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When using Modbus TCP, can change the unit number of Clipper


VincentZhang
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Dear supports,

 

I want to use clipper to read datas from camera by Modbus TCP.

I already install the modbus tcp bin file, I am sure the Modbus TCP on clipper and Camera are ok, because they can connect with PC ModScan tool (a Modbus client and server software). I also use wireshark to watch the datas. I set the modbus setup follow delta manual.

In my test:

Clipper sent frame datas: 00 00 00 00 00 06 00 03 03 E8 00 08

00 00 00 00 00 06 is default

00 is the unit number, clipper want to connect the device

03 is FC03, read multipul register

03 E8, the modbus ref 1000

00 08 ,the length of words 8

 

 

Problem: the unit number of Camera is 2( this number is fixed by camera compny), so can clipper change the unit number 00 to 02?

I hope clipper will sent frame datas like these:

00 00 00 00 00 06 02 03 03 E8 00 08

If that, clipper will read datas form camera successfully.

 

Thank you!

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  • 3 weeks later...

Are you sure that the camera requires a unit id = 2, because based on what I have learned, the unit identifier number is probably ignored. Based on the following description of the unit identifier field, it should not be needed:

 

The unit identifier field was intended to facilitate communication between Modbus TCP/IP Ethernet devices and traditional Modbus serial devices by using a bridge or gateway to route traffic from one network to a serial line sub-network. In this case, the destination IP address identifies the bridge or gateway device to send the message to, while the bridge device itself uses the Modbus Unit Identifier to forward the request to the specific slave device of the sub network. Recall that serial Modbus uses addresses 1 to 247 decimal, and reserves 0 as a broadcast address. Thus, the unit identifier assumes the same assignment for these applications. Further, this is the only way that broadcast messages are supported with Modbus TCP/IP, as

TCP alone only sends unicast (point-to-point) messages.

 

With TCP/IP devices, a Modbus server is addressed using its IP address, rendering the unit identifier non-functional and FFH is used in its place. The hex address FF remains non-significant to a gateway or bridge and will continue to be ignored if the network is later expanded or augmented with serial bridge or gateway devices.

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