Hms-en2mb-r max transactions


#1

Hi

We are designing a project at minute and we will have a complete redundant network with Hot/Standby Controllogix PLC. We are intending to use to EN2MB-R’s, one in each network with only one active at anytime, switching over when the active network goes fault. When will be using DLR function.
I am being told that we will be limited to 64 commands in total. Can you confirm this? I thought it was 64 times 64.
We will have 39 slaves with with 4 commands per slave which amounts to 156 separate commands/transactions.
Can you confirm if this is the case?

Thanks

David


#2

Theoretically, the linking device supports up to 150 transactions. The actual number may however be less depending on the memory requirements of the defined transactions.

If you are using modbus and reading 4 registers that are in a row you might be able to perform a read multiple request, reading all the data at once. This will cut the number of transaction down keeping you under the 150 limit. You will just need to worry about the memory limitations of the device. The device is capable of 500 bytes of Input data and 496 bytes of Output data.

Deryck

Deryck


#5

Thanks for the update. How does the 500bytes relate to the 8KB of I/O data?


#6

Hello @PLC_user,

I will need to look into this I am not sure what the 8KB is in reference to. It might be the total storage or RAM on the device.

Deryck


#7

@PLC_user,

Sorry for the confusion, I realized what i did wrong here. I was looking at data sheets for the Serial linking device not the modbus TCP one.

You are correct in your first post. The HMS-en2MB-r linking device can handle up to 64 different servers, and a maximum of 64 transactions distributed among those servers. This does mean the device would not be able to perform all 156 separate transactions.

What you can do is setup read multiple registers at once, allowing you to read all the data you needed. For example, rather then reading 40001, 40002,40003 you would make one request to read 3 registers starting at 40001.

Deryck