80-Bus News

  

July-October 1982, Volume 1, Issue 3











Page 32 of 51











32

some control over illicit developments of NAS-SYS away from the original philosophy of the design. So in future perhaps it would be wise of me to retain the copyright of any generally useful, silly little routines I write, and be wary about sending them in the direction of Warwick. If Nascom want to make something of it, I can certainly establish ‘prior art’ in connection with that piece.

So on to printer interfaces. Printers fall into two groups, those known as parallel printers and those known as serial printers. These definitions have nothing to do with mode of printing employed, the defintions refer to the method of feeding the computer with the data it is to print. Now the sort of printer we are concerned with, that is those likely to be attached to a home or small business micro are almost invariably themselves under the control of an internal microprocessor. Micro’s are parallel devices, in that data is transfered about internally in parallel chunks usually eight bits wide. So it follows that a particularly convenient method of transfering data into this sort of printer, is in a parallel fashion via an input port. This explains why with few exceptions parallel printers are anything between £30.00 and £100.00 cheaper than the equivalent serial version. In the serial printer the data is sent one bit at a time at a fixed BAUD rate, in a similar (but by no means identical) fashion to the data being sent to a cassette recorder. Because the incoming serial data has to be converted to parallel data within the printer additional components in the form of a UART and its associated clock generator would be used, this explains the higher price for the serial printer, although not always the disproportionate prices charged in some instances.

The description of the electrical signals and timing used to control the printer is known as a protocol. The term protocol can also imply the type of physical plugs and sockets used to connect the printer, although this is often not the case. There are a number of different protocols in use, but fortunately these are mainly international standards or protocols that have been in use for so long that they have become internationally recognised although not necessarily having the authority of national and international standards committees behind them.

The two protocols most likely to be encountered are the RS232-C serial protocol and the ‘Centronics’ parallel protocol. RS232 has been around for many decades and has its origins in early teletype and signalling equipment. It is an international standard (although sometimes known by different names, V24, for instance), and provides adequate defintions for the electrical signals, the physical connections and the data rates, although the last is somewhat anachronistic and in most instances is replaced by a series of higher data rates more in keeping with modern equipment.

The Centronics protocol is more recent in origin, and gains it’s name from the large printer manufacturer of that name. It seems to have become far more popular than the original designers intended because of its simplicity and ease of application to microprocessor based equipment. As far as I am aware it is not an agreed standard, and minor variations on a theme have been noted, it does however lay down the main connections, the type of connectors and the system timing, and seems to have gained general acceptance by a large number of printer manufacturers. It should be noted that recently Centronics themselves have departed from their own standard by changing the type of connector used on some of their more recent products at the cheaper end of their range although the electrical and timing specifications remain the same.

Each protocol has its own advantages and disadvantages. With RS232 the advantages are considerable immunity from extraneous noise coupled with the ability to transmit data over long distances (up to a kilometre or so) using a two or three wire system. Its disadvantages are that interface circuitry is somewhat more complicated when compared with the Centronics, and the data transmission speed is severely limited when data is to be transmitted over long distances. When used over short distances the data rate can be significantly increased, but the limit is still nowhere near as fast as the Centronics. The usual maximum practical speed for asynchronous R5232 data


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