A2 Absolute Optical Shaft Encoder

A2 Features

  • Sleeve bushing or ball bearing versions
  • 12-bit resolution and resolution field programmable from 2 to 4,096 codes per revolution (3,600 factory default)
  • Full 360-degree range, 7 msec update time
  • Low power drain of 18.5 mA max., and 2.5 mA in sleep mode
  • Field programmable parameters such as setting zero position point (demo software available for download below)
  • EEPROM stores downloadable parameters
  • 9,600 baud default data rate adjustable up to 115K baud
  • 12-bit analog voltage output option (0 to +3.599 volts factory default setting. Field programmable up to 0 to +4.095 volts)
  • Multi-turn mode (note: power must be maintained to prevent reset to zero)
  • -25 to 70 degrees C. operating temperature

See more info below

Configure the A2

A2 Product Description

Restriction of Hazardous Substances

The A2 optical encoder is a 12-bit absolute rotary encoder which reports a shaft angle within a single 360-degree rotation of a shaft. Unlike incremental, sometimes called relative, style encoders, the A2 provides true (absolute) shaft position eliminating the need for a home or zero cycle after a supply voltage power cycle. The A2 communicates over a RS 485 style serial bus utilizing US Digital's SEI (Serial Encoder Interface), which allows for simple, quick and convenient networking of multiple SEI devices on a single network. PLCs, motion controllers, and computers can also reside on the SEI bus by using US Digital's SEI to USB interface device. For complete information about the SEI bus, please refer to the SEI Absolute Encoder Communications Protocol webpage.

The A2 is also available with an optional analog output. The analog output option provides a maximum voltage range of 0 to 4.095 volts with 12-bit resolution. The output voltage can be scaled by simple SEI commands to provide user-defined voltage ranges. From the factory, the analog output voltage is set to 0 to 3.599 VDC range. Please note that with the A2 analog output option, only one device may reside on an SEI bus.

Product Specifications

View here or download the specifications


Operating Temperature -25 to 70 C
Vibration (5Hz to 2kHz) 20 G
Electrostatic Discharge, IEC 61000-4-2 ± 4 kV


Max. Acceleration 100000 rad/sec² 100000 rad/sec²
Max. Shaft Speed
100 RPM 10000 RPM
Typical Shaft Torque 0.5 in-oz (S-option)
0.3 in-oz (N-option)
0.05 in-oz
Max. Shaft Loading 2 lb. dynamic 2 lb.
Bearing Life > 1000000 revolutions L10 = (90/Fr)³ *
Where L10 = bearing life in millions of revs, and
Fr = radial shaft loading in pounds
Weight 2.9 oz. 2.9 oz.
Max. Shaft Runout 0.0015 in, T.I.R. 0.0015 in, T.I.R.
Mounting Plate Screw Torque 4-6 in-lbs 4-6 in-lbs
Max. Panel Nut Tightening Torque 20 in-lbs n/a
Technical Bulletin TB1001 - Shaft and Bore Tolerances Download

* only valid with negligible axial shaft loading.


  • Specifications apply over entire operating temperature range.
  • Typical values are specified at Vcc = 12V and 25C.
Parameter Min. Typ. Max. Units
Supply Voltage 7.5 12 16 V
Supply Current @ 12V supply


Analog Output Impedance   51   Ohms
Zero Scale Analog Voltage 0 2 12 mV
Full Scale Analog Voltage 4.066 4.095 4.124 V
Output Noise (Analog version)   10   mV rms
Differential Nonlinearity (Analog version) -1.0   1.0 LSB
Integral Nonlinearity (Analog version) -1.0   1.0 LSB
Absolute Accuracy (SEI interface version)   0.18 0.25 Degrees
Angle tracking speed
  Single-turn mode
  Multi-turn mode
 Position Update Rate (1)      7 msec.

(1) The internal microcontroller takes a snapshot of the disk every 7 msec. and stores the position in memory. It responds immediately to a "report position request" by sending the most recently computed position.


 Parameter  Default value  Volatile?
 SEI address  0  Non-volatile
 Resolution  3600  Non-volatile
 Origin offset  0  Non-volatile
 Baud rate  9600  Volatile
 Mode  0 (1)

(1) Mode is always restored from non-volatile EEPROM on power-up; however, there are separate SEI commands for setting the RAM copy only, or both the RAM copy and the non-volatile EEPROM copy. For an explanation of the Mode bits see SEI Absolute Encoder Communications Protocol.


The analog version of the A2 has a 12-bit DAC on the output which feeds to 2 lines that are otherwise used for the BUSY handshaking pair. This DAC has a full range of 0 to 4.095 Volts which is 1 mV per count. The absolute position value which the internal microcontroller sends to that DAC is the same as the digital value that it sends to the host over SEI. Since the resolution (which represents the number of codes per revolution) is field programmable, the range of the DAC will also follow that setup. The default resolution is 3600 codes per revolution which yields 1 count per a tenth of a degree. This makes the DAC output equal to 1 mV per tenth of a degree or 0 to 3.599 Volts. When the DAC needs to have the full range to 4.095 Volts, the single turn resolution should be set to 4096. This is easily done with the available software which runs on a PC.  Cable CA-MD6A-SS-MD6-6FT and the SEI-USB adapter are needed to interface the A2 analog version to a USB port or RS-232 serial COM port.

Please Note: The BUSY handshaking lines are replaced by the analog output option. This means that only one device will be able to be connected to the SEI bus when using the analog output option.


1 Ground
2 Busy+
3 Busy-
4 Power
5 Data L
6 Data H


Title Date Description Download
EOL Sealed Housing Option - PCN 1021 4/11/2013 This PCN is a formal notification that US Digital is discontinuing the Sealed Housing option for the following products:
  • A2 Absolute Optical Encoder
  • A2T Absolute Optical Inclinometer
  • H1 Ball Bearing Optical Shaft Encoder
  • H3 Ball Bearing Optical Shaft Encoder
  • S1 Optical Shaft Encoder
  • S2 Optical Kit Encoder
The Sealed Housing option provides the encoder with low level capability of surviving in moisture environments, however the encoder is NOT water proof or intended to be used in applications where this is required.
A2/HD25A Product Lines Detector Upgrade 8/18/2014 As part of our ongoing continuous improvement efforts, US Digital is updating our A2 and HD25A product lines design by utilizing surface mount devices for the detector and LED. Previously these were through hole devices. This change is transparent with the exception of minor cosmetic differences for the A2 kit style encoder, and Spacer Tool used in kit assembly process. Download

Additional Information

Product Notes

  • Cables and connectors are not included and must be ordered separately.
  • US Digital® warrants its products against defects in materials and workmanship for two years. See complete warranty for details.


Guides and Additional Documentation


3D Model Downloads

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