The second version, which is currently deployed at RATS's R.Viesti -network, is based loosely on
Arduino concept.
Initial prototype and firmware that this design is based on was made by OH7LZB. Files for the
project can be found at here. Pictures below are from
v1.0 boards, which have a slight error that has been corrected on v1.1
files.
Programming interface on these pagers is quite simple, you can use a simple rs232 - 3.3V converter (for example an old cell phone data cable) with a single diode.
Here's
a sample code plug file that's set up on 144.975 MHz.