Libraries   
9.4 Open user communication library 
  S7-200 SMART 
540  System Manual, V2.3, 07/2017, A5E03822230-AF 
: If Done is TRUE, the CPU received new data so go to the "Receive Check" 
state. 
If Busy is TRUE, stay in the "Receive" state until you receive some data. 
If Error is TRUE, check the error code to determine what to do next. 
In all cases, exit the state machine for this scan. The program continues to the next state on 
the next scan. 
 
: State "Receive Check"... 
Check and process the received data. 
 
: In this program, you echo the data back to the partner. Copy all of the received 
bytes to the send buffer. 
Change the state to "Transmit", and exit until the next scan. 
 
: State "Transmit"...