What is a "Null Modem" cable?
If you need to connect two things via their serial (also known as RS232) ports, you need a cable. There are two sorts of these cables: 'normal' (or 'extension') cables, and 'null modem' cables. You need a Null Modem cable if you're connecting two PCs together. You need a normal cable if you're connecting a PC to a modem. For other sorts of connections you should consult the instructions. If you have nothing else to go on, I suggest using a null modem cable if the things have the same gender of connector and an extension cable if they have different genders.
How should a Null Modem cable be wired up?
Here is the correct wiring for a Null Modem cable. Note that the pin numbers are normally embossed on the plastic part of the connector, but they can be very very small; you might need a bright light and a magnifying glass to see them.
- 1 -- see below
- 2 ----- 3
- 3 ----- 2
- 4 ----- 6
- 5 ----- 5
- 6 ----- 4
- 7 ----- 8
- 8 ----- 7
- 9 -- not connected to anything
There are two options for pin 1. It can either be left unconnected, or it can be connected to pin 6 at the same end, with both being connected to pin 4 at the other end.
What happens if a cable is mis-wired?
If pins 2, 3 and 5 are connected as shown above there is a chance that the cable will work, though perhaps unreliably. That's because pin 5 is ground and pins 2 and 3 carry the data. The other wires carry 'flow control' signals. Their purpose is to regulate the speed at which the data is sent, slowing down one end if the other cannot keep up. If flow control is not working you may find that some data is lost randomly during communication. These signals also make it possible for each end to know whether or not something is plugged in at the other end of the cable.
Mis-wired cables on Ebay
I have bought three batches of null modem cables on Ebay, and two of the batches were wired wrongly.
The first batch was cream in colour with a "Manhattan" logo on the plug. These were wired as follows:
- 1 --- 8 and 7
- 2 --- 3
- 3 --- 2
- 4 --- 6
- 5 --- 5
- 6 --- 4
- 7 --- 1
- 8 --- 1
- 9 -- unconnected
The second batch was more anonymous with mid-grey cable and connectors. They were wired as follows:
- 1 --- 4
- 2 --- 3
- 3 --- 2
- 4 --- 1
- 5 --- 5
- 6 -- unconnected
- 7 --- 8
- 8 --- 7
- 9 --- 9
Note that they all got pins 2, 3 and 5 correct so there is a good chance that they would appear to work when you try to use them. However, because the flow control signals are mis-wired, they may be unreliable or not work in some applications.
My advice to anyone buying a null modem cable on Ebay is:
- Choose a cable where the description says how it is wired up. This doesn't guarantee that the cable will be wired correctly, but it does mean that you can return it more easily if it turns out to be wrongly wired.
- When the cable arrives, check it using a meter. Test each pin in turn and make sure that it is connected to the right pin at the other end. If it's wrong, send it back. Don't leave feedback for the seller until you've checked that it is correct.
I'd also like to encourage sellers to check the cables when they arrive from their suppliers. If you got a batch of really cheap cables, maybe there's a reason why they were cheap!