Viewdata/VideoTex data transmission protocol
              --------------------------------------------

Characters are sent as a text stream.

Character    Action
-------------------
5            ENQ, client should respond with a string describing the client

8,9,10,11,   Write to VDU, but 10 and 11 wrap and remain in the same column
12,13,30     and 8 and 9 wrap between the top-left and the bottom-right

17           Cursor on
20           Cursor off

27,char      VDU char EOR &C0
             As data is expected to be sent over a 7-bit channel, then
             teletext colour characters (128-159) need to be escaped.

32-255       Teletext display characters

The bottom lines of double-height text are not sent, the receiver must
duplicate a line received with CHR$141 in it to create the bottom line.

The text sent is expected to be displayed as teletext characters, so for
example, &23 should be displayed as a pound and not a hash. The BBC's VDU
drivers translate hash, pound, and underline character codes to the teletext
character codes, so for example &23 is displayed as a hash.

The simplest way to display the VideoTex text stream as the expected display
characters to do VDU char OR &80 for received characters 32-127.

Code similar to the following will do this:

char=FNreadbyte
IF char=5 THEN PROCenq:ENDPROC
IF (char>=8 AND char<=13) OR char=30 THEN VDU char:ENDPROC
IF char=17 THEN VDU 23,1,1;0;0;0:ENDPROC
IF char=20 THEN VDU 23,1,0;0;0;0:ENDPROC
IF char=27 THEN VDU FNreadbyte EOR &C0:ENDPROC
IF char>=32 THEN VDU char OR &80:ENDPROC
ENDPROC