different protocols on the same trunk line

[bjwoo]

My application diagram is as follow.

-------------------------------------------------------------- (RS485)
     |                      |                      |                      |
 TriPlc#1          TriPlc#2         I-7022             Modbus Device
     |
  MD-HMI

Plc#1 talk with Plc#2 via Modbus protocol with ReadModbus and WriteModbus command for sending and receiving data(DM data). And each Plc must send AO value to I-7022(the module with 2 AO). Another Modbus Device sends data to each Plc.
The problem is whether it's possible for each Plc can talk to I-7022 module at the same time on Modbus communication environment, because I-7022 use anither different protocol with Modbus.
I found that you can't use com related commands for different protocols at the same 232/485 trunk line at Tri-PLC manual.
Com1 port of Plc#1 will be used for Xserver.

Could you tell me the possibility of communication with each devices via different protocols of each?
If not possible, could you suggest correct system configurations for my application?

Thanks and best regards,
BJ

[support]

MODBUS is a single master protocol at any one time. So most implementation let one PLC as master and the rest of the PLC or devices are slaves. It is theoretically possible to have multiple masters but it requires careful arbitration to prevent conflict of communication sent by two masters simultaneously.

What you show on your diagram means two masters could talk simultaneously on the RS485 bus which will lead to comm failure frequently.

What you can do is to connect the MODBUS device to PLC #1 and PLC #2 via RS485. Then for PLC #1 and #2, use Auto485 converter to convert their  RS232 to RS485, which can then be used to connect as a second RS485 network that includes the M-7022 (which is an I-7022 that has also support MODBUS command, cost slightly more and available only as special order). In that case you have two separate RS485 bus and each has only a single master and the network should work.

[bjwoo]

Thanks so much.
I have another question.

As mentioned at my question above, com1 of Plc#1 will be occupied by Xserver, so Auto485 must be connected to com1 of Plc#2. Then I-7022 will be connected to the second 485. Plc#1 will be master(sends or receives DM data of it to or from Plc#2). Now, all of com1s of Plc#1 and Plc#2 are occupied by Xserver and I-7022(via Auto485).
"Modbus Device" which mentioned at my diagram is the device of wireless SMS messages transfer (for sending commands to Plc, receiving alarms from Plc and sending the messages to operators). So to speak, it must act as another master on original 485 trunk line. As you mentioned at reply, only one master are allowed at single 485. If I connect the Modbus device to the original 485 line and talks with Plc#2, will be any communication problem?

Thanks and best regards,
BJ

[support]

Your configuration is getting a bit complex to comprehend but I sort of know what you are trying to do. If there is a way for the PLC to know (by activating one of its digital input, for example) that the wireless SMS device is going to start sending the MODBUS command to the PLC, then the PLC can stop sending modbus command on its RS485 line and become a slave. This allows it to be interrogated by the wireless SMS device you mentioned. If this is not possible, and the SMS message transfer device can guarantee a time windows where it would not send command, then you can write the PLC # program to monitor its RS485 port until there is no more data and then initiates a data transfer during the window where the other device is not sending data. This is called arbitration.

[bjwoo]

Thanks for your reply.

I'll follow your advices, and it seems to be possible for my application.
I need more study on plc network. I found there are several methods of talking with plcs on master/slaver network at Tri PLC manual.

Thanks again, abd best regards
BJ