Free Certification Practice Tests and Study Guides
Join Us! | Login | Help

Copyright Information

Domain 1.0: Personal Computer Components: Cables and Connectors

:: Return to the Index ::

By David J. Schultz

This paper is meant to help you with passing the A+ exam, in building your own PC, and most importantly, on the job. It is important to know how to recognize which cable is which by looking at it or its connector and how to troubleshoot any problems that may come up with them or their setup. Also my “in the field” will describe some things to remember about these cables. This will help in troubleshooting bottlenecks and other miscellaneous problems.

In reading this, please remember that there are legacy and cutting-edge products. Even though you may have a tendency to turn nose up at the old stuff, you may have to work with it. Being able to recognize it may get you a job and respect from the senior members faster than if you have to look up all the old stuff.

There is also the weird and bizarre factor. There are a million differnent types of cables out there and many of them are proprietary to a particular company or product. We see a lot of this in the laptop world. I am not here to show the weird and bizarre so when you see these types of things, look in the manual.

We need to start with some vocabulary.

  • Asynchronous: Not synchronized. Communication without any timing mechanism. Instead of a timing mechanism you have communication rules. There is a start bit and a stop bit to notify the other end when communication starts and stops.
    • Pros: If one computer needs to send data then it can do so without having to keep stopping during each clock cycle.
    • Cons: With the use of start and stop bits you have 20% overhead. In other words, if you send 100 KB of information then it takes 120 KB to get it all there.
  • Synchronous: In this setup, data is sent as strict blocks of information. Because the timing is uniform, there is no need for a start and stop bit.
    • Pros: No need for extra information.
    • Cons: There is timing so you could say that there are blank spaces.
  • IRQ: Stands for Interrupt ReQuest. This is how a peripheral tells the CPU it needs its attention. There are specific numbers to tell the CPU which peripheral needs time and the priority. The priority is determined from least to greatest (i.e. 0 is the most important and 15 is the least). Most systems have 16 of these IRQs. Some older systems only have 9. This used to limit the numbers of things you can put in a system. Now technology has advanced so there is now IRQ sharing, but be mindful when working with ISA cards and older systems as you can run out fast.
  • Male vs. Female Connector: I’m not going too far into this, but a male connector is the side of a connection that has the prongs--like the power plug on your computer monitor. The side that goes from your monitor into the wall is the male. The outlet is the female side of the connector. NO JOKE.
  • DB: This term you will see in the context of DB-25 or DB-9. What this is describing is the shape of the connector. The connector has a definite shape like a capital D (also could be called a trapezoid with rounded angles).
Now let's take a look at some of the various cable types.

Serial Port

Standard: RS-232
Connector Name: DB-9 (most common) sometimes DB-25
Transfer mode: Asynchronous
Sometimes called: COM ports
Number found on a system: 2 possible of 4
IRQ: 3(Com 2 & 4) and 4(Com 1 & 3)
I/O range: COM 1=03f8-03ff COM 2=02f8-02ff COM 3=3E8-3EF and COM 4=2E8-2EF
Max length: 50 feet
Max data rate: 1.5 Mbps (with 16550A UART)

Quick recognition:
  • On the computer: Male DB-9 connector. Usually 2 of them. Note: this is usually the ONLY male connector on the back of your PC. It is sometimes confused with the VGA connector because of similar physical size. But you can quickly tell the difference because a serial connector on the back of a computer is male and only has 9 pins where a VGA connector has 15 pins and is female (usually colored blue). Also, if you are looking at a really old computer, you might confuse a serial port for the video port. The really old EGA and CGA video connector used a DB-9 connector, but on the box they were female. So again, it is important to remember to look for the male aspect of this connector for identification.
  • On the peripheral you can quickly identify it by its female connector as shown in the image above.

  • Normally used with: This is normally seen on older mice and modems. Two computers can be networked together using a null modem cable. This is a serial cable that has its send and receive crossed over so the 2 computers are not trying to send information to the other's send port.

