Scor­pio News

1. Insert the 74LS00, see figure 2, notch or dot towards you. Turn the board over and solder pin 7 to 0V and pin 14 to the solder pad of R21 with some thickish tinned copper wire, The two wires have now mechanically secured the chip to the board.
2. Interconnect the pine of the 74LS00 as shown in figure 3, using some thin connecting wire, single core preferably.
   
   

   figure 3:Wiring diagram, viewed from component side.

3. Now connect the four additional links as follows ..

   LK1/b	to	74LS00	pin	5	(your original 1MHz clock)
   LK1/c	"	"	"	12	(your 2MHz clock)
   LK1/a	"	"	"	8	(output clock)
   IC5/15	"	"	"	1 & 2	(clock select line)
   (i.e. IC5 pin 15)

4. finally double check all the wiring for any mistakes or shorts.

TESTING

Re-install the FDC card into your system and reconnect the drive cabling. Power up and boot up in your normal disk operating system. Everything should work as before, if not check the voltage level on IC5 pin 15 which should be low. If this is ok then check that the two clock lines are not swapped. I had no problems, everything worked first time when I booted up the disk system.

THE SOFT­WARE

Modifying existing software should be quite straight forward. Changes only need to be made whenever commands that (potentially) move the disk drive’s head are issued to the WD1797 disk controller chip. I include details of 2 versions of software, one where primitive disk routines communicate directly with the WD1797 FDC chip and secondly where the routines communicate indirectly via a single call (i.e. SYS users).

Firstly, for routines that send commands to FDC directly, listing 1 shows the suggested alterations (in lower case) to the existing code. The existing code is presented in a hypothetical form, and you may have to alter it slightly in light of your own circumstances. The ‘FDCBUSY’ subroutine call may in fact be a jump to some common code shared by ‘HOME:’, ‘SEEK:’ and maybe other routines as well. In this case just append the source of the ‘stepslow:’ at the end for your ‘FDCBUSY’ routine.