Wiring Tip 8: Serial Direct Cable Connection

Wiring Tip 8: Serial Direct Cable Connection

The serial direct line uses the computer's COM port. Every computer has at least two COM ports, COM1 and COM2. "COM" stands for "communication". Compared with the parallel port, its pinout is much simpler, but the speed is also slower.

How fast (or slow) are serial ports? The fastest they can go is about 12 to 14 KB per second. That's pretty slow for a network connection. Let's take a look at how serial data is transmitted so you can understand why it's so slow:

The diagram shows how serial data is transmitted. Each numbered colored block is sent from PC1 to PC2. PC2 will receive the data in the order it was transmitted, in other words, it will receive block 1 first, then block 2, and so on to block 7. This is a fairly representative data flow in a serial cable. Serial ports transmit data sequentially over a pair of wires (the remaining wires are used to control the transmission).

Another way you can think of it is as a one-way street where only one car can go at a time (in our example above, only one data block at a time), so you can imagine that this street cannot handle more than one car at a time.

serial port

Many new computers have two 9-pin COM ports; this is the DB-9 positive connector. Older computers have a DB-9 positive connector and a DB-25 positive connector. The 25 positive connector is almost the same as the 9-pin, except it is a bit larger.

Let's take a look at what a serial port looks like:

DB-9 and DB-25 use different pinouts, which we'll look at later. First, let's take a quick look at the COM ports on your new computer:

Note that the COM ports all have DB-9 connectors, but not DB-25. The two blue COM ports in the picture above are LPT or parallel ports.

A computer's serial ports can run at different speeds, which allows us to connect to different devices that communicate with the computer at different speeds. The following table shows the speeds that most computer serial ports run at and how many KB/sec they transfer:

Now let's take a look at the pinout in the DB-9 and DB-25 connectors:

Network cable

The next question is the pinout for a direct connect serial cable. This is a term specific to this type of cable, a so called "null modem" cable, which basically means you have to cross the TX and RX. Because you can have different configurations like DB-9 to DB-9, DB9 to DB-25 and DB-25 to DB-25, I have created different tables showing the various pinouts.

1) DB-9 to DB-9. You use this configuration when you need a network cable with DB-9 connectors on both ends:

2) DB-9 to DB-25. You use this configuration when you need a network cable with a DB-9 on one end and a DB-25 on the other:

3) DB-25 to DB-25. You use this configuration when you need a network cable with DB-25 connectors on both ends:

That's all there is to it regarding direct serial connection via a null-modem cable.

If you are using third party software to connect your computers then you probably won't have any major problems, but if you are using Windows software then make sure each of your computers is using a unique name, this is because Windows treats the direct connection as a "network" connection. This means that you will be able to see the other computers through Network Neighborhood.