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COMPUTER BITS BY JENNIFER BONNITCHA Computer cables and all that rot Modem and serial cables bear a striking resemblance to each other. How can you tell the difference? What really is the difference between serial and parallel printer cables? Why do you need different cables for hard and floppy disk drives? We'll answer these questions and more as you read on. The PC world is full of cables. Apart from generating lots of paper, computers are really good at strangling you with all types of cables - serial and parallel printer cables, video cables, keyboard cables , modem cables, adaptor cables - the list goes on and on. The sad fact is that unless you use the correct cabfe for the job, you can end up feeling very frustrated with little to show for your effort. Cables come in two basic varieties round and flat ribbon. That's pretty trite but one thing is certain - your computer won 't work properly unless all the appropriate cables are correctly attached. A major problem for many people is how to tell the difference between the male and female ends of the cable. The photo of Fig. l shows the differences. On the left is a male connector; it has pins in the centre while the female connector on the right has pin receptacles or sockets. The male pins on a multiple-pin line plug are Fig.1: this photo shows two 25-pin D plugs. The one at left is a male plug while the one on the right is a female plug. 80 SILICON CHIP connected with the appropriate fe male counterparts at the other end of the cable to close a circuit. If you are still confused, think of the typical power outlet - the power plug is a male connector while the power point itself is a female socket. Be aware also that many cables will have the same type of connector at each end, depending on the configuration of the computer and the device you want to attach. So it is possible to have a cable with a male plug at each end. Once you understand the basics of cables and connectors, you will certainly feel more at home with your computer. Parallel cables The parallel printer cable (also known as a Centronics printer cable) enables you to connect a standard parallel printer to your computer. The connection is made by a cable with a multiple-pin plug at each end. The most common is a 36-pin Centronics plug at the printer end (named after the American printer company that pioneered this parallel interface) and a 25-pin "D" type connector to the computer end. The photo of Fig.2 shows these plugs for a typical printer cable. Your computer will probably have only one parallel port which DOS knows as LPTl. A parallel port has eight lines for sending all the bits for one byte •Of date simultaneously across eight wires. This interface is fast and is usually reserved for printers rather than computer-to-computer communications. The main problem with parallel ports is a limit on cable length of about 4-5 metres. Cables longer than this may require amplification (signal buffering) to reduce the risk of introducing errors into the signal. Extender cables are Fig.2: these are the connectors on a typical parallel printer cable. The connector on the left is a 36-way Centronics type while that on the right, which plugs into the computer, is a 25-pin D type. used to extend the length of an existing cable (funny about that!) and usually have a female connector at one end and a mating male connector at the other end. With this type of cable, all wires are straight through as appropriate. Generally extender cables are not recommended for the parallel printer. However, I have used a 6-metre cable with extender without any problems. You can try a longer cable on your computer but remember - you have been warned! The parallel port of most PC, XT and AT computers is unidirectional; that is, data will travel only one way, from the computer to the port, to the parallel device. The newer PS/2 computers have a bidirectional parallel port which enables data to travel to or from the port. This bidirectional capability is not currently used in most applications. Future uses for the parallel port may include taking input from high speed data transfer devices like scanners, bar code readers and video cameras. Pin arrangements Fig.3 shows the pin arrangement of the typical 36-pin parallel connector socket. Note that the pins are numbered from right to left. Some early model computers may have Centronics connections at both the system unit and printer ends. When