80-Bus News

transmission over short distances is 19200 BAUD, for maximum distance, speeds as low as 110 and 300 BAUD are typical. Variations on the RS232 allow for higher data rates but at greater complexity and usually by what is known as synchronous transmission where the system clock frequency is transmitted either as a component part of the data or sent separately along with the data. Synchronous transmission needs the clock to be sent over the same path as the data so that the clock suffers similar phase shifts to phase shifts which occur in the signals when serial data is transmitted over long distances.

The Centronics protocol is almost the reverse of the RS232. When compared with RS232 its noise immunity is poor, and the distances for data transmission should be measured in metres rather than kilometres. It is a multiwire system, typically eleven wires are needed, and if noise is to be kept at bay, these should be ‘twisted pair’ cables needing twenty or more wires in all. However, it is very fast and transfer speeds of 250000 BAUD are typical. The protocol also lends itself to direct interfacing with microprocessor equipment. In home and small business installations, the advantages offered by RS5232 in terms of noise immunity are minimal as the distance between the host device and the printer is usually not more than a metre or so, thus the Centronics protocol offers significant advantages, because of its simpler interface requirements and lower cost.

This leads us to an interesting point concerning both Nascom and Gemini equipment. Both processor cards are equipped with the necessary hardware to cater for either serial R5232 or Centronics parallel, but both operating systems, NAS-SYS and RP/M, are only supplied with the necessary software to drive serial printers, and not the Centronics parallel interface. To be fair to Gemini, RP/M 2 now incorporates Centronics parallel driving routines, but as far as I know RP/M 2 is not yet available as a replacement for RP/M. This anomaly did not come about be accident, but shows the speed of change within the world of microprocessing. When NAS-SYS 3 was designed, a bit over two years ago with RP/M following a year later, only the large Centronics printers, type 101 and larger, were available and these cost four figures even second hand. On the other hand second hand serial printers whilst still expensive were at least available and new ones could be purchased for under a thousand pounds. Now, eighteen months later, an entirely adequate printer with a Centronics interface can be purchased new for just under £200 if you shop around.

The fact that both Nascom and Gemini are equipped with entirely adequate RS232 hardware and software means that I need not dwell on the RS232 side of things, except to cover the aspect of interfacing. Driving the printer is a matter of reading and understanding the manuals. It is all there, if you can not understand the words, then in the case of NAS-SYS try reading the software. In the case of RP/M if you can not understand it try reading my commentary. on RP/M to be published shortly, failing that, the Zaks or Osbourne CP/M commentaries (Zaks for fun reading with inaccuracies, the Osbourne for a weightier and in many instances more abstruse treatment).

Handshaking is an important part of any protocol when applied to printers. It allows the printer to control the rate at which the data is sent. Put simplistically, this is the the case where data is being sent to a printer at, say, 1000 characters per second, where the printer is only capable of printing, say, 30 characters a second. The printer is either going to miss most of the incoming data or tie itself in knots trying to accomplish the impossible. All but the most primitive printers employ a handshake signal. This is a signal returned to the host device to indicate that the printer is ready to accept data. In an RS232 implementation this is known as the Data Terminal Ready (DTR) signal, indicating that the printer is ready to accept incoming data, whilst in the Centronics protocol, this is known as the BUSY signal, indicating that the printer is busy, so do not send data. With RS232 there is another protocol known as the XON/​XOFF signal, this is rarely used and does not concern us here.