Functional Description

6B21The 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.

6B21 FBD

Figure 1. 6B21 Functional Block Diagram

6B21 FCD

Figure 2. 6B21 Field Connection Diagram

Output Ranges

0 to 20 mA

4 mA to 20 mA


RS-485 Interface

Slew Rate
(Software Configurable)
Immediate Change
0.126 mA/second
0.250 mA/second
0.500 mA/second
2 mA/second
4 mA/second
8 mA/second
16 mA/second
32 mA/second
64 mA/second
128 mA/second

Model 6B21

Range Description
(Software Configurable)
0 mA to 20 mA ±0.02 % FSR ±0.05 % FSR
4 mA to 20 mA ±0.02 % FSR ±0.05 %FSR

6B21 Specifications
(typical @ +25�C and Vs = +5 V dc)

Description Model 6B21
Output, Software Selectable
Current, Operating Ranges 0 mA to 20 mA or 4 mA to 20 mA
Current, Overrange +2 mA ±200 µA
Load Resistance 0 to 750 Ω
Protocol RS-485
Baud Rates, Software selectable 300K, 600K, 1.2K, 2.4K. 9.6K, 19.2K
Output Current
Initial Accuracy
   Output Offset ±5 µA (±15 µA maximum)
   Span ±0.02% Full Scale Range
(±0.05%, max.)
Accuracy vs. Temperature
   Output Offset Current ±1 µA/�C
   Gain ±50 ppm/�C
Resolution ±0.02% Full Scale Range
Nonlinearity ±0.02% Full Scale Range
Output Noise, 0.1 Hz to 100 Hz Bandwidth 1 µA peak-to-peak
Bandwidth, -3 dB 3 Hz
Conversion Rate 100 samples/second
Setting Time 1 ms to ±0.1% Full Scale Range
Output Slew Rate
Software Selectable (11 Binary Ranges) 0.125 mA/sec to 128 mA/sec
Common-Mode Voltage (CMV)
Output-to-Input and Power 1500 V rms continuous
Common Mode Rejection (CMR)
Output-to-Input and Power @60 Hz 108 dB
Output Protection 240 V rms, continuous
Ouput Transient Protection ANSI/IEEE C37.90.1-1989
Output Readback
Initial Accuracy @ +25�C
   Output Offset Current ±100 µA
   Span ±0.5% Full Scale Range
Accuracy vs.Temperature
   Output Offset Current ±5 µA/�C
   Gain ±200 ppm/�C
Resolution ±0.5% Full Scale Range
Nonlinearity ±0.5% Full Scale Range
Power Supply
Voltage, Operating +5 V dc ±5%
Voltage, maximum safe limit +6.5 V dc
Current +250 mA
Mechanical Dimensions 2.3" x 3.1" x 0.79"
(58.4 mm x 78.7 mm x 19.1 mm)
Temperature Range
   Rated Performance -25�C to +85�C
   Operating -25�C to +85�C
   Storage -40�C to +85�C
Relative Humidity 0 to 95% @ +60�C, noncondensing
Specifications subject to change without notice.
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