Rockwell Automation Publication 2080-UM002N-EN-E - November 2022 265
Appendix A Modbus Mapping for Micro800
Modbus devices can be 0-based (registers are numbered starting at 0), or 1-based (registers
are numbered starting at 1). When PowerFlex 4-Class drives are used with Micro800 family
controllers, the register addresses listed in the PowerFlex user manuals need to be offset by
n+1.
For example, the Logic Command word is located at address 8192, but your Micro800 program
needs to use 8193 (8192+1) to access it.
EXAMPLE: Modbus Address (n+1 value shown)
8193 Logic Command word (Stop, Start, Jog, and so on)
8194 Speed Reference word
xxx.x format for 4/4M/40, where “123” = 12.3 Hz
xxx.xx format for 40P/400/400N/400P, where “123” = 1.23 Hz
8449 Logic Status word (Read, Active, Fault, and so on.)
8452 Speed Feedback word (uses same format as Speed Reference)
8450 Error Code word
(n+1) To access Parameter
See the respective PowerFlex 4-Class drive User Manual for additional information about
Modbus addressing. See Appendix E – Modbus RTU Protocol, on publication 22C-UM001
.
Performance
The performance of MSG_MODBUS (Micro800 is master) is affected by the Program Scan
because messages are serviced when the message instruction is executed in a program. For
example, if the program scan is 100 ms and six serial ports are used, then the theoretical
maximum for serial ports is 60 messages/second total. This theoretical maximum may not be
possible since MSG_MODBUS is a master/slave request/response protocol, so performance is
affected by several variables such as message size, baud rate, and slave response time.
The performance of Micro800 when receiving Modbus request messages (Micro800 is slave) is
also affected by the Program Scan. Each serial port is serviced only once per program scan.
If the respective PowerFlex drive supports Modbus Function Code 16
Preset (Write) Multiple Registers, use a single write message with a
length of “2” to write the Logic Command (8193) and Speed reference
(8194) at the same time.
Use a single Function Code 03 Read Holding Registers with a length of “4”
to read the Logic status (8449), Error Code (8450), and Speed Feedback
(8452) at the same time.
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