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AB powerflex 4M & V570

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I am writing to enquire about a problem that bothers me. I am new to the "ladder" programming, but as I have one V570module and one VFD powerflex in my possession, I have

decided to link them as master and slave. I have examined the examples of modbus communication and I did the following scheme. It is functioning, but I would like to know is

there a better way to write and read in several registers at once?


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In this case make a read as one larger group even if you only need one or two items out of that read. What slows you down the most, is how many different read and write events you have in your unitronics program to any outside devices.

The read command 3 block will allow you to read many registers at the same time 9as long as all of the registers you want to read are the same. The unitronics will allow you to read up to I think 124 integers (16 bit) or 62 double words (32 Bit). You can also use the code 16 write instead of the code 6 write you used in your example. Code six is one write at a time and 16 allows multiple writes. Again 124 integers and 62 doubles.

Keep in mind these are the limitation on the Unitronics side. I personnaly hardley ever use Allen Bradley VFDs so I am not familliar with the maximun number of reads it allows for a code 6 and writes for a code 16. Sorry but you will have to look those up in the AB Modbus manual.

Just an extra note. Unitronics does not allow any MI to be an unsigned integer and you may have integers on your pwerflex 700 side that will exceed the -32768 to +32676 that a Unitronics MI can have. You can always store this exra data into a ML or other register that will work, however it will require you to make 2 or 3 reads just to get one set of data if you do this. As long as you are picking up an integer (even if it is unsigned on the AB side) you can store this into the standard MI so you do not have to break your reads up and convert it in your ladder logic Or just keep in mind that a number greater than +32767 will show up as a negative number and make your decision acordingly keeping the data in the standard MI.

Good luck and have fun


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