


		[43m   DISK DRIVE TRACK COUNTER PROJECT  [0m


					     by Leslie Ayling
					     P.O. BOX 334
					     Glebe 2037 AUSTRALIA
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    Ed: Another in Les' Cheap Hack series, which have caused so much
   interest among the hardware-minded, locally and overseas. See the
   diagram "DiskTracer_pic" in this drawer for the schematic.
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    Yet another in the on-going series of low-cost DO-IT-YOURSELF projects
   for the Amiga.


    This project will allow you to see at a glance what track your drives
   are currently reading or writing to.  The idea occured to me when I was
   trying to discover the protection scheme on the new Don Bluth game
   SPACE ACE (How do they get those animations into 512k ?? BRILLIANT!),
   I thought it would be useful if I knew where the drive was looking as
   it zipped back and forth across the disk surface reading in various
   bits of code.

	The result is this small hardware project.
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WHAT IT DOES:
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    The unit is basically a 2-digit up/down counter that uses the STEP*
   pulses from the disk drive connector as its clock input and also the
   DIR* line to steer the direction of the count. It keeps count of the
   pulses and gives an accurate representation of the track currently
   being accessed for the selected drive.
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CONSTRUCTION:
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    I built the prototype on a small piece of vero-board with the
   appropriate wires going to the pass through.  For the pass-through just
   get a 23pin plug and socket and solder all the pins in a back to back
   configuration. You can either power the board from the Amiga's 5v line
   on the passthrough or use a separate 5v source if you are concerned
   about the power drain.  Don't forget to supply +5 and GND to the
   appropriate pins on the chips, for convenience I DON'T show this on the
   schematic diagram in this drawer.  (Don't laugh you advanced types!
   Someone couldn't get the accelerator project on MD15 to work for this
   very reason!)

    If the information I have supplied doesn't seem clear, you may need
   some assistance from a hardware oriented person - most user groups have
   one hiding in a corner, muttering to himself about flip/flops and logic
   gates etc.... [Ed: or we may be able to put you onto someone.] Also
   bd sure to connect the ground of the project to the ground on the drive
   connector.
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HOW IT WORKS:
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    The circuit is based around HCF40110 counter IC. A really nifty
   little device that incorporates a BCD up/down counter, 4 bit latch, and
   BCD to 7-segment decoder driver on the one 16 pin IC (really cuts down
   the parts list.

    Refer to the accompanying circuit diagram. The STEP and DIREC signals
   are gated through the 74LS14 and 74LS11 to give appropriate UP and DOWN
   signals to the inputs of the first digit. The borrow and carry outputs
   of the first digit are connected to the up/down of the tens digit in a
   simple cascade setup.

    The borrow of the tens digit is connected through an inverter and
   applied to the reset inputs. This prevents the counter counting down
   past zero.  The manual reset button is there so that on power-up, when
   the counters state is completely random you can set it to zero. After
   that whenever you insert a disk into the drive you are using the
   counter on, it automatically checks the disks track 0 while validating
   (I think so anyway!) and by doing this the counter is automatically
   calibrated (it won't count back past zero remember!)
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A SMALL ANOMALY:
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    For reasons unknown to me, when the Amiga has finished stepping the
   head INWARDS to the centre of the disk, it leaves the DIR* line high
   and pulses it about every 3/4 of a second. God only knows why.
   However it dosen't do it when it has finished stepping the disk
   OUTWARDS!! Your logic probe will show you this interesting phenomenom.
   It had me going for about an hour and a half because counts in the up
   direction were always 1 less than their target #$$@!@^%&*(!!!!!!

    Running the STEP signal through the series of inverters delays the
   signal so that there is no miscount due to the DIR signal. It could
   possibly be a problem within trackdisk.device but who cares?
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USING THE COUNTER:
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    Once it is connected, power up the machine and select which drive you
   want to follow with the switch. Press the manual reset button on the
   counter then insert your workbench disk or whatever you want to use.
   The drive will automatically seek to the boot sector on track 0. Since
   we have just reset the counter to zero, and since it also won't count
   backwards below zero, as soon as the selected drive hits track zero our
   counter will be synchronised to the drive and will follow it faithfully
   on its merry way across the disk surface!

    I didn't bother with the rotary switch and the separate drive select
   lines with my prototype. I just connected the lead to 5v (in other
   words any step pulses for any drive will step the counter) and
   disconnected the external drives. This way only drive 0 will be getting
   any step pulses' I haven't tried that part of the circuit but
   theoretically it should work, anyone who tries it let me know how
   you go/any problems etc.

    If you have any trouble sourcing the HCF40110 chip try AT/M
   engineering at 30 Hotham Pde, ARTARMON in Sydney. They are only about
   $2.50 each. Also don't forget to ensure that the 7-segment displays you
   get are common CATHODE (FND500 or equiv), NOT common ANODE.

    Hope you enjoy and make good use of this one.......regards Leslie
   Ayling.


    P.S. If you find this article on a BBS system I urge you to send a
   $5.00 donation in the spirit of shareware if you find it valuable.
   [Ed: This is the way to encourage Les and people like him to keep
   coming up with good ideas, so think about sending him something.]

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