The Henelec system was designed by three NASCOM enthusiasts who decided to offer it to Henry’s Radio of London, UK. They sold it in two different forms. The first treated the disk as a tape – the user had to keep track of what was where on the disk. The second was a CP/M system.
If you want a lot more information I have scanned the Henry’s Radio leaflet. There are three pages, shown below.
Both the Henelec systems could handle up to three double-sided, single-density 35 track Pertek FD250 series drives (minor alterations to the parameter table at the beginning of the FDC EPROM allowed almost any Shugart compatible 5.25″ single or double sided drive to be supported). These were connected to the computer’s PIO via a drive control pcb containing a WD1771 drive control IC with support logic, bringing the total package count to eight. Drive control was “dumb”: the main processor had to do all the work. 
This kept the cost down compared to imported rival systems which used a separate processor for the drive system so that the main processor would be free to respond to interrupts. A limitation of this approach was that only single-density operation was possible. Formatted disk capacity was about 160kB per drive. 
A separate case was provided which held a single drive (vertically) and power supply, with space for a second drive to be added beside it later. If you have ever seen the old Computhink drive system for the Commodore PET this case and drive arrangement look strangely familiar… The Henelec kits offered a choice of single or double-sided drives. They were not available ready-built. Later the Henelec control card was sold ready-built with two double-sided drives as the Gemini G805 package.
D-DOS
The “Floppy Tape Recorder” system ran on a normal, unmodified NASCOM computer. The drive control software was supplied on a 1k EPROM and D-DOS, the operating system, on a second 1k EPROM. (They were also available combined on a single 2k EPROM if required). D-DOS only had four commands: (R)ead, (W)rite, (F)ormat and retur(N) to NAS-SYS. As mentioned above, D-DOS didn’t support any type of directory handling. It was completely up to the user to keep a record of how (s)he stored the data on the disk. The commands were made available as system calls so that the users could write their own file handling routines. D-DOS was written by Dave Hunt – the D stands for Dave of course! It was offered to Henry’s as a part of the disk package. Another of the contributors, taking an understandable dislike to the ED.COM text editor, modified NAS-PEN to run under CP/M as DISKPEN.
DCS-DOS
This was originally developed by Steve Parrish of Dove Computer Services (hence the name). It was designed for use with the early Gemini disk drives when connected to a Nascom via the Henelec controller (plugged into the PIO). A later version (DCS-DOS 2) was used with the same hardware but was more like NAS-DOS.
CP/M 1.4
CP/M was one of the operating systems available for the Nascom computers. Implementing it was much more involved. The hardware was identical except for a replacement PROM for the NASCOM-2 which changed the memory mapping of the computer to suit the CP/M operating system. The CP/M system used was chosen to be compatible with the SD Systems CP/M format as, at the time, this appeared to have the most commercial software available.
The BIOS program, the machine-specific part of a CP/M system as it handles all I/O processes, was written by the same person who wrote the monitors. He also produced an additional program which he called SYS.com – because he lost his printer driver software whenever he ran CP/M! On a basic system a simple BIOS, CBIOS-S, was used to give basic screen and keyboard control. What SYS.COM did was to replace this by using an intelligent relocating routine to copy a new, custom BIOS over it. This allowed the BIOS to include many extra features which just weren’t available on CP/M systems, e.g. full screen editing. It also included printer driving of course! Of course, SYS development was not frozen at this point. Several enhanced versions appeared later for both the NASCOM and Gemini machines.
POLYDOS (version 1)
D-DOS and CP/M weren’t the only DOS choices around. PolyDos, from the Danish PolyData company, allowed Nascom users to have disk systems without having to change the existing memory map. D-DOS was too simplistic for most users and using CP/M meant moving the memory-mapped screen to a different location in memory – not too difficult on a NASCOM-2, but rather more complicated on the NASCOM-1. PolyDos used the existing screen, which was very useful as it allowed almost all existing software to run as before. It was a very polished product with good documentation. There were many similarities with DCS-DOS but it had a lot of enhancements over that system and cost a lot more (about the same as CP/M 1.4)!
There were 4 versions of Polydos, for different hardware arrangements:
- Polydos 1 – Nascom with GM805 controller
- Polydos 2 – Nascom with GM815 controller
- Polydos 3 – Nascom with Lucas controller
- Polydos 4 – Nascom with GM825 controller
NAS-DOS
Lucas eventually entered the disk game rather late in the day. They produced a disk enclosure with the drives side by side, accompanied by (the rather predictably named) NAS-DOS. NAS-DOS (as mentioned above) extended DCS-DOS 2 to become the official Lucas-Nascom disk operating system. It used the Lucas controller, not the PIO interface that DCS-DOS2 had used. It could handle quad-density double sided drives.
nascom.info 