DW2 24 Bit A/D Dual Channel Load Cell Peripheral/Controller Revision 1507 (Preliminary data)

Overview

The DW2 is primarily an expansion board to allow SPLat controllers to work with load cells. It is also a controller in its own right and has a small amount of digital I/O as well as two channels of high resolution, fully isolated analog input. The actual function of all I/O is controlled by the users program. SPLat Controls provide the board as standard with a program that allows it to be a peripheral for SPLat controllers. In addition to this, it can also be a MODBUS peripheral or be programmed to provide a user defined protocol. It is programmed using exactly the same language and tools as all other SPLat controllers and the programming tools are free.

Features

• 2 channels of 24 bit sigma-delta analogue to digital conversion (Nuvoton NAU7802)
• Full galvanic isolation (500 Volts)
• Built in low noise programmable gain amplifier , gains: 1, 2, 4, 8, 16, 32, 64, 128
• Built in 50/60Hz notch filters
• Built in programmable excitation voltage for bridge sensors
• Supports 4 wire sensors on both ports, primary port also supports 6 wire sensors
• 10 – 30 VDC operation
• 32 bit microcontroller
• Fully programmable using SPLat Controls free development tools
• USB port for programming, data logging, debug etc
• TTL communications interface (3.3 Volt logic)
• RS485 port (Xwire, MODBUS, user defined comms)
• Xwire expansion port for connection to other SPLat controllers/peripherals
• All communication ports are independent of each other
• 4 bi-directional I/O points, can be inputs or outputs (NPN), 250mA per output
• On board DIP switch is under program control
• On board red/green status LED is under program control

Power

The DW2 requires a DC power source between 10 – 30 VDC and the maximum current required by the board in normal operation is less than 100mA. Power is to be applied to the terminals marked “+” and”0V”. The DW2 does not have any specific requirements in regards to ripple, providing the input voltage does not dip below 10 VDC. However, for best results use a DC power supply with a regulated output.

Digital I/O

The digital I/Os are marked “0” through “3”. When used as digital inputs, the input will be activated when the input terminal is connected to the 0V terminal. This is the same terminal to which the power supply ground or negative terminal is connected. Thus the inputs are ground sensing or NPN.

When used as digital outputs, the load is to be connected between the power input “+” terminal and the I/O point terminal. The controller activates the load by switching to “0V” and thus the outputs are NPN. Maximum load current per output is 300mA and the sum of all output currents must less than one amp. Do not use voltages higher than the supply voltage to power the switched load. The board contains protection diodes to clamp inductive loads.

RS485

The on board RS485 interface can be used for Xwire (SPLat expansion bus), MODBUS RTU (slave or master) or user defined communications. Baud rates up to 250k are supported along with most UART profiles. If termination is required, the terminals resistor should be installed into the terminal block along with the connecting wires. For short wire runs and low data rates, termination is not usually required.

Xwire ports

The DW2 has two Xwire ports which allows for easy daisy chaining in multi-board configurations. Both connectors have exactly the same function and are totally interchangeable. Xwire is a half duplex protocol that allows SPLat controllers and peripherals to be connected together to form larger systems.

Status LED

The status LED can be programmed to be off, red or green, under program control.

DIP switch

The on board DIP switch is under program control and can therefore be used for such things as setting the address of the board for communications applications, field selecting options etc.

USB port

The USB port can only be used for program download and as a virtual serial port link to a PC for such things as debug messages, data logging etc.

TTL port

This 3.3 Volt TTL port can be used under program control for full duplex communications. MODBUS and user defined communications are possible and a wide variety of UART profiles are supported.

Load Cell 1 port

This port supports either 6 wire or 4 wire bridge sensors, configured by the option links located just behind the screw terminals for port 1. Half and quarter bridges can also be accommodated using external resistors. To allow the analog to digital converter (A/D) to be ratio metric, the A/D chip provides the excitation voltage for the sensor. This voltage is programmable in the range 2.4 – 3.3 Volts when a single sensor is used and 2.4 – 2.7 volts when both channels are used when using 350 Ohm or greater bridge sensors. The excitation voltage is applied to both A/D channels and is not individually selectable. As a general rule, the excitation voltage should be set as high as possible to extract the most difference signal from the bridge. Shielded cable should always be used to connect the sensor to the DW2.

NOTE1: When using both channels of the DW2, both sensors must be 4 wire types and the 4 wire selector jumpers must be installed.

NOTE2: A future revision of the DW2 will allow higher excitation voltages to be used, in the range 2.4 to 4.5 Volts. The 1507 version of the board will only allow excitation voltages in the range of 2.4 – 3.3 volts for a single load cell and 2.4 – 2.7 volts for two connected 350 Ohm load cells.

Load Cell 2 port

The 2^nd channel only supports 4 wire sensors or half and quarter bridge sensors using external resistors. Shielded cable should always be used to connect the sensor to the DW2. See paragraph for load cell port 1 above for excitation voltage selection and rules.

Mounting

The DW2 is available with or without DIN rail mounting. Please specify at the time of ordering. In all cases the DW2 should be mounted as far away as possible from electrical and magnetic noise sources. The use of a metal enclosure to house the electronics is recommended.