GOING FROM BREADBOARD TO FIELD UNIT
Herewith below pictures of the Master and Slave units. All units are built on standard Arduino 1 shields, using strip pin male connectors, which allow to plug the shields into the Micro board directly. The schematics of these units are given in Part 3.
1. The Slave shield is the simplest and easiest to make, as it only comprises the +2.5V Precision Reference and the RS485 Modem. The Modem is mounted on strip pin female connectors to allow quick assembly/removal.
2. The Master unit comprises not only the Precision Reference and the Modem, but also the 6 Active Lowpass filters and their -2.5v PSU. Obviously all this junk cannot be fitted to just one shield. I made one filter shield, complete with all but the Modem, but, as can be seen, it is far too crowded for reliability. This shield would have to be plugged into another intermediate one carrying the Modem and this assembly would be plugged into the Micro: TOO COMPLEX. I therefore decided to assemble a simple Master shield with Precision Reference and Modem leaving the filter assembly to a completely separate board.
3. NOTE: to simplify the adjustment of the +2.5V Precision Reference Voltage, I have replaced R1 & R2 with a 10 K Trimpot, with the centre pin connected to the LM336 adjust pin (see schematics).
The entire system works fine with the Master and Slave Laptops, but I have yet to connect the external supply to the Slave, as I am afraid of ground loops voltages which might affect and damage the modems. Best leave this test to next week.