Communication processor and Modbus TCP
13.4 Universal serial interface (USS) communication
S7-1200 Programmable controller
System Manual, V4.2, 09/2016, A5E02486680-AK
1151
The USS_Port_Scan instruction is a Function Block (FB) and handles the actual
communication between the CPU and the drives through the Point-to-Point (PtP) RS485
communication port. Each call to this FB handles one communication with one drive. Your
program must call this FB fast enough to prevent a communication timeout by the drives.
You may call this function FB in a main program cycle OB or any interrupt OB.
The USS_Read_Param, and USS_Write_Param instructions are both Functions (FCs). No
DB is assigned when you place these FCs in the editor. Instead, you must assign the
appropriate DB reference to the "USS_DB" input of these instructions. Double-click on the
parameter field and then click on the parameter helper icon to see the available DB names).
Typically, you should call the USS_Port_Scan FB from a cyclic interrupt OB. The cycle time
of the cyclic interrupt OB should be set to about half of the minimum call interval (As an
example, 1200 baud communication should use a cyclic time of 350 ms or less).
The USS_Drive_Control FB provides your program access to a specified drive on the USS
network. Its inputs and outputs are the status and controls for the drive. If there are 16 drives
on the network, your program must have at least 16 USS_Drive_Control calls, one for each
drive. These blocks should be called at the rate that is required to control the operation of the
drive.
You may only call the USS_Drive_Control FB from a main program cycle OB.
Considerations in calling USS instructions from OBs
Only call USS_Drive_Control, USS_Read_Param, and USS_Write_Param from a main
program cycle OB. The USS_Port_Scan FB can be called from any OB, usually from a
cyclic interrupt OB.
Do not use instructions USS_Drive_Control, USS_Read_Param, and USS_Write_Param in
a higher priority OB than the corresponding USS_Port_Scan instruction. For example, do
not place the USS_Port_Scan in the main OB and a USS_Read_Param in a cyclic interrupt
OB. Failure to prevent interruption of USS_Port_Scan execution can produce unexpected
errors, which could result in personal injury.
The USS_Read_Param, and USS_Write_Param FCs read and write the remote drive
operating parameters. 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 functions as necessary, but only one read or write request can be active per drive, at
any given time. You may only call the USS_Read_Param, and USS_Write_Param FCs from
a main program cycle OB.
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