Sharing FT-60 (red cube) between two avionics?

scrockard · #1 05-08-2015, 08:25 PM

I like to know if anyone is successfully sharing a single Electronics International FT-60 Flow Transducer between two different avionics?

I have a trio pro pilot and a grand rapids EIS4000 engine monitor and both want fuel flow input.

This FT-60 is an open collector output with a recommended 1mA pull up current and 15mA max current. The only problem is I have no technical details that I can find for either avionics that says anything about their internal pull up values.

Thanks,
Scott

digidocs · #2 05-08-2015, 10:45 PM

Hi Scott,

Parallel the inputs of the EIS and Trio as you've shown, but don't connect the FT-60. Then put your multimeter in the low current measuring mode and measure the current when one probe is connected to the EIS/Trio input node and the other probe us grounded.

If it's less than 15mA, you're good to go.

David

scrockard · #3 05-08-2015, 10:52 PM

That sounds like good advice... I've put a couple of zero ohm resistors in my circuit to allow for this case (in series with each avionics input).
If they absolutely don't work, then I can choose one or the other avionics input easily or even play with adding slightly more resistance to keep from exceeding the limits of the red cube.
I agree with your observation - if these are just simple pull-ups inside these two avionics, then doing this with a dc current measurement should work.

There's nothing like designing for black boxes! lol.

Thanks
Scott

Walt · #4 05-09-2015, 05:58 AM

The FT-60 is a pulsed output based on FF, therefore direct DC current measuring techniques will not work. I think knowing the input impedances of the devices you want to hook too would be an easier way to determine current loads/limits.

scrockard · #5 05-09-2015, 07:02 AM

Agreed on the square wave output and that messing up rms readings of a digital multi-meter. David is talking about trying to measure this as a static load before the red cube is attached. If this is just a passive pull-up circuit in each avionics, then i might even be able to ohm out that pin on each avionics box before power up and confirm things too?

I do have a scope if needed to measure voltage drops across a resistor. I frankly hope I don't have to go that far.

Scott
(oh - I have initiated a tech support request with one of the avionics mfg but no response back yet. I'll try the other one before Monday. Frankly this forum usually gets good answers and quicker anyway!)

DaleB · #6 05-09-2015, 07:19 AM

Really, this shouldn't be a big problem. OC outputs are pretty useful for spitting switched signals like binary or PWM data out to unknown loads. I'd be shocked to see more than a couple of mA into either box. Certainly you could very easily tell by, as someone else suggested, hanging a meter set to measure mA DC current between each input and ground with everything powered up. Then you'll want to make sure the inputs are pulled up to the same voltage -- 12V, 5V, whatever.

This doesn't need to be a week long MSEE level analysis exercise. This is more likely (depending on what the meter tells you) a five minute check with a meter.

scrockard · #7 05-09-2015, 02:34 PM

It's always good to get a few more opinions on the subject. You guys have all been great help. thank you!

Scott

FasGlas · #8 05-09-2015, 11:39 PM

You could easily isolate the output of the red cube from the two inputs with a logic chip. (TTL/CMOS/etc.) Then you have no impedance conflict.

scrockard · #9 05-10-2015, 12:33 PM

I elected to take my chances and just go with pretty much what David recommended. My reasoning is that the open collectors should in theory allow this type of parallel hookup (with caveats of coarse). I'm allowing the possibility to tweak the resistance a little bit but as DaleB pointed out - "I'd be shocked to see more than a couple of mA into either box". Even though isolating these with active electronics is certainly doable - I'm frankly trying to eliminate that if I can.

I'll let you guys know once I get my "cross connect pcb" back from the board house and I start firing the panel up...

scrockard · #10 05-26-2015, 06:49 AM

I've received responses back from both manufacturers.

Trio ProPilot has an internal 10K with a 5VDC supply.
GRT Sport SX has an internal 4.7K with a 5VDC supply.

Since the parallel resistance and new current draw is well less than any open collector max settings of the red cube, this should work well with one red cube feeding two instruments.

Scott