4MHz Z80A, 64K RAM, twin 200K disks for £1299

Here is probably the first advert of Chris Shelton's Sig/Net, launched around July or August this year. 

Shelton, who had once worked for [=pcw_1980-01_045|Nascom], had started Shelton Instruments back in 1970, and became well-known for his work on microprocessors and microcontrollers.

[picture: ChrisShelton_signet_prac_sep81.webp|Chris Shelton at the launch of Sig/Net, as seen in September 1981's PRAC. The module's interconnecting ribbon cable can be seen at the bottom of the display]

The Sig/Net was an unusual concept at the time as it differed from the usual multi-user approach - where dumb terminals accessed a central computer, and where the central computer's resources, including memory and CPU, were shared - by instead offering satellite stations, which were full computers in their own right, but which shared the central hub's expensive resources, like disks and printers, over a ribbon cable.

Shelton said of the concept, as reported in September 1981's PRAC, that:

~"The system is a new hardware design which offers the expansion potential of bus-based systems at a price comparable with single-board computers. Support for virtually-unlimited hard-disk storage peripherals and users is a major feature. A CPU/RAM module is the heart of our system and is design for multi-user applications by simply adding more - as many CPU/RAMs as there are users or maybe more for task management or resource allocation". 

The 26-way flexible port bus, which connected each module, was said to be the system's defining feature as it offered a much cheaper way of connecting up multiple units - Sig/Net could support up to nine modules per "ring", with up to nine rings being able to connect at the second level, giving 81 units in total[source: "Shelton Sig/Net Review", PRAC, September 19812, pp 61-62] - most of which could also support up to four terminals each.

The downside to the system was that these ribbon cables could only be up to a metre long, however it was possible to connect screens and a keyboard to the satellite module using RS-232, and run that over much longer distances[source: https://www.theregister.com/2013/08/21/unsung_heroes_dr_chris_shelton/?page=3].

The original Sig/Net systems used a custom CP/M-like operating system called McNOS, although here the advert is calling it CP/M. 

The modules came with the full 64K RAM that a Z80-based system could support, plus two 200K floppy disk drives. It retailed from £1,299, which is about [[1299|1981]] in [[now]] money.