Communications
6.2 PtP, USS, and Modbus communication protocols
Easy Book
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Manual, 05/2009, A5E02486774-01
6.2.2
Library of USS instructions
The USS library supports the USS protocol and provides the functions that are specifically
designed for communicating with drives over the RS485 port of a CM module. You can
control the physical drive and the read/write drive parameters with the USS library. Each
RS485 CM supports a maximum of 16 drives.
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The USS_PORT instruction handles actual communication between the CPU and one
drive. Insert a different USS_PORT instruction for each drive in your application. Ensure
that the user program executes the USS_PORT instruction fast enough to prevent a
communication timeout by the drive. Call the USS_PORT instruction in the program cycle
or any interrupt OB.
●
The USS_DRV instruction accesses a specified drive on the USS network. The input and
output parameters of the USS_DRV instruction are the status and controls for the drive. If
there are 16 drives on the network, your program must have at least 16 USS_DRV
instructions, with one instruction for each drive. Ensure that the USS_DRV instruction are
be called at the rate that is required to control the functions of the drive. Call the
USS_DRV instruction only from the program cycle OB.
●
The USS_RPM and USS_WPM instructions read and write the operating parameters of
the remote drive. These parameters control the internal operation of the drive. See the
drive manual for the definition of these parameters. Your program can contain as many of
these instructions as necessary, However, only one read or write request can be active
for any one drive at any given time. Call the USS_RPM and USS_WPM instructions only
from a program cycle OB.
An instance DB contains temporary storage and buffers for all of the drives on the USS
network connected to each CM module. The USS instructions for a drive use the instance
DB to share the information.
The USS_DRV instruction exchanges data with the drive, by
creating request messages and interpreting the drive response
messages. All of the USS instructions associated with one USS
network and CM must use the same instance DB.
Use a separate USS_DRV instruction for each drive.
The USS_PORT instruction handles communication over the
USS network. Typically, there is only one USS_PORT
instruction for each CM, and the USS_PORT instruction handles
the transmission to or from a single drive.
Execute the USS_PORT from a time-delay interrupt OB to prevent drive timeouts and keep
the most recent USS data updates available for USS_DRV calls.
