6B21

Rev. 0 | Page 2 of 8

GENERAL DESCRIPTION

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.

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.

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 VDC from the +5 VDC power supply to

power the current loop and output circuitry.

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Figure 2