DATA TRANSMISSION CODES We have mentioned teletype machines, these were the backbone of operator communication with the computer, but were also used for remote input of data. A phone line could be used to link a storeman in the warehouse with the main computer using a modem (Modulator-Demodulator) in much the same way you dial-up to your ISP (Internet Service Provider). These modems typically worked at 110 Baud, the standard speed of the teletype, and sent or received one character at a time. They modulated the carrier on the telephone line in a string of bits in serial pattern (one after the other) for the code, usually ASCII, complete with start bit, stop bits (1 or 2) and negative or positive parity bit. Other codes were, for example, SOH (Start of Header), STX (Start of Text), ETX (End of Text), used to identify the following or previous characters. Of course these 'control' characters had to be correctly decoded and used to control the data flow. At the computer data terminal this serial data was read into a shift register, gated by the control characters, until all bits were received, then read out from the register in parallel (all at once) to be stored in memory. At the warehouse the data was sent from the modem to the teletypewriter for printing. Little has changed here, except the speed of operation. The codes are much the same, only upgraded for new types of equipment. As new technology is introduced, new access and control codes will be required, but the basic data remains the same. DISK CODES Magnetic disks and, later optical disks use similar recording techniques to magnetic tape, with one major difference - they record in serial format. The blocks on the disk incorporate an address code as well as the data and other codes we have mentioned, as the data on a disk is randomly accessible. We can specify an address and read the data from it. The same applies to an optical disk, which is used mainly for its capacity and portability. The data codes are of standard format. PRINTER CODES Printer codes used were a standard set of ASCII characters, and were restricted to the character set etched on the print drum, or band in use. A line printer was used, and one line of data was sent at a time, to be held in the print buffer and read out as required when the appropriate character was under the hammer. Once again control characters such as line feed and form feed were used. Matrix printers were used for small scale print jobs usually accompanying data terminals away from the central computer. Once again the character set was restricted to the basic ASCII codes, and Control codes with printers from different manufacturers able to be used due to the standard codes (Interfaces were a different matter!) Contrast this with the laser or inkjet printer today, Virtually any font and character set, color and pictures too. Transmission by Network, USB, Parallel or Serial port, or Wireless. Once again mainly control code differences to suit the medium and different handling for serial or parallel. |
