Problem with 4 sensor boards having a common ground

[elec_boy_1]

Hi all,
I have 4 independant sensor boards each running on a 9V battery. Data is transmitted through wires from each of these 4 boards to a collector board that also is powered by a 9V battery. I have connected the grounds of all the sensor boards to the ground of the collector board.
The problem is when any one of the sensor boards are turned on, the other boards also get a little bit of power, even when they are switched off. I figure this problem is because of common ground.
I want a solution.
Note: The wires from the sensor board to the collector board are about 3m long.

 

[Johansen]

Ground Problem

No, it's not because of the common ground.

"the other boards also get a little bit of power"
What exactly does this mean?

Is the collector board sending a logic high to the off board, in this case the protection diodes may be driving the board

 

[elec_boy_1]

Re: Ground Problem

yes the collector board sends logic high. Please suggest a workaround.

 

[FvM]

Re: Ground Problem

So the problem is caused by the interface design.

 

[elec_boy_1]

Re: Ground Problem

The sensor board sends high to the collector board which in reply sends high to the sensor board. This is what happens.
Please suggest a work around. What are the general procedures followed when faced with problem of this kind?

Indications of the problem: the collector board is supposed to send gathered data to a computer via the serial port. When the sensor boards are turned on, then even if the collector board is off data is transmitted to the computer.
In the middle of operations if a sensor board is switched off the LEDs on it continue to glow.
This happens with one sensor board and the collector board.

 

[FvM]

Re: Ground Problem

The general work around is to redesign the interface. The receiver should use a buffer that can be driven without sourcing voltage to the supply. With logic level UART, you could possibly turn off the interface for a sensor as long as it signals break state itself (if there are individual TxD channels for the sensors, you didn't mention that).