TIA485 Signalling Interface Cards¶
The system interfaces with the real world using a series of TIA485 interface cards. These cards are designed in a flexible manner to allow usage across the site and the future extension.
Each card has a total of 16 analog inputs, and 16 digital outputs.
Two part boards¶
The interface card is split into two discreet boards for ease of maintenance.
Back board¶
The back board contains the components that are generally more resilient, including:
- voltage dividers
- LEDs
- TIA485 bus connectors (in and thru)
- Programming connector
- Additional breakout headers
Schematic¶
Click here to see the schematic
Front board¶
The front board includes all the delicate electronics, and will be easily swappable. The boards have been designed in such a way as all the configuration is done on the rear board, meaning when a front board is replaced there are no additional steps to get the board up and running.
Schematic¶
Click here to see the schematic
Analog inputs¶
Analog inputs are provided by a 74HC4067 chip. This allows a single analog pin
to read 16 inputs using a set of four address pins.
Voltage division¶
The input to each of the analog mux channels goes through a voltage divider to bring a potential 50v down to 5v. 50v was chosen to accommodate the highest voltage expected in the field: 48v.
Digital Outputs¶
Digital outputs are provided by 74HC595 chips chained together. The outputs of
the 595s then go through a TPD2005, a driver chip, which converts the 5v logic
level to the appropriate output voltage (24v or 48v).
Monitoring¶
To provide monitoring of the output chips, a second 74HC4067 analog mux device
is included. This allows the system to detect whether each output of the driver
chips is working as expected.
Processing¶
An ATMega328p and appropriate support circuitry is included on the board to
read the inputs, write to the outputs, and generally control the board.
Commmunication¶
The field boards use Modbus to communicate with the head end device. Modbus is
used over a TIA485 bus. There is an on-board MAX1487 device to handle the
conversion between TTL (which the ATMega outputs) and TIA485 levels.
Protocol¶
The devices use Modbus as a communication protocol on top of the 485 bus.
Coils & registers¶
They are configured with:
- 16 coils for digital output
- 16 input registers for analog input from the field
- 16 input registers for monitoring of digital outputs
- 1 input register to monitor the 24v rail
Addressing¶
The addressing for the device is configured using a 8 channel DIP switch, placed on the rear board.
Valid addresses are 1 - 247.