Absolute rotary encoder | Optical | 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 telephonetype cable
  • 12-bit accuracy
  • 7 milliseconds update time
  • EEPROM stores downloadable parameters
  • Field programmable resolution (2 to 4096 CPR)
  • Remotely updatable firmware
Manufacturer:US Digital


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 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. Please note that with the HD25A analog output option only one device may reside on a SEI bus.

Attribute NameAttribute Value
Documentation from Manufacturer
Output Signal Type
Measuring Principle
Resolution Singleturn (bit)
Resolution Multiturn (bit)
SEI Bus, Analog
Housing Diameter (mm)
Shaft Type
Not Specified
Shaft Diameter (mm)
Revolutions - max. (rpm)
6000, 15000
Protection Rating Housing
Not specified
Connector, Cable



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