Siemens Simatic S7-400

RS422 Operation
In RS422 operation, the data is transmitted via four cables (four-wire mode). Two cables
(di󹪜erential signal) are available for the send direction and two for the receive direction. This
means you can send and receive data at the same time (full-duplex operation).
RS485 Operation
In RS485 operation, the data is transmitted via two cables (two-wire mode). The two cables
(di󹪜erential signal) are alternately available for the send direction and the receive direction. This
means you can either send or receive data at the same time (half-duplex operation). After a send
operation, the cable is immediately switched over to receive (the sender becomes high-
impedance). The maximum switchover time is 0.1 ms.
2.4.2 Sending Data with the ASCII Driver
Sending Data
When you send data, you specify the number of user data bytes to be transferred in the "LEN"
parameter of the call of the SEND_440 function block.
When you work with the end criterion "Character Delay Time" when receiving data, the ASCII
driver pauses between two message frames when sending. You can call the SEND_440 FB at any
time, but the ASCII driver does not begin its output until a period longer than the con󹪝gured
character delay time has elapsed since the last message frame was sent.
If you work with the "End-of-Text Character" criterion, you have a choice of three options:
• Send up to and including the end-of-text character
The end-of-text character must be included in the data to be sent. Data is sent only up to and
including the end-of-text character, even if the data length speci󹪝ed in the FB is longer.
• Send up to length con󹪝gured at the FB
Data is sent up to the length con󹪝gured at the FB. The last character must be the end-of-text
character.
• Send up to the length con󹪝gured at the FB and automatically append the end-of-text
character or characters
Data is sent up to the length con󹪝gured at the FB. The end-of-text character(s) is/are
automatically appended; in other words, the end-of-text characters must not be included in
the data to be sent. 1 or 2 characters more than the number speci󹪝ed at the FB are sent to the
partner, depending on the number of end-of-text characters.
When you work with the end criterion "󹪝xed frame length", the number of data bytes transferred
in the send direction is as speci󹪝ed for the "LEN" parameter of the SEND_440 FB. The number of
data bytes transferred in the receive direction, i.e. in the receive DB, is as speci󹪝ed at the receiver
using the "󹪝xed message frame length" parameter in the parameter assignment interface
interface. The two parameter settings must be identical, in order to ensure correct data tra󹪟c.
If an end code is not detected when data is sent, a pause equal to the length of the monitoring
time is inserted between two message frames to allow the partner to synchronize (identify the
beginning of the message frame).
Basic Principles of Serial Data Transmission
2.4 Data Transmission with the ASCII Driver
Point-to-point connection CP 440 Installation and Parameter Assignment
24 Equipment Manual, 03/2022, A5E00057742-AE