David Jones
						   12 Copeland Road
						   Lethbridge Park NSW 2770

	
                 The TANDY 1000 was a very popular PC Compatible,
	with alot of software still being written to utilise it's 16 
	color mode and enhanced sound capabilities, but it's speed 
	of 4.77 MHz is just to slow for many big applications. This 
	TURBO board can give it a new lease of life by increasing 
	the system speed to over 7MHz and with the inclusion of a 
	V20 processor, gives it a Norton SI of about 2.7

                In the Tandy, the main oscillator of 28.64 MHz is
        divided by 2 and fed to pin 14 of the 8284 clock generator
        IC (U45) at 14.32 MHz. This chip also has an internal
        oscillator whose frequency is determined by an external
        crystal (In this case 22MHz or less), which is connected 
	to pins 16 and 17. R1 and R2 are used for stabilisation.

                The turbo mode is selected by shorting pin 13 of U45
        to ground (which is already tied high via a 10K resistor on
        the motherboard). This pin selects which clock (14.32 MHz or
        22 MHz) is fed through to the internal divide by 3 counter
        that produces the CPU clock signal with a duty cycle of 33%.

                The CPU clock is internally divided by 2 to produce
        a 2.38 MHz clock for the 8253 timer IC (U114). But if the
        turbo mode is selected then the clock will run fast. This
        will cause all sorts of problems and the computer will fail 
	to work reliably. The solution is to add a permanent divide 
	by 6 counter driven by the 14.32 MHz clock. This will replace 
	the internal clock from pin 2 of the 8284, and will produce 
	the correct 2.38 MHz signal regardless of the mode selected.

		The divide by 6 counter (IC1 74LS175) is simply a 3 
	stage shift register with feedback, where the Q output of IC1c
        changes state every 3rd clock cycle. Thus dividing the incomming 
	frequency on the clock pin by 6.

                Another problem that must be overcome is that the
        clock for the DMA chip (8237) located on the RAM expansion
        card, must be kept under 5 MHz, or disk drive errors will
        result. The solution is to keep the BUS clock running at
        4.77 MHz, by dividing the 14.32 MHz clock by 3.

                This is done with the Exclusive OR gate (IC2). This 
	decodes the 2.38 MHz shifting signal on the Q outputs of IC1a 
	and IC1b, and produces the original 4.77 MHz clock with a 33% 
	duty cycle. This clock is fed to the BUS via R47a on the mother 
	board (located at the lower right of the 3rd expansion socket). 
	The top lead of R47a (connected to pin 16 of U82 a 74LS244) 
	must be lifted or cut from the board, and pin 3 of IC2a 
	connected to this end of the resistor.

                The circuit can be built on a small piece of Vero
        Board with the 8284 being removed from it's socket (lower right
        of the processor) and mounted in an 18 pin wire wrap socket 
   	soldered to the Turbo board. The IC's should be mounted end to 
	end so the board is as narrow as possible to avoid obstuction
	any expansion cards. Pin 2 of the 8284 must be bent outwards so 
	that it isn't connected to anything, and Pin 10 of IC1 is 
	connected to pin 2 of the wire wrap socket instead. 

		Only a 22MHz or less crystal should be fitted, and this 
	gives a speed of 7.33MHz. If this crystal is a fraction to fast
	(You will get a memory error message at bootup) or you can't get
	a frequency this high, the crystal can be replaced with a capacitor
	of about 28pF or higher. This will result in a remarkably stable
	clock frequency of about 7.2MHz wich can be easily adjusted.

		In Turbo mode not only will the processor and memory run
	faster, but the disk drive will show a significant increase in 
	transfer and access speed. Also the processor must be changed to 
	a V20 to be able to handle the extra clock speed, and as a bonus 
	the V20 will help speed things up even more.