    Ending comments: No discussion of serial would be complete without discussing UART (universal asynchronous receiver-transmitter) chips. This chip is the heart of your serial port. It takes your information and turns it into serial data then back again. These chips have evolved from slower to faster starting with the modern imp.

    If you want the real techie stuff go to:

    Parallel Port

    Standard: IEEE-1284
    Connector Name: DB-25(on PC), Centronics
    Transfer mode: Synchronous
    Sometimes called: LPT
    Number found on a system: 1 (up to 2)
    IRQ: LPT1 =7 LPT2=5*
    Max length: 10 Meters (approx. 30 feet) absolute Maximum
    Max data rate: 4 Mbps

    Quick recognition on a computer:

    This connector stands out as the biggest connector (usually) on the back of your PC. It is usually next to the serial ports. It is a DB-25 connector on the PC and usually a Centronics 36 pin connection to the peripheral. *Warning* There are SCSI 1 connectors that look like this and will actually fit together. Just look for the SCSI symbol to tell the difference. You usually will not see a SCSI connector on a PC. But on an old Apple Macintosh you will. There is also no parallel port on a Mac to make things difficult.

    Real World Notes: I once sold a Mac to a friend. He called me 2 weeks later asking me why the computer did not work. It turns out that someone had given him an old PC printer. Not knowing, he connected the parallel connector to it and turned it on. The Mac did not do anything. It did not even POST. Once he unplugged the parallel connector from the SCSI interface on the back of his Mac it worked fine with no damage.

    Also, if you plug an older device in and it is not recognized, you may need to go into Device Manager on you computer and "Enable legacy Plug and Play detection.”

    *In modern computers they can be set not to use an IRQ for an LPT port.

    Normally seen on: You usually see this on printers and scanners. This has mostly been replaced with USB.

    DIN 6

    Standard: PS/2
    Connector Name: DIN 6 (6 pin)
    Sometimes called: PS/2 port, Mouse port, keyboard port, mini DIN 6
    Number found on a system: 2
    IRQ: 12 and 1
    Max length: 100 ft

    Quick recognition on a computer:

    This will look like a small round connector on the back of your PC. Usually color-coded green or purple. The green is for the mouse and the purple is for the keyboard. If they are not color-coded, look close to the connector for an indicator or look it up in the owner’s manual. Or go by the rule that the mouse is the one on the inside of the PC. If you do attach the wrong connector and start up the PC you will get a "beep" (if the internal speaker is still connected) and an error message ("No keyboard present").

    Normally seen on: This is used for mice and keyboards

    DIN 5

    Connector Name: DIN 5
    Transfer mode: N/A
    Sometimes called: old keyboard connector
    Number found on a system: 1
    IRQ: 1

    Quick recognition: On computer: On the keyboard cable this is a larger round connector with bigger pins arranged in a circular fashion. On some older motherboards, this may be the only built-on connector. It is also considered out-of-date. It was replaced by the PS/2 stile connector, which in turn is being phased out by USB.

    Normally seen on: Only used on keyboards.

    USB 1.1

    Standard: USB 1.1 (by USB-IF)
    Connector Name: USB A/B
    Transfer mode: Asynchronous
    Number found on a system: 2-5
    Maximum number of Devices: 127
    IRQ: 11
    Max length: 3-5 meters
    Max data rate: 12 Mbit/sec (1.5 MB/sec)
    Power: 2.5w

    Quick recognition on a computer:

    On the computer these look like thin rectangular slots.

    Cable Quick recognition:
    The cable has 2 male connectors; one on each side. The difference between the a and b standard is that the 2 power wires are not at the b end. You would see this on a printer that gets its power from the wall.

    Normally seen on: Mice, keyboards, scanners, modems and other low-power peripheral devices. Even some hard drives can be powered by this low current. This can also be found on digital cameras and some camcorders to download the movies and pictures to your computer. Also you can get speakers that use this type of connector. This interface has all but replaced the serial port.

