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Serial cable connection: Tips for networking pros, lesson 8

Serial direct cable connection is examined in this cable networking lesson, which defines the term and explains how it works, as well as the technology required to set one up.

A direct serial cable connection uses the communication ports of your computers. Most computers have at least two communication, or COM, ports: COM1 and COM2. Serial port pinouts are less complex than parallel port pinouts, but the speed is also a lot slower -- between 12 Kbps and 14 Kbps.

That's pretty slow when you're used to a network connection. Here's a look at how serial data is transferred and why it's slower:

Figure 1 gives you an idea about how serial data is transferred. Each colored block that is numbered is sent from PC 1 to PC 2. PC 2 will receive the data in the same order it was sent; in other words, it will receive data block 1 first, and then data block 2 -- all the way to block 7. This is a pretty good representation of data flow in a serial cable connection. Serial ports transmit data sequentially over one pair of wires -- the rest of the wires are used to control the transfer.

Serial data transfer

Serial ports are much like a one-lane road, where the road is wide enough to fit only one car at a time -- it's equivalent to one data block at a time in our example above. It's easy to understand the road cannot process several cars at one time.

What is a serial port connection?

Most new computers have two COM ports, with nine pins each; these are DB-9 male connectors. Older computers would have one DB-9 male connector and one DB-25 male connector. The 25-pin male connector is pretty much the same as the nine-pin connector -- it's just bigger.

Let's have a look at a serial port to see what we are talking about:

DB-9 and DB-25 COM ports

Different pinouts are used for the DB-9 and DB-25 connectors, and we will have a look at them in a moment. But first, let's have another look at the COM ports of a new computer:

COM ports

Notice the COM ports are both DB-9 connectors; there is no more DB-25. The connector above the two blue COM ports is a line print terminal, or parallel port.

The serial port of a computer can run at different speeds, thus allowing us to connect different devices that communicate at different speeds with the computer. The following table shows the speeds at which most computers' serial ports are able to run and how many kilobits per second it translates:

Serial port speeds

Now, here are the pinouts of both DB-9 and DB-25 connectors:

COM port pinouts

Understanding a serial cable connection

All that's left now are the pinouts required to use a serial cable connection directly. There is a special term for this type of a cable: a null modem cable, which basically means you need to have TX and RX crossed over. Because you can have different configurations -- for example, DB-9 to DB-9; DB-9 to DB-25; and DB-25 to DB-25 -- I have created different tables to show you the pinouts for each one:

DB-9 to DB-9. You use this configuration when you need a cable with a DB-9 connector on each end:

Null model cable DB-9 to DB-9

DB-9 to DB-25. This configuration is required when you need a cable with one DB-9 and one DB-25 connector on either end:

Null modem cable DB-9 to DB-25

DB-25 to DB-25. This configuration is used when you need a cable with a DB-25 connector on each end:

Null model cable DB-25 to DB-25

This should cover what you need to know about direct serial cable connection via a null modem cable.

If you're using third-party software to connect your computers, you probably won't stumble into big problems. But if you're using Windows software, be sure you have unique names for each of your computers, because Windows will treat the direct connection as a network connection. This means you will be able to see the other computer via Network Places.

Next Steps

Cabling series, lesson 9: Parallel direct cable connection

Cabling series, lesson 10: USB direct cable connection

This was last published in September 2016

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