Those who have seen my instructable on communication between Modbus RTU and a Raspberry Pi know that I'm planning a project for automating a greenhouse. I have made 2 small PCB's that can be put inside a projectbox. The link to the PCB's I will include later because I didn't receive them yet and I still need to test them.

As replacement for the arduino uno I will use a digistump. This is a very small ATTINY85 based board. Because the ATTINY85 doesn't have hardware serial I have used software serial to make serial communication work. Because most of my connected devices (pumps, solenoid valves,...) work on 24V I will include a converter onto the PCB. You can also use 12V which propably is a better choice.

Supplies

Hardware:

Digistump or digistump derived board
Some PCB or my custom PCB
terminal blocks
RS485 breakout
LDR or other sensor (optional)
10kOhm resistor
DC converter (recom)
Pin headers

Libraries:

Step 1: Installing the Libraries

During testing of the digistump I came across some problems. First I didn't know that the board didn't have hardware serial. I already had installed software serial so I have given this a try. At first this didn't work and after searching the net I found that my version of software serial didn't support the 16.5 mhz digistump. In this case you can overwrite the library in the folder C:\Users\youruser\Documents\Arduino\libraries\SoftwareSerial-master

Download the softwareserial library as a zip
Download the modbus library as a zip
Add the library's via sketch, use library, add ZIP library

Step 2: Installing the Driver

In order to use the digistump board you need to install the drivers first.

Download the drivers
Install the right version of the driver onto your computer
Open device manager
Take view
Display hidden devices
If you connect your digistump you will see the device (image)

Step 3: Adding the Board to the Arduino IDE

In the last step you installed the drivers to use the digistump. Now you still need to add the board to the arduino IDE.

Go to file, preferences
There you can add a link in the inputbox next to additional boards manager URL's
Add this link https://raw.githubusercontent.com/digistump/arduino-boards-index/master/package_digistump_index.json
Click ok
Go to tools, board, boards manager
Select the type contributed
Search digistump
Install digistump avr boards

After installing you will see the board under tools,board.

Step 4: Programming the Digistump

The attached code makes use of some registers that can be written or read. In this code I use the arduino to read the analog value of an LDR and to write the value to one of the registers. In the future I plan to make the code universal for using different sensor types and maybe even to change the default Modbus address.