HP 54601 Programmer's Manual page 44

Programming over RS-232-C
Extended interface with hardware handshake
The TD (Transmit Data) line from the oscilloscope must connect to the RD
(Receive Data) line on the controller. Likewise, the RD line from the
oscilloscope must connect to the TD line on the controller.
The RTS (Request To Send) line is an output from the oscilloscope which
can be used to control incoming data flow. A high on the RTS line allows the
controller to send data, and a low on this line signals a request for the
controller to disable data transmission.
The CTS (Clear To Send), DSR (Data Set Ready), and DCD (Data Carrier
Detect) lines are inputs to the oscilloscope which control data flow from the
oscilloscope (Pin 2). Internal pull-up resistors in the oscilloscope assure the
DCD and DSR lines remain high when they are not connected.
If DCD or DSR are connected to the controller, the controller must keep
these lines and the CTS line high to enable the oscilloscope to send data to
the controller. A low on any one of these lines will disable the oscilloscope
data transmission. Dropping the CTS line low during data transmission will
stop oscilloscope data transmission immediately. Dropping either the DSR or
DCD line low during data transmission will stop oscilloscope data
transmission, but as many as two additional bytes may be transmitted from
the oscilloscope.
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