2 Jan
2017
2 Jan
'17
3:18 p.m.
On 02/01/2017 14:17, "Tony Duell" <ard.p850ug1 at gmail.com> wrote:
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?
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?