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COMPUTER BITS By JENNIFER BONNITCHA Now what's gone bung?!! Just when you thought it was safe to go back to your computer, another problem arises! This time the *#&% printer doesn't want to co-operate. But before going overboard about printers, let's take a little time to examine the communication ports and therefore the two major kinds of printer connections - parallel and serial. The basic communications ports in any PC system are the parallel and serial ports. The parallel port is used primarily for printers and operates as a one-way communications port. The serial port allows bidirectional communication with any system using devices such as modems, mice, scanners, digitisers and of course serial printers. Serial devices are capable of sending and receiving information from the PC while the parallel device can only receive information and so usually does not have too much say at all in what goes on. Synchronous & as~nchronous The asynchronous serial interface is the primary means of system-to-system communications. In this type of transmission, cha racters are "marked" with start and stop bits. The PC with its Asyn- chronous Adapter uses a binary O (zero) for the start and stop bits. The word asynchronous means that no clocking signal or synchronisation is present. Characters can be sent with any arbitrary time spacing, so the intervals between the characters may be completely irregular. A standard start and stop signal frames each individual character. The start bit is a single O bit which precedes each character. It's job is to tell the receiving system that the next 8 bits make up a byte of data. One or 2 stop bits follow the character to signal that the character has been sent. At the other end, characters are recognised by the start and stop signals rather than by the timing of their arrival. The asynchronous interface is character oriented and has approximately 20% overhead for the extra information needed to STOP BIT l J START BIT f _____o_A_r_,,~.._s_,r_s_ _ _ ARITj B(IT 0 0 1 0 0 0 0 0 11 0 Fig.1: basic format of an asynchronous data transmission, in this case for the letter "A". A start bit is transmitted first, then the data, parity & stop bits. identify each character. In contrast, synchronous transmission refers to a communication mode in which each character or sequence of bits is sent down the line in a strictly-timed rhythm. Thus, both the sending and receiving devices must be "in tune" for the information to be received in an intelligible manner. The use of a timing marker at each end means the receiving device can "know" where each ASCII character begins and ends and therefore precisely which character has been sent. Though advantageous at high speed, sophisticated timing circuitry is required at both ends of the transmission. High speed synchronous transmissions also require high cost devices to control the data flow. Parity checking During data transmission, an extra bit of information is usually added to each ASCII character for error checking purposes. This parity bit is usually set at the transmitting end by the program handling communications. Parity may be high, low or no parity and is computed by adding up the number of "1" bits in the data field. The result of this addition is the parity checksum. Parity checking is the most basic method or error detection and most data communications systems use even parity for data exchange. Data checking takes place at the receiving device where the communications software does a checksum on each incoming character and compares its calculation of parity with that sent by the transmitting computer. If any of the bits has been reversed (eg, by noise OCT0BER1990 87 COMPUTER BITS - CTD on a telephone line, radio frequency interference etc], then the receiving side of the system should be able to detect and report the error. Parity checking is the easiest method of error detection since it relies on the statistical fact that having two bits in a character change during transmission is a very rare occurrence. If two bits should change then parity checking won't detect the error. Serial vs parallel Serial transmission means that data is sent over one wire with each bit lining up in a series as they are sent in order. Thus, to move one byte you must send eight signals, one after another. A serial transmission is slower than a parallel transmission, but the speed of the printer is so much slower than the transmission speed that the serial interface mode generally won't limit printing speed. This type of communication is used over the telephone system because it provides one wire for data in each direction. The control program for the serial port is in the ROM BIOS on the computer's motherboard. Serial ports may connect a wide variety of devices such as modems, plotters, printers, other computers, barcode readers, fax cards, scales and so on. Anything which requires a 2-way connection to the PC uses the "industry-standard" Reference Standard number 232 revision c (RS-232c] serial port. This device enables data transfer between otherwise incompatible devices. However, the most common connections to the serial port are modems and printers. Generally, the PC BIOS and operating system handles only two serial communications links: COMl and COM2. You need to ensure that two serial ports within one system do not conflict. Every board supplied is automatically set up as COMl so you need to reconfigure any additional ports (usually by setting DIP switches) for COM2 before 88 SILICON CHIP use. But two ports are barely enough nowadays. With the advent of the PS/2, the use of more than two serial ports became more of a reality. Most pieces of hardware in your computer communicate with the Central Processing Unit [CPU] via signals called interrupts. The signal literally interrupts the CPU and tells it which device requires attention. As the following table shows, the "industry standard" 1/0 addresses of the four serial ports are all unique. However, the interrupts overlap. TABLE 1 Port COM1 COM2 COM3 COM4 1/0 Address 3F8 2F8 3E8 2E8 hex hex hex hex Interrupt 4 3 4 3 Using two interrupts for the same port seems like suicide, however you could use one port from each pair (COMl or COM3; COM2 or COM4) while using the other port in the pair without interrupts. The key to success is the use of "interruptdriven" [eg, communications and mouse software] and "noninterrupt-driven" applications. You could use a modem