keropi wrote:Allright thanks for the info!
Any recommendation of the value the pulldowns must be? Do they need to go to pins 2-3-4-5-6-7 or only needed for the secondary/intensity pins 2-6-7 ?
The rule is not to leave any input of CMOS chip unconnected. This means also situation where the cable is disconnected so all pins float.
So you'd need a pull-down on all eight lines, R,r,G,g,B,b,H,V.
Extremely good question on the pull-up values. I don't know what is optimal. If this was VGA, the HS and VS would be terminated with about 2.2 kohm to ground on today's implementations.
Keeping in mind that TTL high output is very weak, but this is usually driven with a 74LS244 bus driver, it can source about 3mA and sink 12mA. So the strongest pull-down tolerated is about 1kohm.
Since the converter board has very high impedance input, even a 100k would do. Maybe safe between 4k7 and 10k, so relatively high termination impedance.
That said, TTL input monitors might terminate with quite low impedance to get higher frequencies through the cable (it minimizes the R in RC filter, C being stray capacitances of wire etc).
For example IBM CGA monitor uses split termination with 330 ohms to 5V and 470 ohms to ground. That's 194 ohms termination into 2.9 volts.
The low termination value would help with signal reflections (it should match the characteristic impedance of the cable) and also help driving the TTL input buffer chip in the monitor, the IBM CGA monitor has 74S05 that requires the driver to sink 2mA for the input chip and about 10mA for the termination to make signal low, which is a lot. But the driver does not need to source much current at all to make signal logic high.
The monitor would see a disconnected cable as logic high level. It does not matter as because if there is no activity on syncs it keeps blank.
But in this case, as the unused pins need to stay low, it should be a pull-down resistor. If the FPGA is put into CGA or MDA/Hercules modes and ignores unused bits, then these can be pull-ups which suit better to LS TTL chips driving it. Maybe 4k7 pull-up?
I'd try on my converter board but currently I don't have any EGA/MDA sources. I'd try building a signal generator with a LS244 chip and wire to try pullup values and measure with a scope, but I'm afraid I don't have the resources to do that for some time.
Edit: Haha you got the same values but pushed submit first 😀 Yes, those are good values to try.
