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I am looking for a hardware recommendation and connection drawing (for breadboard testing) for a modbus I/O module. The hardware recommendation should include all parts and part numbers (Digikey or Mouser preferred) required to breadboard this project. ALL PARTS MUST BE 3V COMPATABLE. A PCB layout is not needed at this time. All parts should be through-hole (or include through-hole adapter) for easy breadboarding.
This project will require a PIC microcontroller (powered by a 3v battery) to read 4 discrete and 2 (0-3v) analog inputs along with 4 discrete outputs. A max485 or equal needs to be included to provide a rs-485 comm connection. The PIC must be capable of supporting standard Modbus RTU function codes and at least 8 (16bit) holding registers.
The 4 discrete inputs will receive data from a "dry contact" field connection. Optical isolation between the inputs and the PIC is required. Use resistors/zeners/MOVs as needed to protect the battery.
The 2 analog inputs will need to read a 0-3v signal. No special isolation or protection is required as the analog sensor will share the PIC power supply.
Two of the discrete outputs will be latching relays. The latching relays must be DPDT with one pole wired back to the PIC for relay status verification. The latch style is not critical. If the PIC cannot directly drive the relay coil(s), please include the necessary hardware. The relay should be capable of switching a 1-amp control signal at 120VAC.
Two of the discrete outputs will provide 3v at 100ma. The PIC pinouts may already have this rating.
Several companies already produce modbus I/O modules with the ratings and device protection I am looking for. This application is unique in that the analog sensors will be on the circuit board, otherwise this is a pretty standard circuit for industrial control. I do not want to infringe on other companies patents. This product is intended for an industrial environment. Robust longevity and reliability are more important than hardware cost. The final product will most likely be epoxy potted. No programming is required at this time. Please include all misc. capacitors, resistors, etc. needed to complete the project.