Uss Instructions - Siemens SIMATIC S7-1200 Manual

7.10.3

USS instructions

S7-1200 supports the USS protocol and provides instructions that are specifically designed
for communicating with drives over the RS485 port of a CM or a CB. You can control the
physical drive and the read/write drive parameters with the USS instructions. Each RS485
CM or CB supports a maximum of 16 drives.
● The USS_PORT instruction handles actual communication between the CPU and all the
● The USS_DRV instruction accesses a specified drive on the USS network. The input and
● The USS_RPM and USS_WPM instructions read and write the operating parameters of
An instance DB contains temporary storage and buffers for all of the drives on the USS
network connected to each CM or CB. The USS instructions for a drive use the instance DB
to share the information.
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.
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Manual, 11/2011, A5E02486774-04
drives attached to one CM or CB. Insert a different USS_PORT instruction for each CM or
CB in your application. Ensure that the user program executes the USS_PORT
instruction fast enough to prevent a communication timeout by the drive. Use the
USS_PORT instruction in a program cycle or any interrupt OB.
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 CPU executes the USS_DRV instruction at the rate that is required to
control the functions of the drive. Use the USS_DRV instruction only in a program cycle
OB.
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. Use the
USS_RPM and USS_WPM instructions only in a program cycle OB.
Easy to communicate between devices
7.10 PtP, USS, and Modbus communication protocols
159