80-Bus News

from that direction). Antenna sizes are typically those of domestic TV aerials, and consequently most inconspicuous (compare that with your “Silver Rod” lightning conductor!).

Another development on the Amateur Radio scene is the data-handling repeater. This is a land based device which receives data signals on one channel and re-transmits them on another, with a considerable improvement in range (up to 20 miles). One such repeater is GB3MT (Bolton) which will handle ASCII & CCITT No.2 codes and even permit interconversion, i.e. an incoming ASCII (CUTS) transmission being recoded and transmitted in CCITT code and vice-versa. Another facility offered is that of “electronic letterbox”. This allows the reception, storage and later retransmission of messages and data. More of these devices are being planned!

Amateur TV

Television transmissions are permitted by the Amateur licence on a number of bands from 432 MHz upwards, and I have made use of the video output from the Nascom 2 to transmit messages and graphics displays on this band over distances of up to 6 miles with 4 watts PEP. A light-pen project is planned shortly to allow onscreen message writing!

Digital Tape Storage

With so much information on floppy disk systems having appeared in the first edition of 80-BUS News (and jolly interesting it was too!), I feel I must take up the challenge thrown down in the editorial, and write something about my (non-disk) system which uses digital tape storage. I decided to go for this form of mass storage about a year ago, when disk storage seemed too expensive (it still does) and under-developed for the Nascom, and a device called CFS appeared on the market, produced by Grange Electronics. This unit is based upon the Philips DCR which takes mini-cassettes of the kind used in pocket dictaphones.

I don’t intend to inflate this into a full review of the CFS as I am not even sure that it is still being sold, so, briefly, it provides 96k storage (48k per side) in 24 x 2k blocks + directory block (1 each side) on each mini-cassette tape. The unit plugs into the PIO port on the Nascom 2, and is driven by a 2k operating system called CASSOP (the source listing of which is readily available). This O.S. allows tape formatting, file writing, reading, renaming and deletion, and contains subroutines which are accessible from external software. I have so far successfully interfaced several system programs with it, including NAS-PEN, Xtal Basic, Nascom Basic, ZEAP2 and a data-file handling program, via Nas-Sys.

All my software now resides either on mini-cassette, or in EPROM held on the Gemini EPROM card and “paged” in and out of the memory map by a boot-loader based upon David Parkinson’s excellent scheme outlined in INMC80-4. This loader copies Nas-Sys into RAM and then optionally overlays the R,​W and V routine addresses in the jump table with addresses of routines in the loader which communicate with the tape operating system. If the overlay is not invoked, normal audio tape I/O (or data transmissions) may take place.

One shortcoming of the CASSOP system is that no “interlock” is provided to query a “Prepare Tape” command, which is accidentally invoked. Thus the directory can be scrubbed in no time flat (certainly before you realize what is going on and hit the reset button!). Since this has happened to me (twice), I have written a 2k tape salvage program which runs under Nas-Sys 3 (or 1 with simple changes) and allows access to the individual blocks on tape, with screen editing of directory, data blocks and read/​write/​verify etc. I wonder how many other CFS users have had similar problems and could use this program.

All in all, I think the tape system works very well, giving named file facilities and operation under software control, even though it can’t match the random access speed of disks; AND it cost about £170 – a lot cheaper than any disk systems around at the moment (although I wonder if the Sinclair Microdrives could be “bent” to work on a Nascom ??).