this is the case, the cable often has the same male connectors at each end since printers always have female connectors. In the case of the Centronics connector, the female connector is a 18 1 (0----------0) "o---------0) 38 19 Supported & Serviced Australia-wide by Elmeasco Instruments Pty Ltd N S W (02) 736 2888 Vic (03) 879 2322 Old (07) 875 1444 S.A. (08) 344 9000 W.A. (09) 470 1855 You've Gotta Getta Goldstar from : A.C.T. John Pope Electrical806576N.S.W. D.G.E. Systems (049) 69 1625e W.FDixon (049) Fig.3: this diagram shows how the pins are numbered on a typical 36-pin Centronics socket. The pins on the mating 36-pin plug are numbered from left to right. You can see the numbers moulded in to the plastic, right next to the four outside pins. 69 5t 77 • Macelec (042) 29 1455 • Newtek (042) 27 1620 • Novacastrian Electronic Supply {049) 62 1358 • . Obiat PJy L)d 6~8 477f; 9 _Digitel 709 §511 e Geott Wpod 427 1676 QUEENSLAND St Lucia Electronics 252 7466 • BAS. Audiotronics 844 7566 • The Electronics Shop (075) 32 3632 • Solex (Townsville)(077) 72 4466 • Xanthos Electrical (079) 27 8952 S.AUSTRALIA Force Electronics (08) 212 5505 VICTORIA G.B. Telespares 328 4301 e The Electronic Components Shop 670 6474 • Factory Controls (052) 78 8222 • Mektronics 587 3888 • Truscott Electronics 723 3094 W AUSTRALIA Atkins Ca rlyle 48 1 1233 • Leda Electronics 361 7821 PAPUA NEW GUINEA TE (P.N.G.) Pt Moresby 25 6322 Lae 42 6246 ·Recommended prices only NOVEMBER 1990 81 36-PIN CONNECTOR - PRllfTER 25-PIN CONNECTOR 1 STROBE 1 2 DATA 1 2 3 DATA2 3 4 DATA3 4 5 DATA4 5 6 DATAS 6 7 DATA6 7 8 DATA 7 8 9 DATA8 9 10 ACKNLG 10 11 BUSY 11 12 PE 12 13 SLCT 13 14 AUTO FEEDXT 14 COMPUTEB GROUNDS18THROUGH25 31 - INPUT PRIME 16 32 FAULT 15 36 SLCTIN 17 Fig.4: if you are not sure about the details of your printer's parallel interface you should look at the instruction manual. Included will be a diagram like this which shows the exact pin assignments. socket-type - examine your parallel printer if you are still unsure. The standard parallel connector from your computer is more likely to be the 25-pin D-shell socket-type (DB25S). This connector conserves space since it is smaller than the Centronics connector (see Fig.2). Thus the "standard" parallel printer cable commonly has a male 36-pin Centronics connector at one end to mate with the SECONDARY RECV'. DATA DCE TRANB:r.nTTER SECONDARD RCV'. DATA RECEIVER SIGNAL SECONDARY R.T .S. DATA TERMINAL READY SIGNAL QUALITY DETECT RING INDICATE DATA BIG. RATE SELECT. DTE TRANS. BIG. ELE. TIM. 14 15 16 17 18 19 20 21 22 23 24 25 printer's socket-type printer connector, and a 25- pin D-shell (DB-25P) connector at the other to mate with the computer's parallel port. The D-shell connector is so named because it is "D" shaped - although one could argue that the Centronics connector is shaped likewise. Common D-shell connectors are the 25pin variety which have the DB-25 prefix and the DB-9 connectors corn1 2 3 4 5 6 7 8 9 10 11 12 13 PROTECTIVE GROUND TRANB:r.nTTED DATA RECEIVE DATA REQUEST TO SEND CLEAR TO SEND DATA SET READY SIGNAL GROUND RECEIVED LINE SIGNAL DETECT + VOLTAGE -VOLTAGE SECONDARY RECV. LINE DETECT SECONDARY CLEAR TO SEND Fig.5: this is the pin assignment for the RS-232 interface on a modem. Be aware though that they are by no means standard. If you are connecting a modem or other serial device, make sure you find out the exact pin assignment otherwise you could have lots of hassles with plugs and cables. 82 SILICON CHIP manly used for serial connections (eg, modems.) Note that in all cases, the "P" suffix stands for pin (male) while the "S" suffix stands for socket (female). Problem - if one end of the cable has a connector with 25 pins and the other end a connector with 36 pins, how does the message get through? Answer - not all wires are connected. Fig.4 shows a typical parallel interface line diagram. Although there is considerable standardisation, you must check your computer and printer manuals for specific information. As mentioned earlier, while the parallel connection is by far the most popular for printers, the main problem is the limit on the length of the cable. Solution - use a serial print connection instead. Serial cables The Reference Standard number 232 version C or RS-232C serial port (also called RS-232) on your computer is primarily used for devices that must communicate bidirectionally with the system, such as modems, mice, scanners, digitisers - or any device that "talks" as well as