User Manual

Easy to communicate between devices
7.10 PtP, USS, and Modbus communication protocols
Easy Book
Manual, 03/2014, A5E02486774-AF
165
The dynamic configuration changes are not permanently stored in the CPU. After a power
cycle, the initial static configuration from the device configuration will be used.
The SEND_PTP, RCV_PTP, and RCV_RST instructions control the PtP communication:
● SEND_PTP transfers the specified buffer to the CM or CB. The CPU continues to execute
the user program while the module sends the data at the specified baud rate.
● RCV_PTP checks for messages that have been received in the CM or CB. If a message
is available, it will be transferred to the CPU.
● The RCV_RST resets the receive buffer.
Each CM or CB can buffer up to a maximum of 1K bytes. This buffer can be allocated across
multiple received messages.
The SGN_SET and SGN_GET are valid only for the RS232 CM. Use these instructions to
read or set the RS232 communication signals.
7.10.4
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
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.
● 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 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 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. Use the
USS_RPM and USS_WPM instructions only in 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 or CB. The USS instructions for a drive use the instance DB
to share the information.