RS232 Connections, and wiring up serial ports

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RS232 Pin Assignments
(DB25 PC signal set)

Pin 1	Protective Ground
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.

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.

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.

RS232 DE9 PC Loopback test plug
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.

Connecting two PCs together using RS232, without handshaking

When Handshaking is required, generally RTS of one device connects to CTS of the other, and vice versa, and also DSR of one device connects to DTR of the other device, and vice versa. The particular requirements for different equipment may vary.

Connecting two PCs together using RS232, with handshaking

Using a Breakout box or LED box to work out cabling

The first thing to remember, is that there is a good chance the two devices you are trying together will actually work if you can get the cable correct. If you have some other way to actually prove this - for instance by trying each of the devices on another system - do it.

Given a hypothetical example - for instance connecting a standard PC with a DE9M to an CNC industrial turret punch, also with a DE9M, the first thing I would try and do is get a cable that I think would work. In this instance, I would either purchase or build a null-modem cable (DB9F to DB9F) - actually, I would copy the last example, basically the cable used to connect two PCs together with handshaking.

Connector size

DE9 (as pictured) $90.00 DB25 (LEDs only, No jumpers) $76.00

To continue with the example above, if I saw that two ends were driving the same lines, I would assume the null modem cable was not correct, and I would try a one-for-one (or "straight-thru" DE9F-DE9M cable) with a gender changer (DE9F-DE9F in this instance) instead.

If each end drives its own set of LEDs, connect the two ends together. In normal situations, you should see all the LEDs light up - but there are some devices which will not light up all the LEDs. Having said that, if one of the devices is a PC and any LED except RI (Ring indicator) is not lit up, the cable will probably not work.

Normally, other cabling problems will involve handshake lines. An LED box will be an invaluable guide, but there is no trivial test to determine the solution. An LED Box will also show the lines as they change state, although with the D25 models it is usually quite hard to see the serial communications themselves unless the comms are continuous, or at a low baud rate (9600 baud or lower is usually visible). Our D9 Model has superbright LEDs on the data lines, so seeing brief packets is not a problem.

If you have difficulty in obtaining gender changers, null-modem / straight-thru cables, D25/D9 converters (etc) email us, we carry everything you could need to test and debug serial comms, and we can quote by return. Our main business is designing industrial electronics - much of it serially controlled or networked.

Using a 'T' plug and a PC to monitor comms

The gadget below is a quick 10 minute project that is really great for monitoring RS232 Comms using a PC. The circuit is really not that hard to build - the parts can be purchased from Tandy / Radioshack / Dicksmith / Jaycar / Maplin / Farnell - but if you want to buy it ready made off us, just use the button below.

How to monitor RS232 Comms between two devices

There are three sockets on our monitoring gadget. Two of them are connected straight through - you plug them in series with the devices you wish to monitor - and the third goes off to another monitoring PC.

The monitoring PC "Sees" on its serial port both sides of the serial conversation - that is it sees what is sent by the target PC and also what is sent by target peripheral. This can be a positive advantage, because you can see the serial conversation as it progresses between the two devices. Some serial protocols, however, talk "full duplex" meaning that one end can start transmitting while it is still receiving from the other end. This unit cannot monitor full duplex Comms - you will see gobble-dee-gook where the two transmissions overlap.

If you try this unit, you will be surprised how useful it is, and how often it works - mostly because many supposedly full duplex installations still talk half duplex in any case, because that is the sensible way to write the software. Not bad for one diode and a resistor, huh?

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