus Ascii Serial driver in Kepware?

[DanW]

1) You provided a very nice graphic of your network, but it's probably not going to work with the Modbus ASCII protocol, as you have discovered. RULE #1 for Modbus ASCII: Only one master on the network. Repeat: RULE #1 Only one master on the network. HMI's are usually Modbus masters, which poll the slaves for data. Some models can be both Modbus slaves and Modbus masters. There might be Modbus slave-only HMI's, but I'm not sure where. Kepware is a Modbus Master. Count up the Modbus masters - HMI 1 = 1 HMI 2 = 2 Kepware = 3 3 Modbus masters on a Modbus ASCII network, no way. The error codes are legitimate. That's why communications "with (PC to PLC) and (HMI to PLC) on Modbus Ascii individually" works OK. "but when all of three devices wheres come into the network it may display the error message on both HMI and PC time by time." That all makes sense, because RULE #1 has been violated. Mr. Loveless is proposing an alternative, but you haven't answered the question, "Can the HMI's talk Modbus RTU? Have you contacted Techsupport for Kepware? Dan

[Muhammad Daniyal]

Tanks all of you ppl for your help and guideline, we have switched all of our network over Ethernet. most probably this time we should must success in our communication. once again Thank you Dan.