Requirements For Using The Uss Protocol; Calculating The Time Required For Communicating With The Drive - Siemens SIMATIC S7-200 System Manual

S7-200 Programmable Controller System Manual

Requirements for Using the USS Protocol

The STEP 7--Micro/WIN Instruction Libraries provide subroutines, interrupt routines, and
instructions to support the USS protocol. The USS instructions use the following resources in the
S7-200:
USS Protocol is an interrupt driven application. In the worst case the receive message
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interrupt routine requires up to 2.5ms to execute. During this time all other interrupt events
are queued for service after the receive message interrupt routine has been executed. If
your application cannot tolerate this worst case delay, then you may want to consider other
solutions for controlling drives.
Initializing the USS protocol dedicates an S7-200 port for USS communications.
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You use the USS_INIT instruction to select either USS or PPI for port 0. (USS refers to the
USS protocol for SIMOTION MicroMaster drives.) You can also use USS_INIT_P1 to assign
port 1 for USS communication. When a port is set to use the USS protocol for
communicating with drives, you cannot use the port for any other purpose, including
communicating with STEP 7--Micro/WIN.
During the development of the program for an application using the USS protocol, you
should use a two port model, CPU 226, CPU 226XM, or EM 277 PROFIBUS_DP module
connected to a PROFIBUS CP card in your computer. The second communications port
allows STEP 7--Micro/WIN to monitor the control program while USS protocol is running.
The USS instructions affect all of the SM locations that are associated with Freeport
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communications on the assigned port.
The USS subroutines and interrupt routines are stored in your program.
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The USS instructions increase the amount of memory required for your program by up to
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3050 bytes. Depending on the specific USS instructions used, the support routines for
these instructions can increase the overhead for the control program by at least 2150 bytes,
up to 3500 bytes.
The variables for the USS instructions require a 400-byte block of V memory. The starting
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address for this block is assigned by the user and is reserved for USS variables.
Some of the USS instructions also require a 16-byte communications buffer. As a parameter
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for the instruction, you provide a starting address in V memory for this buffer. It is
recommended that a unique buffer be assigned for each instance of USS instructions.
When performing calculations, the USS instructions use accumulators AC0 to AC3. You can
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also use the accumulators in your program; however, the values in the accumulators will be
changed by the USS instructions.
The USS instructions cannot be used in an interrupt routine.
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Tip
To change the operation of a port back to PPI so that you can communicate with
STEP 7--Micro/WIN, use another USS_INIT instruction to reassign the port to PPI operation..
You can also set the mode switch on the S7-200 to STOP mode. This resets the parameters for
the port. Be aware that stopping the communications to the drives also stops the drives.

Calculating the Time Required for Communicating with the Drive

Communications with the drive are asynchronous to the S7-200 scan. The S7-200 typically
completes several scans before one drive communications transaction is completed. The
following factors help determine the amount of time required: the number of drives present, the
baud rate, and the scan time of the S7-200.
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