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