device recommendation needed for wifi - RS485 Modbus RTU bridge

[vinistois]

Hi!  My first post here, going to try to keep it brief!

I have a modbus RTU slave device which I want to talk to with an embedded micro.  The end goal is to use an app like Blynk (what a great app!!) to communicate with the modbus device.  The Blynk app is setup to talk to any arduino, Rpi, Intel board, Photon, Ti launchpad, etc etc, or even just an ESP8266. I am having a hard time choosing, and hoping to tap into some opinions here!

Here is what I'm looking for:

- ease of implementing Modbus RTU (485) communication with a slave device
- flexible power input - the Modbus product uses RJ45s for power and comms, ideally I'd like to plug in a single RJ45 to a board and it will get power and rx/tx from the device.
- easy wifi and/or BTLE, ideally on the board
- Just a few digital i/o's and analog inputs would be a bonus
- Web server functionality would be great, so that I can replicate the functionality of the blynk app from any remote device (with security!)
- MicroSD slot for datalogging
- battery backup and charging circuit also helpful

With all options (I think) I will need differential RS485 transceiver(s). I am looking at the SparkFun SP3485 RS485 breakout. It seems like I will need 2 of these to go full duplex... So does that mean I need a board with 2 UARTS?

I hope to implement the SimpleModbus libraries, as they are shown to work for most people in the research that I have done.

So far I am looking at the particle photon as it appears to have most of what I need.  I could make great use of the relay shield. No SD card, however...  Or the Photon Redboard.  Or the ESP8266, (as in the Adafruit Feather HUZZAH or Sparkfun Thing), also seems to be capable of what I need.  Or even something like the KTA-223 USB/RS485 Relay IO Board


Basically there seems to be many good choices here. How does one pick??

Thanks!





 

[ajb]

As far as I know, none of the development options for the ESP8266 permit low-level access to the UART(s), so I'd be a bit leery of trying to achieve a robust binary serial protocol implementation on it.  A basic MODBUS implementation that only needs to deal with one device might not be too bad, though.  An ESP8266 would certainly be the cheapest/simplest hardware option. 

For full duplex RS485 you only need one UART, but one RS485 receiver (rx) and one RS485 driver (tx).  You could use a pair of MAX485-equivalent transceivers and have one configured as driver and one as a receiver, or you could use a device that has a dedicated driver and a dedicated receiver.  Note that the master device's TX pair must connect to the slaves RX pair and vice versa.