Modbus MSGing Micrologix 1400

I am having an issue configuring my messaging and am not understanding the MSG data I am receiving back from the device.

Summary of devices:
Micrologix 1400 ser A - master Modbus 485 RTU
1763 NCO1 Data cable converter
MSA gas detector (slave address node 2)
RS Logix 500 starter

I have configured channel 0 to be Modbus RTU master
Even parity
19200 baud
1 stop bit
8 data bits
No handshaking 485 network
250ms timeout and delay

I have my MSG blocks set to (MG100:22, MG100:23, MG100:24)
read holding registers 4xxxxx
Data table N7:10, N7:20, N7:30
1 Element
16 Bits (confirmed by MSA)

I am using three MB data addresses: 1, 1000 & 4000
This gives a Modbus address of 40001, 410000 & 440000

After the msg, My N7 data tables are reading in decimal form:
-25536 (yes this is negative) for 4000
-25536 (yes this is negative) for 1000
MSG Error Illegal Data Address for 1

I chose to use 1000 because this is the base address. I don't understand why I could have gotten -25536 here. The default is supposed to be 1000. Then I got the same response for 4000. Does this tell me I am communicating with the device and I am having issues with addressing or does -25536 mean something else?

I've double checked all the channel configurations, swapped the +/- wires. No handshaking. Confirmed baud, parity bits etc... with manufacturer.

If you look at the RSlogix 500 file reference subroutine 20 to find the message blocks I am working with on this controller.

Logic

Here is my compressed logic I hope you can give me some hints!

Messages only trigger on a false to true transition of the rung. You need some sort of triggering.

You have no interlocking between messages. You use interlocking for your messages on port 1. Work up the same type thing.

I have always had to add 1 to the published address when setting up the address for the message.

You have 1000 as the address in two of the messages and 40000 in the other. You might want to re-check this.

Base Address??

bernie_carlton said:

Messages only trigger on a false to true transition of the rung. You need some sort of triggering.

You have no interlocking between messages. You use interlocking for your messages on port 1. Work up the same type thing.

I have always had to add 1 to the published address when setting up the address for the message.

You have 1000 as the address in two of the messages and 40000 in the other. You might want to re-check this.

Click to expand...

OK. I've added some timers to make it so only one MSG block happens at a time.

I rechecked my addresses and have tried various combinations. The only address that gets a response is 1000 (the spelled out base address), and I am getting a weird number, -25536?, 3, 350 & 1 - this is in decimal, would I want to use this numeral system for these Modbus MSG reads when making sense of the data tables?

Base +1 (1001?) should give me factory config data, but I get an invalid address error with this.

Can you confirm that ML1400 uses 5 digit addressing, not 6 digit? I think that is part of the problem. It says in the O&M that the first valid address in 5 digit 41001 w/ a base of 41000.

I've attached the Modbus addressing O&M maybe someone can give me suggestion for how the # I enter (MB Data Adress) corresponds to their addressing.

This really is a guess but I think you might be running into a “base 0 / base 1” issue. Some devices start addressing at 0 (protocol base) and others start at 1 (PLC base). The MicroLogix is going to have a PLC base (1). That would explain (I think) why the registers that are 16 bit words are coming back without an error (but the data is wrong) and the register that is a single bit is coming back with an error.