PET composite video adapter

[Philip Belben]

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.