Re: Franklin Ace 1200 -odd little jumper

[geary@fnord.io.com (Big Bird)]

In article <6I2dnbZj5uZ6DorfRVn-rg@comcast.com>,
Michael J. Mahon <mjmahon@aol.com> wrote:
< Ernest wrote:
< > There is a little jumper wire on my Franklin Ace 1200 motherboard,
< > and I have no idea what it is for. Here is a picture of it:
< > 
< > http://www.apple2clones.com/images/jumper-981.jpg
< > 
< > Does anyone know what this odd little jumper/short is for?
< > 
< > Here is a sketch of where the wires go to, with the chip type
< > listed next to it:
< > 
< > http://www.apple2clones.com/images/chips2-982.jpg
< > 
< > If you hold the SN74LS20N chip with the notch facing left, the
< > wire connects to the second pin from the left on the top.

I've got the schematics here (until I pack the manual up and send it
to Bill Garber).

<  From your description, it sounds like the wires go to
< and pin 13 of the 74LS20.
< The 74LS20 seems to be in location F6.

This is the output of (MAR14 AND MAR15), i.e. the current address is
C000-FFFF. 

< > If you hold the SN74LS08N chip with the notch to to left, the wire
< > connects to the sixth pin from the left on the top.
<
< pin 9 of the 74LS08
< The location number (e.g. H1, for the 74LS08

This is an input pin. The signal comes from pin 6 of the chip at G14,
which is a 74LS259 aka 8-bit addressable latch. The eight outputs from
this chip appear to be selected on/off by the soft switches at
C05X. Pin 6 is the page 1/2 switch (labeled page 2/1 on the schematic).

< > Any guesses as to what this jumper is for?

So, when the jumper is on, these two output signals come together
without a gate. It's been fifteen years since my EE classes, but I
think that makes an implicit AND gate?

These two signals compine as an input for the AND gate at H1
(74LS08). The other input is labeled HRES and is the AND of the C056/7
softswitch and A signal that seems to tell us whether the current scan
line on the CRT should show graphics or text.

The output of the AND gate at H1 goes into pin 11 of chip K1 a
two-input multiplexer (74LS257). The other input to the multiplexer,
pin 10, is MAR14. The select input appears to be the 1 MHz clock signal.

The output, pin 9, goes on to influence the RAM chip select lines, and
a line enabling the keyboard/memory multiplexers.

I still can't tell what the significance of the jumper is. I don't
remember seeing it on my Franklin Ace 1000, but I got rid of it last
year, so I can't check.

Maybe someone can make sense of what I've written.

Mark Geary

-- 
"It's going to be a tough one Sam...Ziggy hasn't got a clue and the
guy in the waiting room keeps asking me if I want a jelly baby."