INMC 80 News


February–April 1981, Issue 3

Page 26 of 55

For example

R 1000 12 210

Read starting at 1000H, the next 12H sectors, starting at track 2H sector 1H from drive 0. The write command is similar:

W aaaa eeee+1 tt ss dd
Initial address to write data from
Last address
First track number
First sector number
Drive to write to

It becomes obvious from the above that a great deal of personal housekeeping is required of the operator, to keep a note of what programs sit at what track/​sector on the disk. It is not by any means intended as a file handling system in its present form and does require a working knowledge of memory storage in general to operate. Thats what CP/M is about anyway.

Does it work?

Obviously this section will interest prospective purchasers of the system, and we have tried to be as honest as possible in our experience. The documentation is easily read and we did as instructed. We can see no problems in that area. The system was connected up and an attempt was made to format a disk. The drive went round but nothing else happened. A quick phone call, and the answer came back that the drive was running fast. Apparently the speed of the drives had not been set correctly at the Pertec factory and unfortunately when the controller tries to format a disk it also reads back the number of bytes on that disk (not quite what it does, but near enough for purposes of argument. Ed.). If the drive is running fast, then obviously the number of bytes written will be less than the required minimum so an error occurs. The solution was to take the metalwork apart, remove the pcb from the base of the drive – taking care not to strain the delicate head leads – and then set the drive to the correct speed using the strobe, provided for the purpose, printed bottom of the drive flywheel. This problem is known to the suppliers, and we are sure that drives will be set correctly in future. (Ed. We are told that they are now, so please do not fiddle !) The drive now worked well, but we were still unable to read the innermost track. It seems that the heads need a bit of bedding in (or may be the disks. Ed.), so we wrote a loop and left it formatting continuously whilst we went off for a few pints. On our return all worked well and no problems have been encountered since.

Overall impressions are that it is an excellent piece of equipment and gives much improvement over using tape as a storage media. For example, it will load 32K in 13 seconds. The only problem is the amount of housekeeping required to operate the system, but we dare say that in time this situation will change. The system is more than adequate for the job in hand and represents a long awaited add-on to implement on the Nascom.

Cost of the system and how does it compare

A single drive with D-DOS will cost you 395.00. A single drive with CP/M is 55.00 dearer. The CP/M with boot EPROMs and new MD PROM costs 95.00. Extra Pertec 250 drives are 205.00.

Doing a comparision with other systems
Two drives, case, power supply and CP/M

Gemini/​Henelec 640.00
Nascom655.00 (pre-receivership price – product still not available)
Apple 650.00 (single sided 80K per drive – not CP/M)
Sharp MZ80675.00 + 99.00 for I/O box + 200.00 for CP/M conversion

It is difficult to draw a comparison on price alone, as these other systems have a lot of in-house software available as well, so we think the only conclusion that can be drawn is that disks for any system are not cheap at the moment.

Page 26 of 55