PRO2020

From: MKELLY@CARLETON.CA
Subject:    PRO 2020 SCANNER MOD

Here is the mod for the Radio Shack Pro-2020 Scanner that I posted
to Info-Hams Vol 89 Issue 314, Slightly condensed.
It adds another 80 memories to the existing 20, in four banks of  20,
and suggests other mods.
It MIGHT be helpfull for other scanners that store frequency info in
a ram seperate from the cpu chip.
The address, data, and WE lines on the old and new ram are paralleled.
Chip enable from the cpu is multiplexed to the old and new ram, with
the mux select being driven by a high order line from a counter. The
high order address lines on the new ram are driven off of the low
order bits of the same counter that drives the mux.  These counter
lines also drive a binary->7 segment decoder/led driver to indicate
which bank the machine is using.  The mux output is inhibited by a
power_on_reset signal to prevent spurious writes during power up.
CE1 from the cpu goes to the input to a 1 to 2 mux (1 input, CE1;
2 outputs, OLD_CE1, NEW_CE1).
The mux is made of 1/2 of a 74hc00 2 input nand and 2/3 of a 74hc10
3 input nand. There is one 3 in nand unused, inputs are tied high.
The 3input nands drive the ce lines on the rams.  The third input on
each of the two 3 in nands is driven by the power_on_reset line that
goes to CE2 of the ram in the original machine.  This signal stays
low until the rest of the system is powered up and stable,  preventing
spurious chip selects (& writes) until the power up sequence is complete.
The top 2 address lines of the new ram (1k x 4 cmos) are selected by a
4520 4 bit binary counter configured to count 0,1,2,3,4 with binary
output 101 decoded with another 1/4 of the 74hc00 looking for a high on
Q0 and Q2 of the counter at the same time.  This signal is inverted
(using the last 1/4 of the 74hc00) and fed into the clear line of the
counter.  The clock inputs are configured so a low on the clock will
increase the count. The line(s) being used for the clock have a resistor
to counter vcc and a capacitor to ground.  The bank select switch is a
SPST N.O. switch that shorts the capacitor.
Q2 goes to the select input of the mux.  When Q2 is low, chip selects
from the cpu go thru to the new ram, and when it is high, the chip
selects go to the original ram, as before, like this:
Q0  Q1  Q2
0   0   0  new ram bank 00  bank display reads 0
1   0   0  new ram bank 01  bank display reads 1
0   1   0  new ram bank 10  bank display reads 2
1   1   0  new ram bank 11  bank display reads 3
0   0   1  old ram (only one bank)  bank display reads 4
1   0   1  generate asynch reset and go back to 0 0 0

CONTINUED IN PRO2020.1