Data Transmission With The Ascii Driver - Siemens SIMATIC CP 440 Manual

2.2.2

Data Transmission with the ASCII Driver

The ASCII driver controls data transmission via a point-to-point connection
between the CP 440 and a communication partner. This driver contains the
physical layer (layer 1 of the ISO reference model.)
The structure of the message frames is left open through the S7 user passing on
the complete send message frame to the CP 440. For the receive direction, the
end criterion of a message must be parameterized. The structure of the send
message frames may differ from that of the receive message frames.
The ASCII driver allows data of any structure (all printable ASCII characters as
well as all other characters from 00 through FFH (with 8 data bit character frames)
or from 00 through 7FH (with 7 data bit character frames)) to be sent and received.
Both RS422 and RS485 operation are possible.
RS422 Operation
In RS422 operation, the data is transmitted via four cables (four-wire mode). Two
cables (differential 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 (differential 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.
Point-to-point connection CP 440 Installation and Parameter Assignment
A5E00057742-02
Basic Principles of Serial Data Transmission
2-11