Panel Server Modbus Gateway Behavior

[Stuart_Cheng]

Hi All,

Would like to know the behavior of Panel Server as a Modbus gateway. According to the User Guide, Panel Server acts as a transparent Modbus-SL to Modbus TCP/IP gateway, that is, it converts Modbus TCP/IP requests received on unit ID X to Modbus-SL requests sent on server ID X.

Wondering if now the panel server has multiple TCP/IP clients (e.g. PME and EPO) and both systems are sending requests to the panel server, the panel server would only sends a single request to the downstream SL device or twice?  

Thanks

[Romain_Polette · Accepted solution]

Hello @Stuart_Cheng , 

Each Panel Server client will open a TCP Source port that will communicate with the port 502 (Modbus TCP) of the Panel Server. Panel Server Modbus Gateway feature will buffer each Client request and forward it separately to the same slave.

PME and PSO may not have similar driver for your slave devices. It means that the Modbus frames will be different (e.g. not the same number of consecutive registers). Modbus TCP is just an encapsulation of the original Modbus Frame. So, Panel Server will forward 2 different requests to the same slave.

regards, 

[Stuart_Cheng]

Hi @Romain_Polette,

Thanks for the information.
I have some questions to follow up. I found the description of Modbus TCP/IP Proxy in the user guide. As you mentioned each TCP/IP client has its own Modbus Frame, wondering how this proxy mechanism works if each client requests different data? Wondering the major difference that makes panel server a TCP/IP proxy but not a Serial proxy.
Thanks

[Romain_Polette]

Hello @Stuart_Cheng , 

In any case, the Modbus frames will never be merged.

Proxy = intermediate. So the Panel Server is a "Serial Proxy" by default.

For answers to the Modbus TCP Proxy, please check the drawings in the "Examples of Modbus Routing" section of the guide.

Example of Modbus TCP/IP Requests for Modbus-SL Devices:

Panel Server is a transparent gateway for Serial Devices (not declared in the Panel Server interface).

This is the "Serial Proxy" feature you mentioned in your comment. Panel Server is managing the connection between SCADA and Serial Devices.

Upstream system send requests to the Serial devices by addressing them with their physical Modbus ID (Slave ID).

Each Client will open a TCP Source Port and communicate with the Panel Server (Destination Port 502). Panel Server translate the Modbus TCP requests <=> Modbus Serial requests.

Example of Modbus TCP/IP Requests for Modbus-SL Devices With Virtual Server ID Different from the Physical Address:

Similar to the previous example, but the Modbus devices are declared in the Panel Server interface, and the Modbus Clients are addressing the Serial Devices with the Virtual ID attributed by the Panel Server.

Example of Modbus TCP/IP Requests for Devices in Switched Topology:

In blue => Panel Server is an ETH Switch

Devices are directly addressed by the upstream software. Each Clients (SCADA, BMS, etc.) will open a TCP port to communicate with port 502 of each Modbus TCP device.

In orange => Panel Server is a Modbus Proxy

Proxy Mode can be useful in case if a Modbus TCP device accept a very limited number of TCP connection. By example, you have 10x SCADA but your device only accept 4 simultaneous TCP connections. In this case, you may have communication timeout between some SCADA and your Modbus TCP device.

With the Proxy Mode, all transactions will go through the Panel Server. It means that only Panel Server will open 1x TCP port with each device.

Modbus TCP requests from SCADA, BMS are not changed. 

Regards,