    Features: To connect many devices, you can use a USB hub. This is a box that you connect into your computer through one of your USB ports and then you can plug many other devices into it. Also, the speed of each USB chain is shared between all devices on that chain. So the more devices you have operating at once, the slower they all will go.

    *WARNING* With enough force you can plug a USB connector in upside-down. This will kill your motherboard (BOOM) or PCI card. Never force anything on your computer!!!!

    USB 2

    Standard: USB 2 (by USB-IF)
    Connector Name: USB
    Transfer mode: Asynchronous
    Sometimes called: USB 2
    Number found on a system: Still being determined
    IRQ: 11
    Power: 2.5w

    USB 2 is really just an upgrade in speed from USB 1.1. So everything is the same, just faster. You can use the same cables but you do need a new hub. The old hub will work, but it will only allow the USB 1.1 speed.

    Firewire (IEEE-1394)

    Standard: IEEE-1394
    Connector Name: IEEE-1394 A B
    Transfer mode: Asynchronous/Isosynchronous
    Sometimes called: Firewire, IEEE-1394 or iLink (Apple computers has trademarked the term “Firewire.” So, if you see it called that, someone is paying for that ability. iLink is what Sony calls IEEE-1394).
    Number found on a system: 2
    Max length: 4.5m (between devices)
    Max data rate: 100Mbps, 200Mbps, 400Mbps (12.5MB/sec 255MB/sec 500MB/sec). Most computers support 400Mbps but most devices are only 200Mbps
    Max # of devices: 63

    Quick recognition: This looks like a USB cable with 2 corners cut out.

    On computer: Looks like a square with 2 corners filled in.

    Normally seen on: You will see Firewire on high-end devices like high res. scanners, high res. printers, hard drives, CD-RW, DVD, DVD-R/W, Video cameras, etc.

    Features: Some on the added features are that you do not need a PC to connect them together. You could conceivably have a video camera connected to a DVD-R and burn your movies without needing a computer connected to it. They can talk to one another without using up the bandwith to the computer and back. You can daisy chain them together so there is no need for a hub as in USB. Also there is more power supplied. So, for example, you have enough to have a hard disk drive running without power coming from the wall. Some companies have set it up so you can connect an MP3 player to your computer using IEEE-1394 and transfer the data plus recharge the player's internal batteries off the firewire cable.


    In this section we are not so worried about things like transfer rates and IRQs. The monitor will only refresh so fast and IRQs would affect your video board not your monitor. The point of this part is so that you can intelligently find and talk about these.

    Standard: VGA
    Connector Name: DB-15

    Quick recognition ona computer:

    This is our old standby and by far the most common of our video connectors. It is easy to find. It is the only small three-rowed DB connector we will find on a PC. It is usually away from the other connectors because it is on a replaceable card. There are, however, motherboards that do have video built in. If so, then this connector will be close to the other connectors.


    The power connector is almost completely standard. This looks almost exactly like an extension cord with the exception of the ground being a slot instead of a round hole. Not much more to say.

    Make sure that the power switch on the back of your computer is set for the correct voltage (115 or 230) for your location.

    Be careful with laptops and some newer Apple Macintosh computers that may have different looking power connectors on the computer.


    Connector Name: 1/8 minijack
    Sometimes called: headphone jack
    Number found on a system: 1-5

    Quick recognition: This the same jack that you have been plugging into your walkman for years. It is a small pointed metal plug with one or two plastic bands imbedded in it.

    On computer: These can be found on the front on most CD-ROM and CD writers. On some CD readers and writers, you can plug a pair of headphones in and play audio CDs independently of the computer. You will also find these on the back side on the PC. If you have a sound card, there will be 3 or more. One for your speakers (this is usually lime green). If it is not color-coded, then consult the owner’s manual that comes with the sound card, motherboard, or PC. The other minijack is the sound-in connector. This is for connecting a microphone or other device that can deliver sound to your PC. This is color coded pink. There is one more plug that goes with this set--the line-in port. It has many different uses.

