6B21 Isolated, Field Configurable Analog Input

The 6B21 is a single-channel signal-conditioning module which provides a digitally-controlled isolated current loop output to a resolution of 12 bits. The output range can be selected to either a 0 to 20 mA or 4 mA to 20 mA process current. It is electrically and mechanically compatible with the 6B Series input modules and backplanes. The 6B21 takes a command from the host computer and converts it into an isolated process current suitable for interfacing with valves and actuators. The 6B21 offers a normal mode output protection of 240 V rms as well as 1500 V rms CMV isolation. In addition the actual current flowing in the current loop is read back by an on-board isolated V/F (voltage-to-frequency) converter.

Software Configuration

The 6B21 can be software-configured for address, output range, baud rate, data format, checksum status, slew rate, and start-up value. All programmable parameters are stored in the nonvolatile memory of the module.

Inside the 6B21

An on-board microcontroller communicates with the host through an RS-485 port to exchange command and status information. An included EEPROM stores calibration constants as well as configuration information. The microcontroller also controls the output DAC through an optically-isolated serial interface. The D/A converter drives the current loop through a protective V/I (voltage-to-current) converter. The output monitor is a V/F converter whose frequency is proportional to the loop current. This variable signal frequency is fed back to the microcontroller through an optical isolator. The microcontroller then scales the frequency signal and returns the current read-back data to the host computer. A DC/DC converter generates 25 V DC from the +5 V DC power supply to power the current loop and output circuitry.