User`s manual

88 www.rabbit.com Serial and TCP Protocols
like Tx and Rx. RTS is an output for a DTE, and tells the DCE whether or not it is willing to accept data.
CTS is an output for DCE, and tells the DTE whether it is able to accept data.
Thus:
In the null-modem cable, RTS and CTS are also swapped over:
Two additional modem control lines were defined to signal the overall readiness state of the attached
devices. These lines, DTR (Data Terminal Ready) and DSR (Data Set Ready i.e., modem ready) are less
important than RTS/CTS, but are nevertheless used often.
The wiring for these is:
And in the null-modem cable, the DTR/DSR pair also crosses over quite
naturally:
DTE
Tx Tx
Rx Rx
GND GND
DCE
RTS
RTS
CTS CTS
5-Wire Straight-Through
DTE
Tx Tx
Rx Rx
GND GND
DTE
RTS
RTS
CTS CTS
5-Wire Null-Modem
DTE
Tx Tx
Rx Rx
GND GND
DCE
RTS
RTS
CTS CTS
DTR
DTR
DSR DSR
5-Wire Straight-Through
DTE
Tx Tx
Rx Rx
GND GND
DTE
RTS
RTS
CTS CTS
DTR
DTR
DSR DSR
7-Wire Null-Modem