ML1400 Modbus RS485 Communication.

Hello!

I would need some help on a question my customer has:

He wants to communicate with our MicroLogix 1400 using Modbus and RS485. I'm not quite familiar in the communication and protocol but I'll make an attempt to explain my problems.

In the current situation, we have a PLC, one HMI and a Frequency Control. PLC and HMI are connected through a switch. PLC and my frequency control is connected on CommPort0 with a 1763-NC01 cable.
The port I need to communicate over Modbus RS485 is also CommPort0? Is it possible to solve?

Unlike ethernet / ip communication which (as I understand it) does not need to be configured by me in the PLC for it to work, I have to program the communication and what information should be passed on to their (my customer) Modbus Master?

/ Tim

Hi,

I will presume that your ML1400 is acting as the Modbus MASTER with relation to the VFD as the SLAVE. So, as long as your client's device can also be configured as a SLAVE, and a different slave address to the VFD, then yes, you should be able to read/write to your client's system.
If they have to be the MASTER, you will run into problems as typically, only one MASTER device is used on a Modbus RTU network.

Regards,

Rob

Ah, I didn't thought of that. I guess that my client has a modbus master since they want our "turn key" machine to be able to connect to Modbus. I think I have read somewhere that also the Comm Port 2 can be used for RS485 modbus, is this maybe a way around this problem? Another solution would be of getting a VFD that is communicating on ehternet/ip and use the Comm Port 0 for the Modbus slave rs485?

Hi,

You can use CH 2 but will need an RS232 to RS485 converter as CH0 has the RS485 connection. But should be possible to set one as MASTER and one as a SLAVE device.

Regards,

Rob

Thanks alot, this helps me in anwser my customer. I'm not sure but I have to program my customer required comunication in my PLC program for Modbus right? It's unlike the ethernet/ip where my customer can access what they want in my program?

According to the ML1400 manual I can use 1761-NET-AIC to convert the RS232 to a RS485 network. Is this the component I should buy and use with my Channel 2 comm port if I want to connect my ML1400 as a Modbus slave on RS485 (assuming the port 0 is occupied)?

Further more, if I want to check if my ML1400 is function correctly as Modbus Slave on RS485, what is the best way? Configure a ML1400 as modbus master and recieve the information?

If I configure the ML1400 as Modbus Slave on RS485, does it matter what label my customer has on his modbus master? I guess no since modbus is a protocol with certain rules?

I would greatly appreciate some input on my latest questions if anyone have the time and knowledge to help a fellow programmer to learn some communication issues

another alternative is to use modbus tcp. not every version of the 1400 supports it, so post your part # for the 1400.

My customer has requested Modbus with RS485 connection, I'm not that good at communication but I think Modbus TCP is made with a RJ45 jacket?

Modbus/TCP is Modbus over Ethernet.

Ethernet typically uses RJ-45, but industrial controllers also use RJ-45 for others for communication transport links, like RS-485.

You need to check your contoller's documentation to see which comm protocol runs on your comm ports.

My customer want modbus over rs485 so I assume they want Modbus RTU. I have checked information about my controller and I just need to get answers to the questions I stated in the last post on 21 februari. Hopefully I can get in contact with my Rockwell support tomorrow but it isn't bad to have some basic knowledge when talking to him.

Hi Tim,

This is just really to summarise what your Micrologix 1400 is capable of regarding Modbus communications...

Ch0 (8-pin mini din) - RS232 or RS485 - Modbus master or slave
Ch1 (Ethernet) - Modbus TCP (series B only)
Ch2 (9 pin D) - RS232 Modbus master or slave

It is possible to configure both channels 0 and 2 for Modbus RTU. Ch0 has a direct RS485 connection, Ch2 RS232 only. Any RS232/485 convertor should work if you need RS485 on Ch2. There are several free Modbus utilities than can be used for testing your logic (Modscan is one). You could connect Ch2 directly to your laptops serial port to test your Modbus slave configuration without the need for a convertor using (say) Modscan. When you are the slave, it's really down to your client to write the logic to read/write the data across the network (you just typically declare an area of memory that will map (say) 40001 to a logix500 address type (eg N7:0) and then give your client some sort of table that outlines the registers you have used for your logic.

Hope this helps!

Rob

Thanks that is exactly the kind of answer I was looking for. I will look into how I do the address mapping and if I have further problems I'll let you know. Thanks alot now I know and can give my customer more details and an estimate price. Much appreciated!

Setting up the slave/server in the Micrologix is very simple once you set that condition for the port. As you will see it asks you to set up as many as 4 numeric files. (It even kindly asks if you want it to set them up for you.) Each will serve as if it were a Modbus area as seen by the external client. If you have a need for longer tables you can link multiple contiguous files together to appear as one larger Modbus file. Your local program controls what information is stored in these files and what it does with information placed in them.

Ah, thanks alot for the help!
I started RSLogix Micro and it was pretty straight forward. Then I searched the net for a good link that explained it and voila:

http://www.plcengineers.com/micrologix_modbus_rtu.html

Hope this can also help someone else.
Thanks for all the help!

rQx,

Thank you so much for this link and your feed back to this issue.

I am sorry you had to figure this out as I am now having to set this up for a customer very much the same way i have read you had to.

Your feed back and link look like the solution I have been looking for.

Thanks again.