The RS232 standard implements an hardware flow control mechanism. While TxD and RxD pins are used to send and receive data, DCD, DTR, DSR, RTS, CTS and RI allow devices which support hardware control flow to maintain a reliable data connection between a transmitter and a receiver.
Signal Pin Pin Direction Full
Name (25) (9) (computer) Name
----- --- --- --------- -----
FG 1 - - Frame Ground
TxD 2 3 out Transmit Data
RxD 3 2 in Receive Data
RTS 4 7 out Request To Send
CTS 5 8 in Clear To Send
DSR 6 6 in Data Set Ready
GND 7 5 - Signal Ground
DCD 8 1 in Data Carrier Detect
DTR 20 4 out Data Terminal Ready
RI 22 9 in Ring Indicator
From the computer side the output pins are: RTS, DTR and obviously TxD.
Both RTS and DTR pins are usable to our scope, I have chosen the RTS one.
The RS-232 standard defines the voltage levels that correspond to the logical “one” in the range of +3V to +15V and the logical “zero” in the range of -15V and -3V but, through a TTL/CMOS converter, we can use 0V and +3.3V/+5V.
If you want to use a real RS232 port (DB9 connector) you can use the MAX232 IC to use a TTL or CMOS voltage level.
In a modern computer without a RS232 port you can use a cheap USB/TTL-232 interface like this, which already has the TTL voltage values.
Not all of these interfaces have all control flow pins so, if you want to buy the new one, check if RTS and/or DTR are present. On my old USB-to-TTL232 interface neither of those was there, but it uses a PL2303 driver that exposes RTS on pin 3 and DTR on pin 2 so I had to solder a wire directly on IC pin.
The output of RTS (or DTR) pin needs to be connected to the Base of a BJT transistor that controls the current flow through a relay: