Viewdata/Teletext

[Adrian Graham]

On 02/01/2017 14:17, "Tony Duell" <ard.p850ug1 at gmail.com> wrote:

> That was quite common. I remember pins in one of the Philips chipsets
> with names like 'After Hours Sync'. Basically if there is a television signal
> it will be synced to it (so you can, for example, overlay teletext on a
> normal picture -- subtitles, etc), if there isn't then it can run stand-alone
> as a display device.

Yep, this is how the Tandata terminals work too.
>> 
>> 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.
> 
> Argh!. My first thoughts were 'defective chip' (but it isn't),
> 'missing master clock'
> (but you have checked that) and 'Needs to be programmed for out-of-hours
> sync' (Some Philips ICs had a bit in one of the internal registers for
> this, this
> one doesn't that I can see). I assume both power supplies are present
> and correct at the chip?

Yep, 12V at pin 40 and 5V at pin 39, 0V on pin 1. I haven't checked current
output but the PSU I'm using normally runs a pair of 5.25" floppy drives.
This PSU is also identical to the one in the Executel that I've not fixed
yet (Astec AC8151).

I've just tried putting the chip on a breadboard with the same pins pulled
high/low and still get nothing at sync out, though admittedly the arduino
providing the clock source is only putting out a 4mhz signal so I'll change
that. Dividers are still working.

Hm, I've just remembered there's a 74LS240N buffer between the CPU and data
pins on the MR9735 that I've not tested yet, need to make sure the outputs
match the inputs. I guess even with nothing to display there should still be
a sync out though.

Cheers!

-- 
Adrian/Witchy
Binary Dinosaurs creator/curator
Www.binarydinosaurs.co.uk - the UK's biggest private home computer
collection?