80-Bus News


November–December 1983 · Volume 2 · Issue 6

Page 21 of 67

Accessing Prestel with a Nascom 2

By Robin Luxford

Although Prestel was originally designed to reach the home via the television receiver and an adaptor with a numeric keypad, this method seems to have severe limitations; unless you buy a special printer you cannot have hard copy and any form of magnetic storage comes even more expensive.

However, with the availability of surplus Post Office approved modem cards, and the 300 baud service now being run by Prestel, it becomes a fairly straight forward matter to access Prestel with your home computer.

The three programs PREST, SAVEPRES and PRINPRES were written for a Nascom 2 running CP/M 2.2 with a Gemini IVC card, using the N2 keyboard, although the principles are applicable to any system.

PREST makes your computer run as a dumb terminal storing incoming data until all the RAM up to the start of the BDOS is full. When you are up to your last page of RAM the screen inverts as a warning but in practice you have to be connected to Prestel for a very long time to get that far. When you hit CTL/C to exit, the program sends the sign-off string to Prestel then waits a few seconds to see if there is a mailbox message for you before warm-booting. Because the mailbox facility is only avaiable at present on the Enterprise computer and the 300 baud service is only available on the Kipling and Dryden computers, I have not yet been able to develop any routines to handle mailbox messages. At the start of each new page Prestel sends a string of 40 DELs; in order to save buffer space and also to simplify subsequent handling of files by GEMPEN, the first DEL is converted to 07H and the rest of the string is suppressed.

After exiting from PREST you can make a disk file fom the buffer by running SAVEPRES or print it on CP/M List device by running PRINPRES.

If you are connecting to a telephone jack via a unit such as the GEC LTU 11 line coupler (also available on the surplus market), you can have the added convenience of being able to dial the Prestel number automatically. The PREST program has routines for doing this, also for sending your Customer Identity number automatically when the Prestel computer replies.

The only hardware mod. to the Nascom required is to connect the uart rx clock to the tx clock so that when the tx clock is switched to 300 baud, the rx clock will also be switched to it. The 300 baud “cassette” position of the rx clock will not work. The simplest way of achieving this is to connect the end of R19 nearest to IC29 to TP5 and switch LSW2/6 to EXT. LSW2/5 must be in th EXT/​TTY position of couse and LSW2/7 must be in the TTY position to receive from the modem.

Acknowledgement is due to tha author of the DUMBTERM program published in this magazine some time ago, who I think was David Parkinson. Some of his program appears in the source code of PREST, mostly the upper case lines.

Page 21 of 67