SPI devices in parallel.

[jmgelba]

I agree with Henrik. Capacitance is the enemy here. Noise is also a problem - a 50ft caable makes for a nice antena when running around 50+ dc/dc power supplies. Clock and data speeds are low as its only one byte every so often. This is just for dimming control set via push buttons on the master PIC.

Unfortunately due to the cost of each assembly, I cant just build a few and test to see if it works. Each assembly where the pot will be located is several hundred US$.

[HenrikOlsson]

In that case, what difference does the cost of a dual (or two single) differential receiver(s) on each board make? Probably around a buck or two.
If the boards are already designed and ready for production and you can't spin it again then you perhaps you could fit the receiver circuitry inside the connector housing used to for the communication lines (or is it screwterminals?).

/Henrik.

[jmgelba]

EL5371 driver and EL5372 receiver should do the trick? I have never done anything with data transmission over cables before. I assume I would need a driver on each of the dc/dc pcb's too? So it would go:

Master-driver --> cable --> DC/DCreceiver-driver --> cable --> DC/DCreceiver-driver --> cable --> DC/DCreceiver-driver .....

http://www.intersil.com/content/dam/...n73/fn7307.pdf

[HenrikOlsson]

Nah, I don't think those are suitable, they look more like differential analog drivers - not really what you want.

I was thinking more on line of a DS9638 on the master and a DS9637 (or similar devices) on each of the slaves:


/Henrik.

[AvionicsMaster1]

Terribly sorry. I assumed most digital pots CS line requirements would be the same. The question wasn't about power supply it was about control and I again assumed that any pot chip would work about the same. Obviously bad assumptions on my part though I assumed a worst case scenario and replied on that.

I stated I wasn't an engineer and had no experience but I assumed a shield on the CS line and if necessary the power supply lines. He could also use a twisted pair on the power supply which should reduce noise collection.

I again assumed that a small conductor cable wouldn't create a great capacitor and with the slowish switching speed I assumed that wouldn't be a problem.

So now I've been straightened out and I apologize for leading anyone astray. Please let us know how you proceed and more importantly, your solution.

[jmgelba]

Dont worry about it.

I'm going to try the drivers and will be using standard cat5 cable and connectors. Makes things really easy.

[jmgelba]

Just a thought, connect a 1K resistor to the output of each receiver and connect the other end of the resistor to the base of a 3904 transistor, I think I should be able to turn on an LED each time the line goes high without loading it. It would be really nice to have a simple led showing activity on the data lines at each light, as a cable and connector diagnostic.