A guide to the common connections on modern computers.

The typical layout found on todays motherboard now has a selection of connection. ASUS on their latest motherboards are producing an adapter that fits in one of your spare 5.25 inch external drives to give you front panel access to on-board audio ports, infrared module and I/O ports. So perhaps for all of us it might be worth our while getting up to date with what is what.

Typical arrangement at the back of a motherboard.

Protocol	MB/s
IrDA-Control	0.009
Serial	0.02
Parallel	1.0
Bluetooth 1.1	0.125
Bluetooth 2	2 to 12
USB 1.1	1.5
USB 2.0	60
SCSI 1	5
Fast SCSI 2	10
Fast Wide SCSI 2	20
Ultra SCSI	20
Ultra Wide SCSI	40
Ultra2 SCSI	80
Ultra 160 SCSI	160
FireWire (IEEE 1394)	50
FireWire (IEEE 1394b)	100

Our simple chart gives you the throughput you can expect from the various types of connections. We would recommend as you can appreciate using an SCSI adapter card for such devices as your hard drive especially the latest Ultra 160 SCSI which works so fast. With this you can then connect up a whole array of devices to it. SCSI also takes a lot of loading of your CPU improving your computers performance.

Infrared connection.

The infrared port, (IrDA Data, IrDA Control or IrDA Air) is now very common for all sorts of remote control devices, and has a bidirectional data rate of up to 75 kbps over 5 meters. A newer standard, IrDA-Data with only 2 meters range can go up to 9,600 bps to 4 Mbps. It is normally an internal module cabled to a motherboard header.

Serial connection.

The RS-232 9-pin com ports we are all used to. These are capable of data transfers up to 115 kbps. It is a relatively slow, low-bandwidth ideal for modems, mice and such like. But with modern serial drivers you can use leads of up to 15 meters, so this is a very handy connection type.

Parallel connection.

The bidirectional Enhanced Parallel Port (EPP) was designed primarily for parallel storage devices such as CD-ROM's, tape drives and a few external hard disk drives. The Extended Capability Port (ECP) is also bidirectional but this is for more modern faster printers and scanners. Both of these types EPP & ECP are about 8 times quicker than the old Centronics standard, at about 1Mbps.

USB connections.

USB 1.1 and USB 2.0 (Universal Serial Bus). This is a replacement for the now old Serial RS-232 connection. USB 1.1 with data rates up to 1.5MB/s, is capable of bi-directional data flow and is ideal for slow to medium peripheral devices such as keyboards, mice, digital cameras, printers, modems and so on. As USB is a hub-&-spoke scheme you can connect up to 127 devices in a daisy chain using simple four-wire leads of up to 5 meters long and USB hubs. The devices can be hot-swapped and will automatically identify and configure without having to power down or reboot the computer. The cable is made of four wires, two for data transfer and two for transferring the 5 volt power to the USB device. As they are powered from the computer bus you can tap a small amount of power of them, 0.5A.

USB devices are organised in a tiered array so not every device needs to be directly connected to the host (computer). A device can be connected into a hub, then to another hub and then to the computer. The device is given a unique address ID when connected and co-exists with all the other USB devices and the operating system will load the necessary driver. There is no need to configure IRQ's, addresses or DMA channels. All devices connected to a USB port run off a single IRQ, this avoids the problem of too few IRQ's in PC's today.

USB 2.0.

Is a new standard. It will work at about 60MB/s(480Mbit/sec), a big jump from USB 1.1 (1.5MB/s) and will be backward compatible. Will work at about 40 times the speed of USB 1.1.

SCSI connections.

SCSI (Small Computer System Interface) is as can be seen being continually up-rated in speed and is the fastest interface for connecting up your internal and external SCSI devices.Three types of SCSI signals:

Single-Ended (SE), most devices use "normal" SE signalling and supports transfer rates to 40MB/s and cable lengths up to 3 meters. Includes most 50-pin (Narrow) devices such as scanners and Zip drives.

High Voltage Differential (HVD), differential SCSI provides reliable signalling in high noise situations over a long bus length of up to 25 metes and data rates up to 40MB/s. HVD hardware cannot be mixed with other SCSI signal types and a differential bus requires differential terminators.

Low Voltage Differential (LVD), introduced with Ultra2 SCSI, a typical multimode LVD/SE SCSI system. LVD can transfer data at up to 80MB/s and with cable length up to 12 meters. LVD-rated equipment is required for Ultra SCSI standards.

The two ends of a SCSI bus must be properly terminated by a small device, basically resistors (Passive) and active has a voltage regulator that ensures the correct termination voltage, finally, Forced Perfect Termination (FPT) which is better than Active termination, but all are designed to dampen the electrical signals reflected from the ends of a cable, the first and last devices on the SCSI bus must be terminated. The Daisy Chain configuration is where devices are connected in a series. It must be said that the quality of the leads is important. Buy the best leads you can, especially with these high data speeds.

SCSI-1, the old 8-bit (Narrow) 50-pin connection with rates up to 5MB/s.You will find these quite often connected to low to mid-range devices such as scanners that do not require high speed data transfer. As you can see the connection is quite large compared to the others above connections above. A cable length of about 6 meters.

