How to read data from RS485 port of a device using raspberry pi

Question

I need to get data from RS485 port of a transducer in to raspberry pi to connect it to IOT. Any suggestions

rew · Answer

Hi Ravi,

I sell a board that is meant to do DMX. DMX works based on RS485. I'd like to know if we can hack it to work with your RS485 sensor.

I was going to say I would send you a board for free. But the most practical thing is that you order the cheapest item from my shop so that I have your contact details. So its "almost free".

I'll be glad to work with you attempting to get it to work. For example, in your case it might be better for you if we leave out the bulky DMX connectors. So that will be the first thing we'll need to discuss. How does that sound?

P.S. The "uart" on the pi3 isn't that good. The processor has two UARTs and one of them is "reasonable" the other one is "mediocre". And the better one was assigned to bluetooth... You can "steal" it back, so that the better uart now goes to the GPIO pins like on the PI2 and earlier models.

balearicdynamics · Answer

Hello Ravi, PI3 has a good support for the UART serial connection. If I am not wrong the difference between the RS232 and RS485 is that the second is based on differential currents on the signals to compensate / avoid some kind of external interferences and work with higher currents than the 5V of the 232. What you need I think it is a small circuit to convert the 485 industrial serial connection to the TTL level of the UART in the PI, as well as you should do if you will use a 232, with a Max chip (just for an example). Accordingly with these notes: serial - Difference between UART and RS232? - Electrical Engineering Stack Exchange In my opinion, also to avoid to spend a lot of time to make a conversion circuit by yourself, FTDI already provide USB to 485 adapters together with the USB to 232 based on their FTDI232 and FTDI485 chips. You can get the chip specifications to integrate the conversion circuit in your own project or just buy an adapter (at least to test the prototyping phase and develop proper software on the Raspberry PI). Below are the links: https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&sqi=2&ved=0ahUKEwj4u-ey-7DNAhVKL8AKHU7EB-… https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=4&cad=rja&uact=8&sqi=2&ved=0ahUKEwj4u-ey-7DNAhVKL8AKHU7EB-… The FTDI converter datasheet (pdf) : http://www.ftdichip.com/Support/Documents/DataSheets/Cables/DS_USB_RS485_PCB.pdf Depending on the quality and current range of your RS485 source I suggest to evaluate to eventually optoisolate the signals. Enrico