Block CCTV signal flowing in reverse

austin48111

I have a circuit where I'm using a couple ADG426 1x16:1 switcher units with a BS2P40 controlling them. The issue I'm seeing coming up as I plan to test this design is that when certain feeds are active they may flow to not only the input but to the outputs. Is there a way I can keep signal flowing in only one direction? I've been looking at electrolytic capacitors and diodes. Diodes make the most sense but it seems like the forward voltage drop would be too much when dealing with the low voltages involved in CCTV. The other option I see is the electrolytic capacitor. They are obviously polarized but what happens if signal flows into them from the other direction?

Another option I see would be SPST relays but running 16 relays off the few pins I have remaining seems unlikely without another IC communicating via I2C or a decoder.

Really, all I need is a way to block backwards signal. I'm certain there are other options but I'm ignorant as to what they may be.

Phil Pilgrim (PhiPi)

Please explain your problem a little better (schematic). I should think that your multiplexers would block signals that are not selected. At least that's what they're supposed to do.

-Phil

austin48111

This is a drastic oversimplification but it communicates the exact issue I expect to have when I prototype on a much smaller scale. There are two inputs. The BS2 detects if there is video present on the input (not shown in diagram). If there is not it enables pin S9 which goes to the drain (pin D). What my concern would be is that yes, I would get my desired output through but it would also feed back up the trace to the input. As such I need a way to block it. I'm debating using a reed relay or something but would prefer something which does not use so many pins (16 inputs = 16 relays = 16 pins).

Let me know if this helps:

Phil Pilgrim (PhiPi)

Why not use one of the other MUX inputs for your primary signal and monitor the input instead of the output?

If that's not a possibility, this circuit might work to block a ground-referenced DC video signal from back-feeding:



It's certainly not as good as using the MUX, though.

-Phil

lanternfish

Why don't you have both inputs routed through the MUX so that the normal input is selected until such time as no signal is present and the MUX is switched to the alternat input. Or have I missed something?

austin48111

This is a simplified version of what I'm working on... greatly simplified. There are 20 total ports being routed in and 16 total ports going out. I'm at the limit of what the BS2P40 can handle in terms of I/O and in fact need to add a PFC8574 to add additional I/O already. I could just keep expanding and have a ton of I/O but as my first functional layout I want to keep as few chips and variables on this as possible. I've decided to use the PFC8574 to free up pins for the BS2P40 to fire SPST reed relays to block the signal. I suppose I don't need to worry about it flowing backwards if there's nothing there to flow back on.

schill

Without knowing more about your application, are you sure you need more data lines. If you are using devices like the ADG426 you should be able to share address lines. A0 - A3 can be shared by two (or more ADG426 or other devices). Each one needs its own WR (inverted) line to latch the data. For example, 6 lines could control two ADG426 (or more if you use a decoder for the WR lines). You may also need lines for EN (if you are using it). I guess the point is, don't forget that you can share the address lines.

To me, it still sounds like you should be able to route all of your inputs through the multiplexer(s). If you have 20 inputs, are you using a 16-input multiplexer and a 4-input? I think adding additional multiplexer inputs will be much easier and cleaner than using relays or other devices. You can probably just tie the outputs of the multiplexers together for your output since they have EN pins to disconnect all inputs completely (although I didn't read the spec sheets very carefully so I may be wrong).

I have done some stuff with switched video like this (it's been a while), but I used matrix-based chips with multiple inputs and multiple outputs that could be tied together anyway I wanted to.