Siemens SIMATIC S7-200 - USS Protocol Instruction Requirements; Resource Usage; Global Symbol Table Configuration

Using USS Protocol Instructions to Communicate with Drives
11-2
S7-200 Programmable Controller System Manual
A5E00066097-02
11.1 USS Protocol Instruction Requirements
The USS Protocol instructions require the following resources from your CPU:
• Size 1250 bytes to 1750 bytes (depending on the instructions used)
• Port 0 (only)
• 8 subroutines and 3 interrupts
• V memory size 400 bytes and a16-byte buffer for some instructions
Resource Usage
USS Protocol Instruction uses resources as shown below:
• Port 0: When Port 0 is being used for USS Protocol communications, it cannot
be used for any other purpose, including communicating with
STEP 7-Micro/WIN. The USS_INIT instruction controls assignment of Port 0 to
USS Protocol or PPI. Once Port 0 has been assigned to USS usage, Port 0 can
only be used again for STEP 7-Micro/WIN communication by a programmed
reassignment via another USS_INIT instruction, or by setting the MODE switch
to STOP. Stopping communication to the drives will cause the drives to stop. It
is recommended that during program development of USS Protocol
applications, the developer should use a CPU 226, or EM 277 PROFIBUS-DP
in conjunction with a PROFIBUS CP card in the PC. This allows a second
communications port to be used for STEP 7-Micro/WIN monitoring of the
application while USS Protocol is running.
• All SM locations associated with Freeport communication on Port 0 will be
affected by USS protocol instructions.
• User program space: In addition to the space occupied by each instruction,
there is a user program space overhead of up to 1750 bytes occupied by USS
protocol support routines
• V memory: A 400 byte block of V memory, starting at a user assigned location,
is reserved for USS variables. A 16-byte communications buffer, starting at a
user assigned location, is required for some instructions. It is recommended
that a unique buffer be assigned for each instance of USS protocol instructions.
• 9 subroutines and 3 interrupts are taken by USS protocol.
Global Symbol Table Configuration
After you enter an address for the first symbol, the table automatically calculates
and assigns the remainder of the symbols in the table. Figure 11-1 shows the USS
tab of the Symbol Table. You should assign a starting V location for the table,
which will occupy 400 bytes.