Tech Note

DIN Rail Mount Single-Channel Data Acquisition Modules Offer 15-Bit Resolution & 500Vrms Isolation

Dataforth Corporation, Tucson, Arizona announces the introduction of its new SCM9B-D100 Series of isolated and software configurable single-channel data acquisition modules. Twenty-seven models of the SCM9B-D100 make up a family that converts analog input signals to digital data and transmits via RS-485 to a controller, which may be a computer or other processor-based equipment. The modules interface directly to thermocouples, RTDs, thermistors, strain gages, voltages, currents, timer and frequency inputs, and digital input signals. One model provides digital output signals. In addition, the modules perform signal conditioning, scaling, linearization and conversion to either linearized ASCII data values or Modbus RTU data values.

Features such as address, baud rate, parity, and echo are selectable using simple commands over the RS-485 port. The selections are stored in nonvolatile EEPROM, which maintains data even after power is removed. Configuration software is included and is described below.

Data is acquired on a per-channel basis so the user only buys as many channels as is required. The modules can be mixed and matched to fit any application. They can be placed remote from the host and from each other. Up to 247 modules can co-exist on a twisted pair of wires over a distance of 4000 feet (1220 meters) by using RS-485 with repeaters.

All modules are supplied with screw terminal plug connectors that allow system expansion, reconfiguration or repair without disturbing field wiring.

Theory Of Operation

Each Dataforth module is a complete single-channel data acquisition system. Each unit contains analog signal conditioning circuits optimized for a specific field input type. Input signals are converted to digital data with a microprocessor-controlled integrating A/D converter. Offset and gain errors in the analog circuitry are continuously monitored and corrected using microprocessor techniques. The D100 converts the digital signal data and stores the resultant data in a memory buffer. The modules continuously convert data at the rate of 8 conversions per second and store the latest result in the buffer.

Host processors may request data by sending a query to the module. The D100 will instantly respond by communicating the memory buffer data back to the host processor. Each module is identified by a unique user-programmable address that allows modules to be interrogated in any order.

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