Ascii Mode - Siemens SIMATIC S5 CP 521 Manual

Technical Description
2.3.2

ASCII Mode

The possible methods of transmission depend on the following:
• Type of interface (TTY or RS-232-C (V.24))
• Whether handshaking has been configured when the RS-232-C (V.24)
interface is used (handshake OFF or ON).
• Type of ASCII mode (transparent or interpretative)
TTY interface or RS-232-C (V.24) interface with handshake OFF
The CPU only uses the RXD line. If the CP receives data from the peripheral device,
the latter must maintain logic "1"* on the RXD line, otherwise the CP 521 will
send the "Permanent line break" error message (XD
• Transparent ASCII mode
No XON/XOFF protocol is possible in transparent ASCII mode. Messages must be
of fixed length. It is not possible to transmit messages with endidentifiers.
• Interpretative ASCII mode
If you have programmed an XON/XOFF protocol in ASCII mode, the CP sends 20
bytes to the peripheral device before the 1 Kbyte receive buffer overflows XOFF.
The CP then only sends XON to the peripheral device again when the receive
buffer contains more than 256 free bytes.
If the peripheral device sends more than 20 bytes after an XOFF from the CP, the
message data already received is rejected. The CP sends the "Receive buffer
overflow"error message (XB
Note:
Immediately after startup, the CP 521 assumes that the communications
parnter is ready to receive, even if no XON is sent.
* For the TTY interface logic "1" means: Current
For the RS-232-C (V.24) interface logic "1" means: U -3 V
2-8
) to the CPU.
H
) to the CPU.
H
GWA 4NEB 812 6030-02b
CP 521