Acorn BBC Master 128, Руководство по обслуживанию

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Master Series Service Manual
Teletext
The Teletext modes do not generate a bit mapped display, but a
character cell one. The character/graphics ROM within a SAA5050 device
generates RGB signals according to the desired character/graphics
information within the reserved memory space. Each byte of memory is
therefore just a definition of the character/graphics symbol required.
Other SAA505X devices may be used when different languages are
required. Only 1 Kbyte of memory is needed for either of the Teletext
modes, although 20K is reserved for it in mode 135. The MOS uses the
spare 19K to speed up inter-filing system file transfers but the user
may use this memory if no such transfers are to be done.
The VIDPROC has to be re-programmed to use the SAA5050 RGB outputs.
The 6845 is still used to generate the cursor. As a delay of 2.75 us
will occur after a character is read from RAM, before outputting the
appropriate RGB signals, the 6845 has to be programmed accordingly.
The "start" of screen signal is given a 1.5 byte-time offset, and the
SAA5050 has a further 1 byte-time offset to restore the correct
cursor/data phase.
The VIDPROC has further adjustment which allows for the cursor to be
adjusted to pixel accuracy.
Hardware Scroll:- Scrolling may be achieved in any mode by re-
programming the 6845 start of screen address to an integral number of
video lines further down the memory map than the nominal start of
screen.
This of course causes the linear address generator to attempt to
display an end of screen which is out of the reserved video area. To
overcome this effect, hardware scrolling is provided with a variable
address wrap-around.
In effect, when the address generator would otherwise attempt to
access out-of-screen RAM, its addresses are modified to point to the
gap between the original start of screen and scrolled start of screen.
When this is done, only the end of screen needs to be written over in
RAM. (If this is not done, the entire screen appears to "roll-over").
The amount of modification to be used is controlled by two nodes; C0
and C1.
Video Output:- Three outputs are provided for displaying video
data. These are:
a) PAL/NTSC encoded, UHF carrier. On channel 36 with 1.5mV into 75 ohm.
b) Composite video. This is a iv peak to peak signal.
c) Digital Red, Green, Blue outputs. These are approximately 75 ohm
outputs.
For use with NTSC, the modulator has to be changed from UM1233/E36 to
a VHF equivalent. Provision is made for selection of either one of two
channels with V.H.F. A Molex type link has to be inserted for this.
25 Issue 1