    Normally seen on: Any PC with sound. The male end you will find on any set of speakers or set of headphones.


    Male and female

    They come on most sound cards. It is a DA-15 port female on the PC and male on the device. This port on the PC will usually be colored gold. This is quickly being replaced by USB. Adapters are available to convert from the DA-15 to USB.


    Telephone cable

    RJ-11 is a common telephone cable. The end is called an RJ-11 connector and the cable is called category 1 (Cat 1).

    Ethernet cable

    Standard: Conectors EIA/TIA 568a/b Cables Category 3, 4, 5 or 5e
    Connector Name: RJ-45
    Transfer mode: Serial
    Sometimes called: Unshielded twisted pair (UTP) Shielded twisted pair (STP just UTP with some more shielding), 10BaseT, 100BaseT, 1000BaseT. Crossover cable
    Number found on a system: 1 (sometimes more)
    IRQ: Would be assigned to the Network interface card.
    Max length: 100 meters (328 ft.)
    Min length: 1 Meter (3 ft.)
    Max data rate: 10/100/1000 Mbp/sec

    Quick recognition on a computer:

    On computer: On the computer it looks like an over-sized telephone jack.

    Normally seen on: Networking equipment for connecting 2 computers.

    Tidbit: The A and B standard are very important to look for. If you have a cable that has never worked, look at the colors at the end. If they match, they are a straight-through cable. This is used for connecting dissimilar devices together i.e.… a PC to a Hub or a switch to a router. But you want to use a crossover cable for like devices i.e.… PC to PC, switch to switch, or router to router.

    *Warning* 80% of all network problems are caused by this little cable. If you can't get the network to work, check the cable before you go and start changing the setting.

    There is also a Plenum-grade Ethernet cable. This is a cable that is necessary for fire code. The issue is that if there is a fire and the PVC outer coating of non-Plenum grade cable catches on fire, it will release deadly gasses. In that case, if you have an enterprise grade network where you have literally thousands of these cables running through the walls, this can cause deadly results in a fire. Before wiring a building, check with the Fire Marshall about needing this cable.


    Connector Name: DB25, IDC50, Centronics 50, HDI30, DB50, HPDB50, HPDB68, HP Centronics 60, HP Centronics 68, SCA 80-Pin, and VHDCI68
    Number found on a system:SCSI devices can be daisy chained, so the number connected to a system will vary
    Max length: 1.5 meters to 25 meters depending on the type
    Max data rate: 360 MB/sec max

    Quick recognition: This is a big connector

    On computer: This is the biggest connector you will see on a system

    Normally seen on: Hard drives, CD-ROM drives, tape drives, scanners etc.

    50 Pin Centronics: This is connector is at the device end of some peripherals

    25 Pin D Sub: This one you have to look out for. You can plug a parallel connector into this and they will match up perfectly. But this is found on an Apple Macintosh. To help you out, you will not find a parallel connector on a Mac making this easy. If you are working on a Mac, it is a SCSI interface not a parallel connector.

    DB-50: This is a rare connector that has 3 rows of pins. It was used on HP and DEC computers. It is not very common.

    50 Pin MicroD (High Density): This is a connector that has 2 rows of squared off holes. It is used on 8-bit fast SCSI.

    68 Pin MicroD: This looks like the 50 Pin MicroD but longer and with more pins. This interface is used on all SCSI Wide connectors.

    It is beyond the scope of this paper to discuss ALL of SCSI. I just want to focus on recognizing these connectors. Most times you will not see SCSI on a PC. But this is the standard on most servers as it is fast - Up to 360MB/sec. This is a Gigabyte every 3 sec. Wow, that is fast! SCSI has grown up as a technology and it has many implementations from SCSI-1, SCSI ultra wide, SCSI-2, SCSI-3, SCSI 160, SCSI 360 and more.