nRF52840 and RS-485 protocol

[Nikosant03]

Hi everyone,

I have some devices that are communicated through RS485. nRF52840 DK supports RS485 communication?

Do you have suggestions of how should I connect those devices with nRF52840?

Thanks in advance 

Nick

[TANGyouyou]

I'm sorry to disturb you, so I want to know whether you have solved this problem now.  The RX TX on NRF52840 cannot be directly connected to the RO DI pin of MAX485 chip, right?  So what's the solution?  Looking forward to your reply 

[hmolesworth]

Use the same supply (VCC and GND) for the MAX485 and the nRF52840 (typically 3 volts) and then the pins can be connected directly together (nRF52840 Rx to MAX485 R0, Tx to DI. IO pins control /RE and DE, again direct connection.

MAX1487-MAX491

[TANGyouyou]

My NRF52840 is powered by 1.8V voltage. How can I communicate with MAX485?  I have thought of using ilM2881-5 RS485 chip that is compatible with 1.8V voltage when I consulted the data, which is compatible with 1.8V voltage for transceiver. I wonder whether this scheme is feasible. If it is feasible, can THE RX TX of NRF52840 be directly connected with the RO DI pin of LTM2881 chip?  How to determine the control pins in NRF52840?  Looking forward to your reply https://www.analog.com/media/en/technical-documentation/data-sheets/2881fj.pdf

[hmolesworth]

Yes; as long as both RS485 and nRF52840 VCC and GND are the same then direct connections are ok. 1.8 volts on both is perfectly ok.

As an aside, the internal core runs at 1.3 volts so the external voltage (1.8 volts in your case) is typically stepped down with the on-board DC-DC convertor from 1.8 volts to 1.3 volts. Were you to use an external 3 volt supply, the 3 volts would be stepped down to 1.3 volts in the same manner but note the overall power consumption would be the same in both cases as fewer mA would be required at 3 volts than at 1.8 volts. The only difference would be if other devices on VCC consumed more power at 3v instead of 1.8v. You can ignore this comment, but this is often overlooked and if you happened to be using a 3.3 volt coin cell then an external 1.8 volt regulator is often pointless.

The /RE and DE can be controlled by any two i/o pins; they can also be connected together and controlled by a single i/o pin. The only reason to use 2 pins is if you want to listen (validate) your own transmission. My other post (link above) shows how to do this. See rs485-transmit-enable

Edit: I should add that RS485 allows very high baud rates. For symmetric transmit and receive baud rates the nRF52840 only allows up to 1MBaud, but if you can accept asymmetric baud rates you can use higher baud rates on transmit (eg 4MBaud) while only allowing 1MBaud on receive. If that is of interest I can post details.

[TANGyouyou]

I am sorry, I still have a little doubt, NRF52840 and RS485 communication is only nrF52840 output voltage is 1.8V, find compatible with 1.8V transceiver RS485 chip, (as I said before, I use the CHIP LTM2881,  Related Datasheet also in the previous answer) is level compatibility only ok?  Then nrF52840 UART communication protocol and RS485 communication protocol is matched, how to consider this?  Looking forward to your reply 

[hmolesworth]

It works, the data sheet naming can be confusing.

Pin VCC: (Pins B6-B8): Supply Voltage. Recommended operating voltage is 3V to 3.6V for LTM2881-3 and 4.5V to 5.5V for LTM2881-5. Internally bypassed to GND with 2.2µF
Use here at 3.3 volts (not nRF52840 VCC)

Pin VL (Pin A7): Logic Supply. Interface supply voltage for pins RO, RE, TE, DI, DE, DOUT, and ON. Recommended operating voltage is 1.62V to 5.5V. Internally bypassed to GND with 2.2µF
Use here at nRF52840 VCC = 1.8 volts and the digital signals between nRF52840 and RS485 digital inputs are compatible:

VL = nRF52840 VCC = 1.8 volts

VIL Logic Input Low Voltage 1.62V ≤ VL ≤ 5.5V Max value = 0.4 volts
VIH Logic Input High Voltage DI, DE, RE = 0.67•VL = 1.2 volts

VOL Output Low Voltage = 0.4 volts
VOH Output High Voltage = VL-0.4 = 1.4 volts

(Note DIN is not the same as DI)

Note do not connect RS485 VL to RS485 VCC, they are different voltages