80-Bus News

Prestel Update

by Robin Luxford

A review of the Henelec Prestel Terminal program and some installation notes on the GEC LTU-II kit.

Although the 300 BAUD Prestel service has been accessible to CP/M 80-BUS computers for some time (see 80-BUS NEWS Vol 2. iss 6.), the appearance of the new Henelec Pretzel 2 software from Henry’s has made the 75/1200 BAUD service readily available. Since the 300 BAUD ports on the Prestel computers strip all the graphics characters and replace them with asterisks, the graphics are pretty dull. The full set of characters and mosaics loaded into the Gemini SVC/IVC video boards by Pretzel 2 makes the display of pages far more interesting and represent a fair compromise as the SVC/​IVC can not reproduce colour. Pretzel 2 also reprograms the SVC/​IVC to the Prestel screen format of 40 columns by 24 lines, making text and graphics appear in the correct aspect ratio, a considerable improvement over the normally cramped output of the 300 BAUD service displayed using a normal terminal program.

Pretzel 2 has facilities for saving and reloading pages to and from disk (either on or off line), and also for dumping the current screen image to printer. The program contains a screen editor allowing the user to alter and experiment with pages without being connected to the Prestel computer. On line the system works in the approved Prestel full duplex mode where all keyboard input except the program’s command codes is echoed to the screen via the Prestel computer. Off line all keyboard entry is direct.

My only criticisms of the software are the lack of a directory facility, coupled with the fact that the program stores the screen images individually rather than in the form of a random access file. The latter is wasteful of disk space as each image is only 1K long but with a Gemini CP/M block size of 4K, three quarters of the space allocated is wasted. The former is simply inconvenient as the number of saved pages soon mounts up, and it is difficult to remember what names have been used or may be deleted.

The program is written entirely in Z80 machine code and is supplied on disk as a fully commented Z80 source file suitable for the Microsoft M80 assembler. All documentation is supplied on the disk along with some twenty demonstration pages. The user has to patch the I/O section to accommodate the modem used and select the various options for Nascom/​Gemini processor cards and either SVC or IVC and then assemble the program for use. One assembly option is to include the users’ identity number and password which are then sent automatically on log-on. Minor text changes would make the source file suitable for most other Z80 assemblers. Another advantage of having the source file is that it is possible to patch in any auto-dial facilities which the user may require such as those described on page 22 of 80-BUS NEWS vol 2. iss 6.

Overall I have found Pretzel 2 an excellent piece of software particularly at the £30.00 charged for it. The well chosen graphics more than make up for lack of colour on the screen and its storage and printer facilities offer much more comprehensive use of Prestel than an adaptor connected to a TV set.

My modem is the 75/1200 BAUD modem available as a surplus unit from the same source as the program. The GEC LTU-11 modem was designed for a GEC Prestel terminal, but slight hardware modifications make it entirely suitable for use with either Nascom or Gemini processors. The unit is supplied in two parts, as a printed card about 6 x 4 inches, without circuit diagrams but with full descriptive manual, and a sealed box containing the auto-dial relays and isolation transformer, measuring some 3 x 5 x 2 inches (oddly enough the isolation box is supplied with circuit diagrams but no description). The modest power supplies are derived from the computer. The modem card expects TTL level inputs and outputs so a simple interface is required to make it suitable for the RS232 from either a Gemini or Nascom. Additionally the data to and from the card is inverted so use is made of a simple 74LS04 to buffer the data, and provide the inversions.

Data out from the RS232 is clipped using two resistors and zener diode to convert the + and – 12V RS232 swing to a swing between 0 and 5 volts, this is then inverted by one of the inverters in the 74LS04. Data output from the modem is inverted by another inverter in the 74LS04 and fed directly to the RS232 input. Although this input is not at RS232 levels, both the Gemini and Nascom input circuitry cope quite adequately. Dialing information is taken from a port and fed directly to the dialing/​isolation box which requires TTL level inputs. A word of warning about the dialing​isolation box. All inputs including the power supply are protected by what appear to be fusible zener diodes, which go short circuit if a voltage across any line exceeds 6V. These devices, which are probably included to protect the computer from lightning strikes on the telephone line are, i) very fast, and, ii) very unforgiving. So care is required to ensure correct working voltages on the dialing​isolation box.

The whole system works extremely well and complements the Pretzel 2 software nicely. As the whole lot, modem and software cost less than £60.00 and provided a couple of evenings entertainment sorting it all out, I consider it money well spent.