6B21 Isolated, Field Configurable Analog Input


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.

Inputs and Communications

6b21 (1)

Figure 1. 6B21 Functional Block Diagram


Figure 2. 6B21 Field Connection Diagram

Output Ranges
0 to 20 mA
4 mA to 20 mA

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

Available Models

 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)

Model 6B21 
Outputs, 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)
±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
+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
-25°C to +85°C 
 -40°C to +85°C
Relative Humidity  0 to 95% @ +60°C noncondensing 

Specifications subject to change without notice.