Best way to do I2C/TWI over long distance

Question

I have a project that requires to do I²C/I2C/TWI over a long distance (30 to 40 meters).

I've seen some people suggesting lowering the clock-rate to somewhere around 500 Hz, to mitigate the effects of the capacitance of such a long line I assume? The components I'm using require at least the standard 100 kHz clock-rate. I did some further research and found among answers to another question a suggestion to use a P82B96 level shifter. In the datasheet they give examples of using them on lines of even 100 meters:



I've come across another way of shifting levels through a breakout board from adafruit, which is just a mosfet (bss138) with two pull-up resistors (one for each side/voltage). They got the idea from an application note from NXP (AN10441), and two of the channels on there could be used like this:

Now I wonder: which solution is best? Or is there something I have overlooked? And also, is 5V enough to ensure a good connection? Would there be an advantage to using an even higher voltage like 12V?

Answer 1

I think you are on the right track with something like the NXP P82B96. If you have a look at Figure 14 and the associated text the datasheet discusses using cable length of up to 250 m with data rates over 100 kHz.

There are many I2C temperature sensor available that can give you accuracy of a few degrees celsius without calibration and without any added analog circuitry. Since you are bringing the data back for processing anyway, filtering out noise would be very easy.

If you are concerned about the wiring capacitance you might be better off with unshielded twisted pair rather than a shielded cable.

Answer 2

You can probably get away with the DS18B20 sensors by using shielded CAT5 or similar cable. It's only thermal measurements, so if some readings get corrupted you can make your program robust enough to deal with that (it's good practice anyhow).

For chain restaurant use, I've designed what are called RTU (rooftop unit) controls that work on multiple passive sensors (they use interchangeable NTC thermistors, which require no calibration for comfort heating). It's much easier to filter noise out of a passive sensor. Another set of installations (a major North American automotive complex) used platinum RTDs with signal conditioners- very accurate and stable devices. You should decide the sensor first, then look at how it might be interfaced- any of these sensor options are suitable for your application (though the RTDs might be overkill).

In case it's not clear, I'm suggesting a "star" configuration of sensors around your processor.

If you really want to use a "bus" (other than one-wire or I2C), you'll need smarts at the ends, which means a processor in some form. If you're going whole-hog you might want to consider using wireless sensors that communicate via some ISM band that's legal in your locality. Alternatively, you could use one sensor per Pi and have them talk to each other via your WiFi intranet.

Answer 3

Adafruit now (2021) sells the "LTC4311 I2C Extender / Active Terminator". It works by monitoring the SCL and SDA lines and then injecting current during the positive transitions to help overcome capacitive line loads and sharpen the waveform. I've not implemented any yet myself, but the description says they've been able to read sensor data at 100kHz over 100m of Cat5 cable, and 400kHz at a few meters.

https://www.adafruit.com/product/4756

You only need one of these near the head end of the bus, with no other wiring modifications. Seems like this could be a much simpler and cheaper off-the-shelf solution than other options like RS-485 or the PCA9615-based extenders that SparkFun offers.

Answer 4

Extender: NXP PCA9617A works as both level translator and extender and supports Fast-mode Plus (1Mbps).

Isolator: ISO1540, ISO1640, ADuM1250