The scanner rom was primarily a support ROM for wapping editor although it
would work standalone I'll see if I can dig out any tech info. Do you have a
copy of the driver ROM?

I just found some technical info for the RH digitiser:
(Correction: It used a 6821, not a 6522)

----
The digitiser occupies 4 bytes in page &FC of the 1 MHz bus starting at
&FC7C and these 4 bytes are the data and control registers of a 6821 PIA.
These are allocated as follows:

Port A is an output which specifies the vertical column to be digitised
       (0 is leftmost and 255 is rightmost).

Port B is an input which is the intensity value of the pixel being
       sampled.(Only the top 6 bits are valid, the two least significant
       bits should be ignored).

Control line A1 is an input connected to the field sync signal divided by
       two.

Control line B1 is an input connected to the 'pixel valid' signal and should
       be configured to be positive edge triggered.

Control line B2 is an output which controls the video switching (High = BBC
       video to monitor, Low = video signal to monitor)

Consult a 6821 data sheet for programming details.

The digitiser works by taking successive vertical strips of information from
each field and combining them to produce a picture which is, in fact, made
up of pieces from 160 seperate pictures.

To start sampling a video signal, the software should set up the column to
be scanned in port A and then poll control line A1 until a positive or
negative transition occurs. For a frame scan both transitions are valid, but
for field scans only one transition is valid (depending on which field is to
be sampled).

The software should then poll control line B1 until a positive transition is
detected. This means that the pixel has been sampled and a valid intensity
can be read on Port B. This value should be read and stored within 64 micro
seconds (the scan-line time) before polling control line B1 again for the
next pixel. This should be repeated for 256 lines before incrementing the
column value and repeating the whole process until 160 columns have been
sampled.



EDIT: found some tech info on the hand scanner as well:
----
Hardware registers (all read only) :
data = &FCE0 : REM reads data byte, b7..0 = left..right, 1=white
rdy  = &FCE4 : REM bit 7 high when data ready (test for rising edge)
sync = &FCE8 : REM bit 7 goes low at start of line data
yclk = &FCEC : REM bit 7 = vertical pulse

The scanner interface continuously outputs stream of bytes representing the
current line in view. 'sync' indicates the start of the line by going low.
The 'rdy' signal pulses when a new data byte is ready (every 21 uS). The
first 3 bytes after sync should be ignored. Vertical movement produces 200
yclk pulses for each inch moved. yclk stays low if the scanner button isn't
pressed. Reading yclk resets the sync flag.