The Industrial Raspberry Pi Forum

Hi all,
Kyle here, working on the shop floor with a few RevPi setups. We’ve got three modules hooked up along the production line, each running 8 analogue inputs for temp and pressure. When we start scaling, some of the readings start jumping, especially on the modules further down the line… In an ideal world I’d stick to a single master module, but the Modbus bus hits saturation once we poll all inputs every 100 ms. Better to split sensors across modules or handle the spikes in software with buffers and smoothing? Signals are pretty fast, so losing data would mess with real-time reporting.
Cheers,
Kyle

Hi Kyle,

can you be a bit more specific on what exactly jumping means?
Constantloy diverting temperatures? Or scatter? What are the actual values? What would you expect instead?
What type of sensors do you use? What's the cable length and sre they shielded?

Hey, thanks for the quick reply!

By “jumping” I mean the readings fluctuate a few degrees or bars up and down for a second or two before settling again, not constant drift, more like short bursts of noise. The sensors are 4–20 mA PT100 converters for temperature and 0–10 V for pressure. Cable runs are around 10–15 m each, all shielded and grounded on one side.
What’s odd is that the noise only shows up once we add the third RevPi module, I’m guessing it’s either bus load or grounding differences between modules. I’ve seen some folks on gritt.io discuss similar Modbus timing and interference issues, so I might try a few of those suggestions (longer polling intervals, staggered reads, etc.).
Appreciate any other ideas, especially if you’ve run multi-module setups under similar timing constraints.

Cheers,
Kyle

This is interesting. I'll have development look into this.