■ FIGURE 9
is the only eight bit up/down counter
that I found. It is good to 100 MHz, if
that is important to you. You might
choose different gates, depending on
your speed requirements.
The binary number 50 is 110010.
That number is tied to the parallel
input of the eight bit counter to make
it start at 50, and is decoded at the
output to switch from counting down
to counting up. The binary number
200 is 11001000. That number is
decoded at the output to switch from
counting up to counting down.
I used the fact that a NAND gate
is also an inverted NOR in IC2C to
clock the flip-flop and toggle the
counter up/down pin. The RC network, R1 and C1, puts a temporary
ground on the preset input of IC3B
so that the counter starts counting
up. The RC network, R2 and C2, puts
a temporary high on the preset of
IC3A so the ground on CLR is active,
setting Q low temporarily which
loads binary 50 as the start count.
Figure 9 is the same circuit using
the 4000 series CMOS which can
operate with VCC up to 15 volts DC.
The power is much less but so is the
speed. I estimate 2 MHz max at 5
volts; 5 MHz at 15 volts.
For a manual reset, just connect
a two pole momentary pushbutton
switch across C1 and C2.
USB TO PARALLEL
QAt long last, I just bought
a new computer with an
Intel Core2 Quad CPU, and
when I got it home, I found
that there was no parallel port to
connect to my three printers (through
my A-B-C switch box). Is there some
way that doesn’t break the bank that I
can connect my printers through the
USB ports on my new computer?
— Paul Tilson
AYou should become familiar
with Google (www.google.
com) — that is what I used
for this answer.
Sewelldirect.com has a USB to
Centronics parallel adapter (part no.
SW-1302) for $13.95. You can hardly
buy the cable alone for less. It is not
compatible with all-in-one printers or
Cablestogo.com has a USB to
IEEE1284 adapter, part no 15898, for
$24.99. This part has bidirectional
communication capability. IEEE1284
also supports Centronics parallel in
Compatibility mode. NV