Managing Flow Control - Siemens SIMATIC S7 System Manual
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Baud rate: The default value for the baud rate is 9.6 kbits per second. Valid choices are:
300 baud
600 baud
1.2 kbits
Parity: The default value for parity is no parity. Valid choices are:
● No parity
● Even
● Odd
● Mark (parity bit always set to 1)
● Space (parity bit always set to 0)
Number of stop bits: The number of stop bits can be either one or two. The default is one.
Flow control: For the RS232 communication module, you can select either hardware or
software flow control, as described in the section "Managing flow control (Page 239)". If you
select hardware flow control, you can select whether the RTS signal is always on, or RTS is
switched. If you select software flow control, you can define the ASCII characters for the
XON and XOFF characters.
The RS485 communication module does not support flow control.
Wait time: The wait time specifies the time that the communication module waits to receive
CTS after asserting RTS, or for receiving an XON after receiving an XOFF, depending on the
type of flow control. If the wait time expires before the communication module receives an
expected CTS or XON, the communication module aborts the transmit operation and returns
an error to the user program. You specify the wait time in milliseconds. The range is 0 to
65535 milliseconds.
8.4
Managing flow control
Flow control refers to a mechanism for balancing the sending and receiving of data
transmissions so that no data is lost. Flow control ensures that a transmitting device is not
sending more information than a receiving device can handle. Flow control can be
accomplished through either hardware or software. The RS232 CM supports both hardware
and software flow control. The RS485 CM does not support flow control. You specify the
type of flow control either when you configure the port (Page 238) or with the PORT_CFG
instruction.
Hardware flow control works through the Request-to-send (RTS) and Clear-to-send (CTS)
communication signals. With the RS232 CM, the RTS signal is output from pin 7 and the
CTS signal is received through pin 8. The CM 1241 is a DTE (Data Terminal Equipment)
device which asserts RTS as an output and monitors CTS as an input.
Hardware flow control: RTS switched
If you enable RTS switched hardware flow control for an RS232 CM, the module sets the
RTS signal active to send data. It monitors the CTS signal to determine whether the
receiving device can accept data. When the CTS signal is active, the module can transmit
data as long as the CTS signal remains active. If the CTS signal goes inactive, then the
transmission must stop.
S7-1200 Programmable controller
System Manual, 11/2009, A5E02486680-02
2.4 kbits
4.8 kbits
9.6 kbits
Point-to-Point (PtP) communications
8.4 Managing flow control
19.2 kbits
28.4 kbits
57.6 kbits
76.8 kbits
115.2 kbits
239
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