I'm using four SRF08s and they all work fine when connected straight into my Duemilanove but when I try running them on longer cables I hit problems. I'm powering the board with a 9V adapter but once the cable length exceeds around 6m I stop getting readings and the LED on the sensor doesn't flicker, and that's just with one never mind four. However, the LEDs do flash to indicate addresses on power up. Any one had any experience using these sensors with long cables? Any advice on the subject would be very much appreciated as I'm running out of time. Thanks.
What protocol do you use? I2C?
you might need to lower the pullup resistors.
Yes I2C. I'm having some success. I'm getting results with longer cables now but could just do with a bit more. I'll keep trying. Thanks very much for your advice.
Some ideas.
Have a look at the waveform with an oscilloscope. For the cost of 2 sensors you could buy a basic oscilloscope on Ebay.
Consider something like this I2C - RS-485 adapter
The parts should only cost a few dollars and a breadboard will tell you if it's going to work.
Arduinos are a minor cost compared to the sensors. How about adding one next to each sensor and then using the Arduino PWM outputs to transmit the readings. If the cables are very noisy then consider building a current loop driver circuit. For an extra $20 an Ethernet board will give you > 100m over cat5 cable.