III.2 Circuit Description

In the following section I'll describe the functioning of my GAL-programming
device. I'll refer to my circuit diagram, so if you haven't ordered that,
you can skip this section.

The hardware is connected to the Amiga's parallel port. The connected data
signals are D0-D4 and the BUSY-Line.
 
IC1, IC3, IC4 and IC5 are eight way "serial in/parallel out" shift registers.
The outputs of the shift-register from IC3, IC4 and IC5 are connected to the
Textool-Zero insertion force socket for the GAL. Therefore it is possible to
(besides VCC and GND) define each of the GAL's pins with a level (HIGH or
LOW).

The possible outputs of the GAL (pin 14 to 23) can be read through IC7 and
IC6. IC7 is an eight way "parallel in/serial out" shift register.

IC1 is so to speak the switch centre, this IC selects IC3, 4 and 5 (OE).
Furthermore, this IC switches the programming voltages for the GAL (VCC,
Edit-voltage) on or off.

Since the ICs 1, 3, 4, 5, 7 can be individually accessed, a separate clock
line is provided for each IC. These clock lines are selected via IC2, a 1 out
of 4-decoder, by the parallel port's data lines D0 and D1.

D3 determines whether a read (low) or write (high) operation is to occur.
It must be ensured that D3 does not go low until the IC to be accessed is
selected through D0 and D1. Otherwise an IC gets an unwanted clock-pulse
and at the next Strobe-pulse (D2) the wrong (once left shifted) data is
presented at the outputs.

The Strobe-pulse for the shift-register is derived from D2 . When D2 goes
high, the data in the shift registers is transferred to the output registers
of ICs 1, 3, 4, 5.

Through IC7, D2 can be made high (D2 = high), so that the data on Pins P1-P8
are transferred to the internal shift register and may be read through the
BUSY line by clocking the relevant clock-line.

D4 transfers the individual bits from the Amiga to the GAL-Burner.

Since Pins 2 and 4 of the Textool-socket may be supplied with the programming
voltage of up to 16.5 Volt, we have to protect IC4 with the diodes D2 and D3
against over voltage.

The programming voltage is derived from IC9, a switch mode voltage regulator.
This voltage can be precisely adjusted with the trimpots R40-R44.

The relay K1 connects the supply voltage of the GAL to pin 24 of the
Textool-Socket, Relay K2 connects the output Q7 from IC3 or +5V supply voltage
(according to the GAL type) to Pin 22 of the Textool-Socket. Both relays are
driven by IC1.

The LED shows whether voltage is supplied to the Textool-Socket or not
(controlled by software). When the LED is on, a GAL may not be inserted or
removed from the socket.


Parallel-Port:

 D0-D1:  Selection of individual ICs by Clk
 D2:     Strobe-pulse for IC 1, 3, 4, 5
 D3:     write = low, read = high
 D4:     Data line for "Write Bits"
 BUSY:   Data line for "Read Bits"

IC1:
 Q1   make 16.5V (but don't switch it!)
 Q2   switch edit-current on pin 2 for GAL20V8
 Q3   switch edit-current on pin 4 for GAL16V8
 Q5   switch Vcc
 Q6   OE for IC3, 4, 5
 Q7   controlles LED on : high, low = off
 Q8   not used

IC3:
 Q1-Q8  pin 16-23 of the Textool-socket over R3-R8

IC4:
 Q1-Q8  pin 1-8 of the Textool-socket

IC5:
 Q1-Q3  pin 9-11 of the Textool-socket
 Q4     pin 13 of the Textool-socket
 Q5-Q6  pins 14, 15 of the Textool-socket over R9, R10

IC6:
 a      read level at pin 13 of the Textool-socket
 b      read buffer of IC7
 c      read level at pin 23 of the Textool-socket

IC7:
 P1-P8  read level at pin 14-21 of the Textool-socket