105BCommunication processor
12.3 Point-to-Point (PtP) communication
S7-1200 Programmable controller
System Manual, 11/2011, A5E02486680-05
551
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.
Transmission resumes when the CTS signal becomes active. If the CTS signal does not
become active within the configured wait time, the module aborts the transmission and
returns an error to the user program. You specify the wait time in the port configuration
(Page 549).
The RTS switched flow control is useful for devices that require a signal that the transmit is
active. An example would be a radio modem that uses RTS as a "Key" signal to energize the
radio transmitter. The RTS switched flow control will not function with standard telephone
modems. Use the RTS always on selection for telephone modems.
Hardware flow control: RTS always on
In RTS always on mode, the CM 1241 sets RTS active by default. A device such as a
telephone modem monitors the RTS signal from the CM and utilizes this signal as a clear-to-
send. The modem only transmits to the CM when RTS is active, that is, when the telephone
modem sees an active CTS. If RTS is inactive, the telephone module does not transmit to
the CM.
To allow the modem to send data to the CM at any time, configure "RTS always on"
hardware flow control. The CM thus sets the RTS signal active all the time. The CM will not
set RTS inactive even if the module cannot accept characters. The transmitting device must
ensure that it does not overrun the receive buffer of the CM.
Data Terminal Block Ready (DTR) and Data Set Ready (DSR) signal utilization
The CM sets DTR active for either type of hardware flow control. The module transmits only
when the DSR signal becomes active. The state of DSR is only evaluated at the start of the
send operation. If DSR becomes inactive after transmission has started, the transmission will
not be paused.
Software flow control
Software flow control uses special characters in the messages to provide flow control. You
configure Hex characters that represent XON and XOFF.
XOFF indicates that a transmission must stop. XON indicates that a transmission can
resume. XOFF and XON must not be the same character.
When the transmitting device receives an XOFF character from the receiving device, it stops
transmitting. Transmitting resumes when the transmitting device receives an XON character.
If it does not receive an XON character within the wait time that is specified in the port
configuration (Page 549), the CM aborts the transmission and returns an error to the user
program.