HD25A Absolute Industrial Rugged Metal Optical Encoder

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  • HD25A
  • HD25A
  • HD25A


  • NEMA size 25 package
  • Anodized milled aluminum housing with O-ring housing seal
  • 3/8" diameter shaft
  • Up to 15 devices on a single 6-pin telephone-type cable
  • 12-bit accuracy
  • 7 milliseconds update time
  • EEPROM stores downloadable parameters
  • Field programmable resolution (2 to 4096 CPR)
  • Remotely updatable firmware

The HD25A is a rugged absolute encoder designed for heavy duty industrial use. This absolute encoder is a non-contacting optical rotary position sensor which reports the shaft angle within a 360...

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Product Specifications

Operating Temperature -25C to 70C
Storage Temperature -40C to 100C

98% non-condensing
100% condensing (NEMA IP65)
Vibration (5 to 2kHz) 20G
Shock, 11 mSec 60G

Size NEMA size 25
Housing and Cover Material Anodized aluminum
Weight 16.91 oz.
Shaft Material Stainless steel
Shaft Diameter 0.3748 in.  (+0.0000 in. -0.0003 in.)
Shaft Optional Flat Size .80 in. long x .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 Total Indicated Runout 0.0003 in.
Bearing Life in Millions of Revs. (124 / load in lbs.) ^3
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.
Supply Voltage 5.5 12 16 V
Supply Current



Analog Output Impedance 950 1000 1050 Ohms
Zero Scale Analog Voltage 0 0.0005 0.003 V
Full Scale Analog Voltage 4.079 4.095 4.111 V
Output Noise (Analog version)   10   mVrms
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 this value which is always the most current position.

 Parameter Default value
 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.



A DataH Blue w/ White stripe
B Busy+ Brown w/ White stripe
C Analog+ Orange w/ White stripe
D Power Green w/ White stripe
E NC Gray w/ White stripe
F Common White w/ Gray stripe
G Case Ground White w/ Green stripe
H DataL White w/ Blue stripe
I Busy- White w/ Brown stripe
J Analog- White w/ Orange stripe
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

Product Description

Restriction of Hazardous Substances

The HD25A is a rugged absolute encoder designed for heavy duty industrial use. This absolute encoder is a non-contacting optical rotary position sensor which reports the shaft angle within a 360 range. The HD25A can be used for automation, motion control and robotic applications. As opposed to incremental encoders, the HD25A reports the absolute position rather than just the change in position. The HD25A is a true absolute encoder over one revolution, and also has a multi-turn mode. When powered up, it does not require a home cycle, even if the shaft was rotated while the power was off. In multi-turn mode, it tracks the position in a 32-bit counter as long as the power supply is maintained.  Internally, an infrared LED flashes through a circular bar code onto a linear array sensor. The microcontroller decodes the image into a unique position. All user programmable parameters such as resolution, origin, direction, and mode are permanently stored in an internal EEPROM. 

The interface of the HD25A has 2 separate outputs, one utilizes our digital SEI (Serial Encoder Interface) bus and the other is an analog output. The SEI bus is a simple, quick, convenient network of devices interfacing to a RS232 serial port. The SEI bus supports 1 to 15 devices on a single cable up to 1000 feet long (RS-485 like). The analog output provides an analog voltage proportional to the angular position, with 12-bit resolution.

The AD2B adapter provides an interface to an RS-232 port and the SEI-USB provides an interface to a USB port. One of these products is required in order to interface the HD25A to a PC via our SEI bus. The wall-mount PS-12 power supply furnishes the power for all devices on the SEI bus.

Our absolute encoders may be used in many stand alone applications that do not require a PC interface. For these applications we provide detailed communications protocols for all of our absolute products (see the SEI Absolute Encoder Communications Protocol page).

US digital provides software free of charge for all products which require software for operation. The software can be used as is or it can serve as an example for creating your own custom application. The absolute encoder software utilizes our SEI Explorer software to configure and communicate with the product from a PC platform.

Analog Output:

The HD25A has a 12-bit DAC and 2 dedicated analog output line (Analog+, Analog-). This DAC has a full range of 0 to 4.095V, which is 1mV per bit. The value which the internal microcontroller sends to that DAC is the same as the digital value that it sends to the host. Since the resolution (which represents the number of codes per revolution) is field programmable, the range of the DAC will also follow that setup. Our default resolution is 3600 CPR which yields 1 count per tenth of a degree. This makes the DAC output equal to 1mV per tenth of a degree or 0 to 3.599V. When the DAC needs to have the full range to 4.095V, the single turn resolution should be set to 4096. This is easily done with the available software which runs on a PC. See referenced documents information on the AD7 HD25A Breakout Box.

Available Options:

  • Shaft seal to protect the encoder from liquids. Note that this option adds about 3.5 in-oz. of shaft drag.
  • 3/8" diameter shaft with or without flat.


In applications where a failure could result in an unacceptable loss, we recommend that your system be designed to include two redundant encoders with the outputs from both continuously compared to make sure that they agree. If there is a discrepancy, the system should be designed to automatically shut down as a fail-safe measure to minimize the risk of damage or danger. This product is not certified for applications where a failure could result in a costly, dangerous, or life threatening situation.

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.

Cable/connector assembly not included and must be ordered separately.

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