HD25A Absolute Industrial Rugged Metal Optical Encoder

HD25A Features

  • NEMA size 25 package
  • Anodized milled aluminum housing with O-ring housing seal
  • 3/8 in. diameter shaft
  • Up to 15 devices on a single 6-pin telephone-type cable
  • 12-bit resolution
  • 7 milliseconds update time
  • EEPROM stores downloadable parameters
  • Field programmable resolution (2 to 4,096 CPR)
  • Remotely updatable firmware

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HD25A Product Description

Restriction of Hazardous Substances

The HD25A is a NEMA 25 sized absolute encoder designed for industrial applications. The HD25A optical encoder is a 12-bit absolute rotary position sensor, which reports a shaft angle within a single 360-degree rotation of a shaft. Unlike incremental, sometimes called relative style encoders, the HD25A provides true (absolute) shaft position, eliminating the need for a home or zero cycle after a supply voltage power cycle. The HD25A 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 as well by use of US Digital's SEI to USB interface device. For complete information about the SEI bus please refer to the SEI Communications Protocol webpage.

The HD25A also provides an analog output. The analog output 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. 

Product Specifications

View here or download the specifications


Parameter Value Units
Operating Temperature -25C to 70C C
Vibration (5 to 2kHz) 20 G
Shock, 11 mSec 60 G
Electrostatic Discharge, IEC 61000-4-2 ± 4 kV


Parameter Value
Size NEMA 25
Housing and Cover Material Anodized aluminum
Weight 16.91 oz.
Shaft Material Stainless steel
Shaft Diameter 0.3748 in.  (+0.0000 in. -0.0004 in.)
Shaft Optional Flat Size 0.80 in. long x 0.03 in. deep
Max. Acceleration 100000 rev / sec²
Max. Shaft Speed
  Non-sealed (mechanical)
  Sealed (mechanical)

15000 RPM
6000 RPM
Max. Shaft Torque

< 0.5 in-oz
3.5 in-oz typical
Max. Shaft Load

40 lb.
35 lb.
Max. Shaft Runout 0.0003 in. T.I.R.
Bearing Life @ 4 Pound Load 2.3 x 10^9 revolutions
Moment of Inertia 2.8 x 10 ^-4 oz-in-sec²
Technical Bulletin TB1001 - Shaft and Bore Tolerances Download


  • 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


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 -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.


In addition to the SEI interface, the HD25A also provides a 0 to 4.095 V analog output (1 mV per count). The absolute position value which the internal microcontroller sends to the 12-bit 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 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.


A Data H
B Busy+
C Analog+
D Power
F Ground
G Case Ground
H Data L
I Busy-
J Analog-


Title Date Description Download
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


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