13 Oct
2008
13 Oct
'08
1:25 p.m.
Ethan Dicks wrote:
P.S. - I found another type of composite circuit using
a CD4011 as an
inverter and a CD4066 to mix the signals, but for some reason I don't
have all the components labelled on my drawing - the cap on hsync in
particular. One interesting variation with that one is that the 'LS02
circuit has a 1.5K pulldown on hsync between the cap and the gate,
while the CD4011/CD4066 circuit has a 10K pot. How much adjustment on
hsync is likely to be needed between the PET and the first gate of the
composite circuit?
It's the 4011/4066 circuit that I recall. From (very rusty) memory:
The circuit as given in "The Pet Revealed" by Nick Hampshire doesn't
work, but I can't off-hand remember why not. Maybe he got two signal
names swapped.
You use a 4011 to combine the syncs, and maybe a second gate to invert
the composite sync. The output of this drives the control input of the
4066, which gates the video line from the PET to the monitor.
You may need a pull-dowm for when the 4066 is high impedance. I think
the only capacuitor (apart from on the supply lines to the chips) should
be on the composite output - the syncs that appear on the PET user port
are at logic levels; only the video output is at video levels.
I don't think any component values are critical as long as you don't try
and draw too much current. The pull-down should be large compared to
the on-resistance of the transmission gate, so several K ohms.
Before you ask, I do possess copies of TPR and circuit diagrams for the
PET. If you want me to find them, it may take some time to dig them
out, since I have no idea where they are.
HTH
Philip.