Raspberry Pi Analog Input Module, 8 Channels 4-20mA Input, RS485, Modbus Interface
The VP-EC-8AI is an industrial version Raspberry Pi 8 Channel Analog Input Module by VP Process Inc.
Each input accepts a 4-20 mA Input into a 150 Ohm load resistor. The interface supports RS485 Modbus RTU and plugs directly into the Raspberry Pi's GPIO port (26 Pin). Raspberry Pi versions supported are from the original to the new RPi 3.
The 12 bit Analog to Digital convert (ADC) is the popular Microchip MCP3208 and is controlled via the SPI interface. The dipswitch is read by the GPIO lines, and the UART is driven by the Rx and Tx lines and a RS485 direction line, also from the GPIO lines.
Another feature of the VP-EC-8AI is the 8 channel Green LED's used for status indication. In the test code provided, the channel LED lights if the input mA is above a preset level, typically 2.5 mA.
This interface can be DIN rail mounted by adding the optional DMB-4774 enclosure.
- Power: 12 to 24 VDC
- Network: RS485, N81, Modbus RTU Protocol (or other protocols as required by the Raspberry Pi)
- Modbus ID: Dipswitch Selectable
- Analog Inputs: 8 Channels 4-20 mA into 150 Ohm load, PTC Fuse Protection
- Resolution: 12 Bit ADC (MCP3208 A/D Chip, SPI Interface)
- Dimensions: 3.4" wide x 6.15" long
- Power On: BLUE LED
- Analog Input: 1 thru 8: GREEN LED ON when input above 2.5mA (74HC595 Driver)
- RS485: GREEN LED Rx, RED LED Tx.
- DIN Rail Enclosure Bud Industries Model: DMB-4774
Sample Source Code
This sample code uses the Wiring Pi library and the Geanie C compiler. The code has been tested for:
- Reading the Dipswitch
- Reading the 8 channels of AD Counts from the MCP3208 AD Converter
- 20 mA into a 150 Ohm load resistor = 3.0 VDC
- MCP3208 uses the 3.3 VDC as a reference, therefore 20 mA = 3/3.3 * 4096 = 3723 AD Counts
- A1 thru A8 LED driver using the 74HC595 Serial Shift Register
For the source code listing, please see the Post:
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