Data logger - separate power supply with isolation? - Page 1
Forum section: electronics / beginners

author: Sikanda
author_rank: Newbie
timestamp: September 18, 2024, 06:08:48 pm
content: I want to build a small data logger with a Raspberry Pi Zero W. The devices have either a 4 to 20 mA output or a RS485 bus connection. The supply voltage for the devices is 24VDC.The data logger has 4x M12 connectors where each connector has its own RS485 bus and a 4 to 20 mA input. Between the data logger and the devices are long cables. As the datalogger will be used in an industrial environment, I thought it would be a good idea to isolate the "sensor" interface from the Raspberry Pi. I have drawn a diagram to make it easier to understand. How would you plan the design? Would you leave out the isolated voltage and just provide ESD protection or would you plan the setup differently? Is my design too complicated?

author: shabaz
author_rank: Frequent Contributor
timestamp: September 18, 2024, 07:33:50 pm
content: Although it might not be essential in every circumstance, having isolation for RS-485 seems a reasonable thing to do. Same goes for ADCs. But personally if I were doing that, then I'd use a separate isolated DC-DC module for the ADC, i.e. not share with RS-485. Or maybe leave space on your board and wire links, so you have options.Attached diagram shows a slightly different thing, which was a controller using a Pi 3, and adding digital input/output isolation was easy of course, but if you look at the right of the diagram (green arrow) there was also an isolated supply and isolated SPI. Was very handy for subsequently adding a thermocouple board.Also, unless you've already ruled it out, you could consider a microcontroller, e.g. Pi Pico instead of Pi, unless you really need network or other Linux capabilities. Pi Pico can log to Flash or to micro SD for instance. Anyway, it's just a suggestion in case it sounds appealing.

author: PGPG
author_rank: Frequent Contributor
timestamp: September 18, 2024, 10:48:05 pm
content: I understand these 4 devices are powered from data logger and have no other connection to building earthing system. True?If not true then you should consider isolation between them.I would use microcontroller having 4 UARTs and one SPI and one more UART or SPI to communicate with Raspberry. if you want to isolate Raspberry from your own supply than you will have to isolate only one link.

author: thm_w
author_rank: Super Contributor
timestamp: September 18, 2024, 10:54:22 pm
content: Having one isolator for I2C means all the RS485 connections are then common.I guess you are planning to use something like SC18IM704PWJ which is not cheap.But neither is 4x RS485 ISO chips, Si85853D-IS $4 each.Plus Pi zero W only has one UART anyway.Can you have all the RS485 devices on one bus? Or you think that is too risky.

author: Sikanda
author_rank: Newbie
timestamp: September 19, 2024, 07:14:12 am
content: QuoteRaspberry Pi seems a bit over the top. But it is mainly for the ease of programming. I can plug in a USB stick, download the data and have it running. With a microcontroller, I would have to do a lot more programming.QuoteYes, all devices are powered by the datalogger.QuoteThat's a nice idea. I will think about it.QuoteYes, I was thinking about the SC18IM704PWJ, but maybe I will go for the one microcontroller suggestion from PGPG.The advantage of the SC18IM704PWJ would be that I could use a Linux driver for the chip and easily access each RS485 port.QuoteI rather have a direct link to each device. If for some reason there is a problem with the wiring the whole measurement gets disturbed.

author: ArdWar
author_rank: Frequent Contributor
timestamp: September 19, 2024, 08:11:54 am
content: If you're already using SPI isolator then might as well use UART to SPI bridge (SC16IS752, or MCU for the whole 4UART+ADC as mentioned above) instead of I2C.Personally I'd prefer to stay away from I2C isolator unless there are good reason to use one.