SCSI-2, (Fast SCSI or Fast-10). This is again 8-bit (Narrow) & uses a 50-pin High Density connector & can work at up to 10MB/s with a cable length of about 3 meters. Ultra SCSI (Fast-20) also 8-bit works at up to 20MB/s using the 50-pin High Density connector with a cable length of about 1.5 meters. While Ultra2 SCSI is the fastest 8-bit version providing data transfers of up to 40MB/s.

Wide SCSI, (Fast Wide SCSI or Wide Fast-10) is the 16-bit (Wide) version of SCSI-2 working at 20MB/s. It uses a 68-pin High Density connector. Wide Ultra SCSI (Wide Fast-20) 16-bit and double the data speed at 40MB/s. Uses the 68-pin connector again. Lead length of about 1.5 meters for 4+ devices or 3 meters for 3 or less devices. Ultra2 SCSI, (Ultra2 Wide, Ultra2 LVD, U2W, or Fast-40) is again 16-bit, another variant using a 68-pin connector with lengths up to 12 meters, and now with a data rate of up to 80MB/s.

Ultra 160 SCSI LVD, (Ultra3 SCSI), 16-bit wide, this new and quicker standard uses a 68-pin connection and will transmit data at up to 160MB/s.

Future SCSI, Serial SCSI. Will allow data transfer up to 100MB/s through a six-conductor coaxial cable. It will solve many of the termination delay problems of older SCSI, and will be more plug-and-play in nature, such as automatic SCSI ID assigning and termination.

IEEE 1394, (FireWire or iLink).

Is a high speed serial link and can work at up to 50MB/s using a simple 6-pin cable. This makes it quite ideal for data intensive transfers such as video, and is now seen in digital VCR's and digital video cameras. Data transfers between these devices can bypass the computer thus eliminating the operating system to maximise the data transfer speed.

It can handle up to 63 devices and a typical FireWire interface will have one internal and two external connectors as in this picture of a Western Digital external FireWire hard drive.

FireWire devices like USB are also hot-swapable so you can attach or detach with the need to shut down your computer, but unlike USB are NOT powered from the computer bus.

IEEE 1394b, (FireWire or iLink) will be the next generation. Giving high-speed data transfer at up 100MB/s. FireWire has mostly been taken up by Apple so this next IEEE 1394b is expected to be available in the summer.

Bluetooth.

Is wireless technology for the home, general consumer, handhelds and internet linked cell phones. It is a short range radio technology frequency operating in the 2.4GHz ISM band. It has a range of about 10 meters at about 1MB/s transfer rate. As you can see it is slower than IrDA-Data but by increasing the power of the transmitter it can be extended up to 100 meters. Bluetooth will support up to nine devices through a piconet (two or more Bluetooth devices sharing a channel) and it can reserve bandwidth for digital voice transmission and will be support up to three simultaneous full-duplex conversations.

Bluetooth 2.0 to provide between 4, 8 & 12Mbit/sec while Bluetooth 1.2 to give 2 to 3 Mbit/sec. Bluetooth 2.0 will be able to work in non-switching narrow-band channels, have better mode functions.

RAID.

IDE RAID (Redundant Array of Independent Disks). It is not an interface type like IDE or SCSI, there are no special RAID drives, but RAID is a protocol, or system of using existing IDE or SCSI drives. RAID uses additional hard disks to improve performance, add reliability, or both simultaneously.

ATA IDE connections.

Ultra ATA 66 IDE cable with the Light Blue connector uses the same standard 4-pin Molex power connection (+5v, +12v, & gnd) but now uses a 80-pin data/control signal cable. 40 of the leads being paired and grounded at one end to prevent ground isolation loop from occurring. This pairing of the 40 leads reduces the signal interference between the cables and hence allows for quicker settling times. It is this reduced settling time, now half, that gives the increased data rate, that is along with the increase in rpm.

Ultra DMA ATA 33  -  60ns cycle time  -  33MB/sec transfer rate. ATA-33.

Ultra DMA ATA 66  -  30ns cycle time  -  66MB/sec transfer rate. ATA-66

Ultra DMA ATA 100 - 100MB/sec transfer rate.

Ultra DMA ATA 133 - 133MB/sec transfer rate.

Serial ATA.

Serial ATA to have data rates up to 150Mbytes/sec transfer rate and work in full-duplex mode.

PCI.

PCI-X. The next update from the present PCI system uses a parallel bus that can work as fast as 133MHz which offers a bandwidth of 1.1GBytes/sec with each of the 64 wires (64-bit) carrying about 17MBytes per second.

Intel's 3GIO (Third Generation Input/Output) often called Arapahoe or Serial PCI is a serial point-to-point interconnect. Probably to be called in the end PCI 3.0.

Likely to be six times faster than PCI-X. Versions will be defined with a single wire or two, four, eight, 16 or 32 wires (1 6032-bits) for higher collective speeds. 3GIO will carry data at least 12 times the pace of PCI-X. Thus a single wire (bit) of 3GIO could carry 2.5Gbits/sec. The 8-wire (8-bit) version could carry 20Gbits/sec. First versions of 3GIO will have wire communicating at 12 times the speed of the wires of PCI-X. Later versions will have faster wires.

AGP4X communicates at 1.1GB/sec and Fast Ethernet cards generally work at 120MB/sec as a comparison.