Learning Lab: Modbus RTU... C-more EA9 to Yaskawa V1000?

I'm getting ready to do a lab on communicating directly from an Automation Direct HMI to a Yaskawa V1000 using Modbus RTU. Surprisingly, I've never worked with Modbus, and I probably should know how. We do work with a lot of Yaskawa drives, and they all come with embedded Modbus RTU. I chose the EA9 because it's inexpensive, and I have one available in my shop to play with. I'm not really finding much about configuring a Modbus connection in the EA9, which makes me wonder if this has a chance of working. My laptop with the software is at my shop, so I haven't looked at the communication config yet. For those who know more about this, do you think it'll work? An HMI is really a master, and the VFD is a slave device... so in theory, it should. Right?

Should be as simple as setting up one of the COM ports in the C-More for Modbus RTU. Then all you need is the mapping of the drive's data to Modbus addresses which should be in the drive's user manual.

Alright, the Modbus lab is done. It wasn't as easy as I anticipated. There was a little frustration with figuring out what values the HMI was sending to the VFD. Looking around the internet, I found some Modbus utility programs that displayed the values, which ended up helping considerably.

Here's a short video that I threw together to demonstrate the communication:

https://youtu.be/cSr3zFHJMr8

Nice job on the video.


You ran into two out of the four big Modbus RTU Gotchas, the dreaded one offset (zero based register addressing vs one based register addressing) and hex vs decimal for the process values.



The other two are


1. RS-485 A/B (+)/(-) line drivers are not always the same polarity as designated by different vendors. Two different vendors might label the A/B driver lines the opposite, so although A should connect to A, B to B, (+) to (+), (-) to (-), sometimes the labeling is backwards. Connecting a line backwards will not damage the driver, but it will never work. So when you're mixing different vendors or models of cevices and you can't get it to work, try swapping the A/B ro (+)/(-) lines on one end.


2. 32 bit Floating point (FP) word/byte order

The slave provides the data in a specific word/byte format, eigth 8 bit bytes for a 32 bit FP value. There are four IEEE word/byte order formats, two of which are quite common and the other two actually quite rare. Nonetheless, seeing a completely ridiculous value like 7.23*5.40287^-16 when your expecting 54.9 can throw you. The master needs to use the other FP word/byte order format.

Thanks! I knew about the word/byte order from another post, so I checked that right away. The RS-485 polarity came up when I bought a USB to RS-485 adaptor from Automation Direct. I used this when working with the utility programs. The raw leads are red and green, with no clear indication which was + or -. I had to look at the pinouts for the RJ45 jack to make sure it was wired correctly.

The adaptor is here: https://www.automationdirect.com/adc/shopping/catalog/cables/drives/usb-485m