Modbus data transfer into an ML1400

H2Otbaker

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I have a module that I am getting a 4-20mA signal into and it is communicating over Modbus/TCP to a Micrologix 1400. When the value from the module comes in, I can't get a steady value or a value that makes since. In my input register, with 12 mA going into my module from a Fluke source meter, I get 32,767 in the N11 register file in the Micrologix 1400. The module is web based so I can see the milliamp values change but what i'm seeing in the register files doesn't make since. I'm sure there is away to have the logic make since of the data coming over from the module but i've tried several things and i'm stumped.
 
Do you have a link to information about this module? Is the value a signed 16 bit integer or is it something else?

Also please save the MicroLogix 1400 programa and data when the 12ma signal is being set by Modbus/TCP then zip the .RSS file and post it here also.
 
Last edited:
The module is a Moxa iologik e1242. I forgot to mention when I apply 20 mA to the module I get a negative reading in the PLC. It's a 16 bit signed integer. The Moxa module moves the raw data value 1 word into the Modbus address i'm pulling from with the PLC.

thanks
 
I'm reading the AI raw data as status input registers, referenced address 0000 which is 30001 for the defined address. I am looking at this Modbus address in the PLC message statement and have designated N11:0 for 4 elements as my input registers. I'm reading off of the 2nd channel on the module so the reading is coming up in n11:1. When 4mA is applied to the Moxa module, I have a 0 in my register but when I apply 12 mA the register reads 32,767. At 20 mA the register goes to 4 but is erratic and varies up and down a few numbers even into the negative.
 
I see that you are reading registers from the Moxa unit (Moxa unit is a server/slave). Is the Moxa unit also initiating message passing with the Micrologix (is it also acting as a Master/client?) If it is not acting as a Master/client then you do not need to designate data tables under the Chann 1- Modbus tab.

In ladder 2 Rung 6 you are requesting data from Modbus register 30001 for a size of 4 registers (registers 30001 through 30004) into your file N11 (N11:0 - N11:3).
But in the Moxa manual I see the Raw values for this unit are starting at address 30513 (pages A-22,23). Where did you get 30001?
 
there's a check box, to enable user defined modbus addresses in the module web interface. I changed it to 30001. 30513 is the default address but I get the same reading no matter what address I use under input registers (3xxxx), and it won't let me use duplicate address under the same defined address in the module.

There is the capability to do peer-2-peer with the moxa module but i am not using that option. I'm just reading or pulling from the moxa with the plc.

I'm assuming the PLC is acting as the master, because I have 6 of these modules spread out and are communicating back to the ML1400
 
Not having any experience with this module then I will have to bow out of further comments. My Modbus comm experience is mostly with AB drives. Good luck.
 
This is what i do to handle unsigned on my end. If that is your issue something like this should fix it.

unsigned.jpg
 

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