program on COMl and a serial printer assigned to COM3 using the DOS Mode command. Even though each uses the same interrupt, COM3 will never try to use its driver circuit to control the shared interrupt line. The parallel port has eight lines for sending all the bits for one byte of data simultaneously across eight wires to the receiving device. This interface is fast but with eight wires required to send the information, the cost of cables starts to increase and so this method of communication is usually reserved for the short distance to printers. The only real problem with parallel ports is the cable distance limitation due to the lack of signal error checking. One byte of infor- mation is sent in one transmission since one bit can be sent over each of the eight wires. Parallel ports are unidirectional. Data travels in only one direction from the computer to the port and then to the parallel device. The newer PS/2 computers have a bidirectional parallel port however, so data can travel to or from the port. Problems with serial and parallel ports can often be detected using diagnostics software and a wrap plug for each type of port. To test any serial or parallel port cards, run your computer's advanced diagnostics communications adapter tests. Typical error codes displayed are shown in Table 1. For serial ports only, check for a voltage reading of - 10.BV to - 12.9V DC between pins 4 and 8 [ground] and the system board power connector. If the voltage measurement isn't within the range specified, the power supply may be defective. Many problems stem from using the wrong serial port card in a system. The main difference in serial ports is in the Universal Asynchronous Receiver Transmitter [UART] chip which is the primary port circuit. The system ROM BIOS is designed specifically to support certain chips and so some chips can only be used in certain systems. If you use a port with the wrong UART, problems such as port hanging, incompatibilities with software, lost characters or total function failure are not uncommon. Printer problems And so finally to printer problems which can manifest themselves in a myriad of ways. Starting with the most obvious problems, check: • printer power on and power cord securely connected to both the printer and power outlet; • printer is "on-line" or "selected"; • printer is not out of paper; • the printer cable is securely connected at both the computer and printer. Check again for security, even if you think all is OK; • the DOS Mode statement has from tractor feed to cut sheet feed, etc. Paper smudge on a dot matrix printer usually means the paper thickness control is set incorrectly. If the print head is too close to the paper, smudges occur as the printer moves across the paper this typically happens when a new ribbon is installed or a thicker than usual paper is used for printing. Change the thickness control as required. COMMON ERROR CODES 9xx 901 10xx 1001 11 XX 1101 12xx 1201 Parallel printer adapter errors Parallel printer adapter test failure Alternate parallel printer adapter errors Alternate parallel printer adapter test failure Asynchronous communications adapter errors Asynchronous communications adapter test failure Alternate asynchronous communications adapter errors Alternate asynchronous communications adapter test failure been set correctly for a serial printer - much of today's software will set the communications parameters automatically, but check just in case. Next, you could try a little more problem determination: • Perform a Power On Self Test (for most printers, turn the power off and then, while holding down the Line Feed (LF) or similar button, turn the power back on). A sample pattern should now print. If not, check your printer manual for further information. • If the printer test functions correctly, try a "screen dump" of any information [other than graphic images) from your computer. Press Print Screen or Shift PrtSc to send the information to the printer. If you get no response, try something simple [swap printer cables, for example). If the print screen works correctly but you still get "junk" reports, check the software configuration. Is the program set up correctly for your printer type [serial or parallel) and model [not just manufacturer) of printer? For laser printers, is there an operator message to indicate the need for toner or a new OPC drum? For dot matrix printers (especially those using film type ribbons), is the ribbon exhausted? BIT PULSES _ _ _ _ _ _A _ _ _ _ _ __ ( 1 0 0 Q Q Q ' For programmable printers [typically dot matrix and laser), check the system settings. It is possible that someone may have changed the inbuilt settings and thus altered the mode of operation - eg, changed the printer program BIT PULSES _ _ _ _ _ _A...__ _ _ _ __ (1 0 0 0 0 0 ' 1 TO PRINTER Parallel Transmission Fig.2: in parallel transmission, the data bits are all sent to the port simultaneously via separate lines. The data travels in one direction only, from the computer to the device. Missing characters Characters "missing" or not fully printed on dot matrix printers usually indicate that the ribbon is not correctly seated or a pin in the print head may be broken. Check ribbon operation before panicking further. Inkjet and thermal wax printers can suffer from clogged jets and insufficient temperature of the print head, thus causing problems with the dye or wax mixing. The appearance of vertical streaks on laser printouts is a sign that the corona wires need cleaning. Refer to your printer manual for more details. While most printer problems are operator-related [sorry team, but that's the truth, the whole truth and nothing but the almost truth), there are times when a call to a technician is the only answer. Use common sense and you should be able to avoid any unnecessary expense. And always remember to try the simple things first. Go for the least expensive items [like printer cables) and work your way upwards in cost. By their very nature, it is often the simple, low cost items which fail first. Next month, we'll talk about cables and give some tips on avoiding cable mixups. ~ Serial Transmission 1 ----c?:__ _ _-Jl_,..lQ)=====:j) ) TO PHONE '-'--------- LINES Fig.3: in serial transmission, the data is sent one bit at a time over one line (ie, each bit is sent in sequence}; Serial transmission is the method used for sending data over the telephone system. Because there are two lines, data may be sent in either direction. OCT0BER1990 89