about micrologix-1400

getco · Mar 15, 2013

hello friends

i am working with a project in which i want to communicate my PLC(ML-1400 SERIES-A) with popular micro-controller called ARDUINO using modbus protocol
so my idea is i have made arduino as a modbus master and PLC(ML-1400 SERIES-A) as modbus slavei have downloaded the modbus master library for arduino

now my question is whether i need to do any sepreate programming for PLC to work as a modbus slave or only my simple programm which i have written for tank level control is sufficient??
i have also attach the screen shot of my channel configuration
is this sufficient or i need to do separate programming for configuring my PLC as modbus slave

waiting for the reply

Lancie1 · Mar 15, 2013

Whether I need to do any seperate programming for PLC to work as a modbus slave, or only my simple program, which i have written for tank level control. is sufficient??

I don't see your simple program, but it must have special characteristics using Message (MSG) Instructions, as shown in the MicroLogix 1400 Programming Reference Manual, Appendix D, Protocol Configuration, Modbus RTU Protocol, Modbus RTU Slave, starting at page 580, and also in Chapter 22 Communications Instructions.

Message instructions are used to transfer information between the data files in the Modbus RTU Master and the Modbus RTU Slaves. Refer to Chapter 22 for detailed information about configuring a MSG instruction for Modbus Communications...

The Modbus RTU Slave driver maps the four Modbus data types—coils, contacts, input registers, and holding registers— into four binary and/or integer data table files created by the user....

The coil and contact files can contain up to 4096 coils or contacts in each register when the data table file is configured for a maximum size of 256 words. Each input register and holding register file can contain up to 256 registers when the data table file is configured for a maximum size of 256 words. With the “Expanded” box checked, the controllers can be specifically configured to use up to six 256-word data table files for a total of 1536 Modbus Holding registers.

I have also attached the screen shot of my channel configuration. Is this sufficient, or do I need to do separate programming for configuring my PLC as modbus slave?

Comparing it with the example in the manual, it appears to be sufficient.

getco · Mar 16, 2013

Thank you so much sir for your reply

i have read the manual but on page number 354 its written that it can be used only with the modbus master

also on page number 388 they have given the example but it is for modbus master

i am confused please tell me whether i can use MSG instruction as a modbus slave or not

thank you so much for your time

waiting for your reply

bernie_carlton · Mar 16, 2013

As a Modbus Slave the PLC does not initiate any messages (MSG instruction). As a Modbus Slave the operating system reacts automatically to external requests from a Modbus master providing the contents of (and accepting new values for) the registers which have been designated in the Modbus configuration. This is done automatically without any ladder messages needed.

getco · Mar 17, 2013

thanks sir for your reply

so it means that i need not to do any special ladder programming as i have selected PLC as modbus slave

so now i can start programming in arduino to communicate with my PLC

thank you so much

i will be back if i have any doubts

Lancie1 · Mar 18, 2013

so it means that i need not to do any special ladder programming as i have selected PLC as modbus slave

The "special programming" will need to be done in your Ardunio, where you tell it exactly where all the addresses are in your MicroLogix 1400. In other words, your Ardunio Modbus Master has to both write to addressses in the ML, and read data from the ML, in order for it to communicate.

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