receives information from the PC. The asynchronous serial interface is, in fact , the primary system-to-system communication device. It is character oriented and thus has about 20% overhead for the extra information needed to identify each character. Data is transmitted through the cable one bit at a time, with each individual character framed by a standard start and stop signal as opposed to a parallel transmission of eight or more bits at a time. The recommended cable length is 15 metres, however much longer cables generally work without any problems. Your computer may have no serial ports or several. DOS numbers them consecutively as COM1, COM2, COM3 and so on. The original PC and XT computers and most clones of that era used the DB-25P connector and so required a serial cable like the one shown in Fig.1. The introduction of the AT computer saw a change to the computer connector with the advent of the DB-9P connector which has 9 rather than 25 pins for the serial port. Fun with RS-232 The serial port requires a communications or RS-232 cable to connect Table 1: Modem Pin Assignments and Description PIN NAME DIRECTION DESCRIPTION 2 T.D. To DCE Transmit Data: data is transmitted out from the terminal to the modem etc 3 R.D. From DCE Receive Data: data coming into the terminal from the device 4 R.T.S. To DCE Request To Send: the signal form the DTE informs the device that the terminal has data to send 5 C.T.S From DCE Clear To Send: indicates to the computer or terminal that data may be sent to pin 2 for transmission. Signal appears in response to asserting RTS 6 D.S.R. From DCE Data Set Ready: indicates to the terminal that the device is connected 7 Gnd Both Signal Ground: all signals on other pins are referenced to this level 8 C.D. From DCE Carrier Detect: indicates that a valid carrier has been received 12 H.S. From DCE High Speed: signal is low when the device has established communications 15 TCLK From DCE Transmit Clock 17 RCLK From DCE Receive Clock 20 D.T.R. To DCE Data Terminal Ready: signal from the computer or terminal which enables/disables operation of all inputs/outputs of the RS-232 22 R.I. From DCE Ring Indicate: changes level when a ring signal is detected 24 XTCLK To DCE External Clock input to device a serial port to another serial device and here is where the fun really begins! The "standard" serial cable can be either straight-through or a nullmodem cable. Most IBM/clone systerns use pin-type connectors for the serial port so you must use a serial cable with a mating socket-type connectar (DB-9S or DB-25S) at one end and at the other a connector suitable for connecting to the other device. Most devices have the socket-type connector (DB-25S). so the cable usually has a mating DB-25P connector. When you connect one PC to another, the cable needs to have a sockettype connector at both ends. What happens when you want to connect serial devices with differing connectars? You use either an adapter cable or one of the many "gender benders " supplied by most electronics stores. Fig.6: this is a typical RS-232 "gender bender", suitable for adapting male to female connectors or two cables with the same male or female plugs. They can save you the trouble of having to change a plug or the expense of buying a new cable. Fig.6 shows a typical example of the gender changing devices available. Armed with your serial cable and assorted adaptor devices, the other important information is the serial interface itself. Data terminal equipment The source or destination of signals on a communications network is known as Data Terminal Equipment (DTE). A device such as a modem which establishes, maintains and terminates transmission while possibly converting signals - is a Data Communications or Data Circuit-terminating Equipment (DCE). The main difference between the two is how the signals at the DB-25 connector are arranged. When you connect two DTE devices (ie, computer-to-computer), you need a null-modem cable, while DTE to DCE (ie computer-to-modem) requires a "normal" straight through modem cable. Fig.5 shows the standard computer RS-232 pin assignments while Table 1 provides a brief description of a typical serial connection - a modem. What it all boils down to is that cables are part and parcel of cornputer life. Confusion is overcome by asking questions, experimenting and labelling your cables as required. Acknowlegement Our thanks to Phil Gleeson of PC Marketplace for the illustrations used in this article. ~ NOVEMBER 1990 83