Does the "Portable PC" connect its video to
the RCA jack, or to the 4 pn
berg?
Assuming yoy mean the 5155, it uses the Berg connector inside. 2 of the
pins (ground and composite output) go to a twisted pair cable which goes
to the monitor.
The repaining pin on the Berg header is +12V IIRC. This is not used in
the Portable PC (the monitor is connected directly to the PSU). I always
assumed this was a handy power feed for an RF modulator should you want
to use one with a CGA card.
-tony