I put together that circuit (using all four gates in the 74LS86 chip) to experiment with
Now that I know it works (on my STFM and STE), It will be possible to simplify it somewhat
The advantages of using a logic chip are:
- It will not excessively load the outputs from the custom Atari Glue chip.
- It amplifies and cleanly mixes the horizontal and vertical sync signals to produce a clean signal for the GBS-8200 video converter.
- It is more likely to work reliably (famous last words ) with all models / versions of the ST range of computers, and it may well work with other computers that also produce horizontal and vertical sync signals.
I do like very simple circuits (and well done to spiny getting the two resistor method to work
), but the simple resistor circuit may damage the output driver in the Atari Glue chip. Time for a quick bit of maths and a thought experiment:-
Now some of the time one of the sync signals will be high (+5V) and the other will be low (0V), with the two signals being connected together you have a total of 5V across the two resistors.
When using an STF(M), if these resistors each have a value of 47 Ohms, the total resistance is 94 Ohms. Current = voltage / resistance. So 5V divided by 94 Ohms = 53.2mA
Now that is likely to be more than the designers at Atari intended. Maybe more than double what was allowed for
Now earlier in the thread I did suggest using 75 Ohm or 100 Ohm resisters. Even when using these values (if they work) results in a theoretical current of 33mA or 25mA
Now I don't know what the specification is for the output section in the custom Atari Glue chip. But then I don't expect anyone else does except for some former Atari staff.
For the sake of less than £5 worth of parts, I would rather not risk it.
I now know that 74LS86 chips work. Looking at the Hsync and Vsync signals on my oscilloscope, I think that either a 74HCT86 or a 74HC86 is very likely to work. I have a 74HC86 and may try it tomorrow.
On eBay 74LS86 chips are 5 for £1.00 (plus £2.50 P&P), see http://www.ebay.co.uk/itm/220659545904
I bought chips from posttrade before and like their service.
There are a number of sellers of 74HC86 chips, at the moment the lowest price is £2.02 for one with free P&P.Mark
Falcon, Atari 520STFM, Atari 1040STE, more PC's than I care to count and an assortment of 8 bit micros (nearly forgot the Psion's)