On 02/01/2017 14:17, "Tony Duell" <[email protected]> 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?
