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RS232 connections, and wiring up serial ports
RS232 Pin Assignments (DB25 PC signal set)
Pin 1 Protective Ground
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Pin 2 Transmit Data
Pin 3 Received Data
Pin 4 Request To Send
Pin 5 Clear To Send
Pin 6 Data Set Ready
Pin 7 Signal Ground
Pin 8
Received Line Signal Detector
(Data Carrier Detect)
Pin 20 Data Terminal Ready
Pin 22 Ring Indicator
The connector on the PC has male pins, therefore the mating
cable needs to terminate in a DB25/F (Female pin) connector.
RS232 Pin Assignments (DE9 PC signal set)
Pin 1
Received Line Signal Detector
(Data Carrier Detect)
Pin 2 Received Data
Pin 3 Transmit Data
Pin 4 Data Terminal Ready
Pin 5 Signal Ground
Pin 6 Data Set Ready
Pin 7 Request To Send
Pin 8 Clear To Send
Pin 9 Ring Indicator
The connector on the PC has male pins, therefore the mating
cable needs to terminate in a DE9/F (Female pin) connector.
[Some people call this a DB9... DE9 is the real name, however]
Wiring up something nice and simple, for instance a plain old "dumb terminal", is just a matter of
connecting Tx, Rx and Ground, right?
Usually Not. While the normal PC hardware might well run with just Tx, Rx and Ground connected, most driver software will wait forever
for one of the handshaking lines to go to the correct level. Depending on the signal state it might sometimes work, other times it might
not. The reliable solution is to loop back the handshake lines if they are not used. We specifically chose to use the Personal Computer (PC) as the
frame of reference for the signals on this page. RS232 predates the PC, but even with EIA-574 and the later TIA-232-F the definitions still use DTE (Data terminal
equipment - such as a PC or equivalent) and DCE (Data circuit-terminating equipment - such as a modem or 'dumb terminal' or any other device) as the frame of
reference. We are trying to flatten the learning curve: On this page "PC" is used in place of the technical term "DTE".
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Handshake looping a PC serial connector
When the lines are handshake looped, the RTS output from the PC immediately activates the CTS input - so the PC effectively controls
its own handshaking... which leads in to what each of the pins on the RS232 connector (or EIA574 in the case of the DE9) actually does
- we have a proper description of all the pins further down this page.
RS232 DE9 PC Loopback test plug Build it yourself using the photo to the
right as a guide - or if you prefer we will do it for you:
$18 + shipping
The PC loopback plug is a useful diagnostic tool. The loopback plug connects
serial inputs to serial outputs so that the port may be tested. There is more
than one way to wire up a loopback plug - but this is the most common.
RS232 DB25 PC Loopback test plug
Connecting together two serial devices involves connecting the Rx of one device to the Tx of the other, and vice versa. The diagram
below indicates how you would go about connecting two PC's together, without handshaking.
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