Specifications

46
Schneider Electric
20 mA CL – RS232 – RS422 –
RS485 links (continued)
RTS/CTS or XON/XOFF flow control
The hardware method is recommended: it uses signals from the RTS/CTS pins in the
RS-232 serial interface to order the DTE to stop transmitting. This method is the
most secure and most widely used.
The software method requires that the modem send two different ASCII codes to the
terminal to order it to stop (Xoff) or continue (Xon) to transmit:
v XON Ctrl-Q ASCII 11 Hex.
v XOFF Ctrl-S ASCII 13 Hex.
All IBM PCs and compatible computers permit flow control by RTS/CTS, but some
Apple computers do not.
If its buffers are full, the modem can signal the DTE at any time to wait for a certain
period before transmitting (and vice-versa).
Definition : Flow control allows the
receiver to order the terminal to stop
transmission when its buffer memory has
reached 90% of its capacity. There are two
types of control: hardware and software.
Regardless of the method used, flow
control makes it possible to increase the
transmission speed of the terminal (DTE) to
a speed greater than that of modem
throughput on the line.
Guide to Integration
Modbus / Jbus