INMC 80 News


February–April 1981, Issue 3

Page 25 of 55

and a 26 way ribbon cable terminated in a plug to fit a Nascom 2 PIO socket. The case and finish are of a very high standard. Along with the disk drive is a power supply which gives +5V at about 1 amp, and +12V at about 2 amps. These supplies are adequate for two drives and the controller card. We noted that the controller card also requires a –5V supply, but this is derived from a voltage doubler on the controller card fed from the clock. Again the power supply and controller card are constructed to a very high standard. The controller board uses the Western Digital 1771 Floppy Disk Controller (FDC) and is configured to give double sided single density operation on up to three drives if required. It is possible to get double density, however, a number of problems exist which make this unfeasible. The only reservation we have about the controller board is that it uses the internal data/​sync separator of the 1771. Western Digital themselves recommend an external chip for this function. However, we have had no problems in this area, and the board has been made so that one could be fitted at a later date if necessary. We also received the complete circuit diagrams of the controller card (because we asked for it). We understand the circuits are not normally supplied. The Pertec 250 drive, as we have already said, is used in double sided, single density mode in this system, and is configured as follows:

35 tracks per side
18 sectors per track
128 bytes per sector
all to IBM standrds
161.28K maximum formatted capacity

Cable is supplied to connect two drives. We don’t know what you are supposed to do if you wish to run three drives. Worth clarifying with the dealers if you have that number in mind.

The Software

D-DOS is normally supplied in two EPROMs assembled to run at B000H, but we understand is also available, to special order, in either 2716, or on tape assembled to run at 8800H if you already have a Bits & PCs Toolkit sitting at B000H. The first 1K (B0000-B3FFH) consists of control software for the FDC. This includes a number of low level routines to control the PIO. It also initializes the FDC controller, drives, and positions the heads on track 0. Re-try software is also included which in the event of a read error makes five attempts at reading then moves the head out, repositions it, and has five more tries. If this fails, then it returns to the command level with an error message. No listing is supplied of this 1K but it is well worth disassembling because a number of routines exist which are not otherwise mentioned; for example, a power on boot which will load track 0 sector 1 into RAM at 1000H and then execute it. Assuming you have the ability to program EPROMs, alter the jump at B000H to B385H to make use of this feature. We hope our mentioning this doesn’t upset the supplier too much. With this in mind, you will have to find the other routines yourselves.

The second EPROM (B400-B7FFH) consists of the main DOS and gives you four simple commands to control the disk, as follows:

Return to NAS-SYS
Read from disk
Write to disk
Format disk

The middle two need five parameters to be specified as follows:

R aaaa nn tt ss dd

Which translates thus

Initial address to read data to
Number of sectors to be read (128 bytes per sector)
First track to read from
First sector to read from
Drive to read from

Page 25 of 55