Modbus Communication

This webinar will cover the basics of implementing Modbus RTU with Unitronics controllers.
Split into three convenient videos, with example programs included, as well as Q&A session.

Downloads Vlp MODBUS slave: modbus-slave

Downloads Vlp MODBUS master: modbus-master


Questions /Answers

Q: what is the pt# for the “port expander” to allow daisy chaining?
A: This part number is MJ10-22-CS66, it is to connect pins 1 and 6 of the RS-485 port to the RS-485 networkQ: Does the MB0 only go high on the 1st scan
A: SB2 is the system bit that goes high on only the first scan. We use it to initialize and set up communications, or for any thing we want to scan at only the first scan of the ladder. In this case, MB0 will be high when there is modbus communication on the specified port, port 1 in this case

Q: Is there a pinout for that simple cable?
A: yes, the bottom of every installation guide has the pinout for the serial port 
Q: So if I want to read ML 9, is that 7009h or 7019h?
A: it should be 7009

Q: When you want to read a 32-bit register, do you have to “skip” 2 pointer values per address since modbus is 16-bit based?
A: You have to use an even length, but the scan EX block is compliant with 32 bit.

Q: can it be master to one and slave to another?
A: Each com port is independent of each other. If we have two com ports, one can be master and one can be slave. It is a popular function to bridge modbus networks in this manner. One master com and one slave com

Q: Can we have multiple R.H.R or multiple P.H.R FBs in a program?
A: yes you can. You can have as many as memory will allow you to create. you can specify the information statically in heac one (leek address of controller, or address of vector) or you can create them i=with indirect values (such as the address, we can use one read command, and change the address of the controller we read from, this way we only need one read). It is up to the user to decide which works best for their system. The time delay Jason mentioned is important in this case.