Adrian Graham witchy at
Mon Jan 2 07:56:16 CST 2017

Hi folks,

Happy new year to all!

Is anyone here familiar with the innards of Viewdata? I'm still deep in the
bowels of this Executel viewdata phone system and one thing that it should
be doing but isn't is puzzling me.

The teletext chip is a Plessey MR9735, datasheet here:

It can work in either 'off hours' or 'on hours' meaning effectively 'day' or
'night' mode when there's no broadcast TV signal at night, this is the 80s

The surrounding circuitry forces Off Hours by pulling high all the incoming
TV signals apart from Line Flyback which is pulled low. The datasheet says:
"When the incoming transmission is turned off, (i.e. Goes 'Off-hours'), this
is recognised by the [On Hours Detector] after at least 300ms of missing
sync. Pulses. An internally generated Composite Sync signal is then switched
to the Composite Sync Out pin."

Since Sync In is pulled high there are never any negative sync pulses (I've
watched this on a logic analyser) so after 300ms Sync Out should become an
internally generated pulse, but this doesn't happen and Sync Out remains a
steady 5V meaning the TV picture is unsync'd.

I know the MR9735 itself is fine as I have a pair of Tandata viewdata
terminals which also use this chip in Off Hours mode and I can swap them
around. The chip itself is receiving a steady 6MHz clock to pin 21 and the
clock divider outputs at pins 20 and 19 are working.

Any clues?

Binary Dinosaurs creator/curator - the UK's biggest private home computer

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