INMC 80 News


February–April 1981, Issue 3

Page 23 of 55

Disks again


In the last issue we devoted some space to disk systems, in particular the Gemini G805. At the time we said we didn’t intend to review it in that issue, as the people supplying the hints and tips had too much of a vested interest in the system and therefore any review forthcoming from that source could hardly be objective. Having said that, a few days later, a letter arrived from the Merseyside Nascom Users Group, containing a review on the Gemini system using D-DOS (the simple ‘floppy tape recorder’) software. Well although their review arrived before the last issue went out, it was too late to include it, so we have had to hold it over until this issue. Before we start on the review, we’ll give a little of the history of the Gemini system.

Designing a Disk System
or How To Go Loony in Six Easy Lessons by Dave Hunt (a suitable case for treatment)

About March last year three people decided that it was about time that Nascom pulled its finger out and got the long awaited disk system on the road, failing that, they would have to do something themselves. Well it seemed that although the Nascom disk system had been seen in the flesh, it was still no nearer completion, so Peter, Richard C (not Beal) and I started to look around to see what made disks work, and why Nascom were taking so long. Bolstered by a bit of spare loot from Naspen royalties (Oh! What a give away.), and the fortuitous publication of a design discussion about a simple disk drive system in one of the more obscure industrial magazines, Peter and Richard beavered away for about a month, and finally phoned me, saying “Come on over, we’ve got something that works”, and it did!! There it was, a huge 10 amp power supply, running a hand knitted 4″ square vero card, surrounded by a rats nest of wires attached to an N2 at one end, and a Shugart drive at the other. It was a rather different approach than the American designed disk controllers available, in that instead of giving the controller card a list of instructions and then letting the controller card take over the bus and get on with it, it made the Z80 do most of the work. This meant that the Z80 couldn’t do something else whilst disk access was taking place (as the American designs envisaged), nor could system interrupts occur during disk access. But, what the heck, it brought the chip count down to 8, and made it cheap compared along side its imported rivals. 99.99% of applications would not require the Z80 to be doing anything else whilst disk access took place, and anyone giving priority to system interrupts would just have to put up with it {or buy an American one).

One problem caused considerable debate amongst us. We chose the Western Digital 1771 FDC chip to do all the work, and Western Digital recommend the use of an external data/​sync separator. Now the trouble with data/​sync separators is that they usually use monostables and these usually need ‘twiddle pots’. ‘Twiddle pots’ aren’t a clever idea when it comes to kits, because constuctors usually can’t manage to set them correctly without sophisticated test gear. A lot of effort went in to designing a consistent data/​sync separtor which didn’t need adjustment, and after a lot of brain scratching Richard C came up with one. We wrote special test routines which could count (and then recover from) ‘soft errors’ and set it to work. In theory we should get on average about one error an hour, yet after churning back and forth all day, still no errors. Fantastic. Then someone muttered darkly that Tandy use a 1771 and get away without any data/​sync separator at all. So we took it all off and repeated the test. 72 hours later, still no errors (except by picking up the drive and shaking it). Moral: who needs data/​sync separators anyway!!! Just to hedge our bets, we left pads on the pcb to connect a data/​sync separator, just in case.

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