Communications
6.2 PtP, USS, and Modbus communication protocols
Easy Book
Manual, 05/2009, A5E02486774-01
79
The USS_RPM instruction reads a parameter from the drive.
Execute the USS_RPM from the program cycle OB.
The USS_WPM instruction modifies a parameter in the drive.
Execute the USS_WPM from the program cycle OB.
Minimize the number of EEPROM write operations to extend the
EEPROM life. Use the "EEPROM" parameter to disable writing
to the EEPROM.
Calculating the time required for communicating with the drive
Communications with the drive are asynchronous to the CPU scan. The CPU typically
completes several scans before one drive communications transaction is completed.
The USS_PORT interval is the time required for one drive transaction. The following table
shows the minimum USS_PORT interval for each baud rate. Calling the USS_PORT function
more frequently than the USS_PORT interval will not increase the number of transactions.
The drive timeout interval is the amount of time that might be taken for a transaction, if
communications errors caused 3 tries to complete the transaction. By default, the USS
protocol library automatically does up to 2 retries on each transaction.
Baud rate
Calculated minimum USS_PORT call Interval (milliseconds)
1200
790
2400
405
4800
212.5
9600
116.3
19200
68.2
38400
44.1
57600
36.1
115200
28.1