EM1 Transmissive Optical Encoder Module

EM1 Features

  • Two-channel quadrature with optional index
  • Improved replacement for HEDS-9000 Series
  • Single 5 VDC supply
  • Resolutions from 32 to 2,500 CPR
  • Internal decoupling capacitor
  • Sink/source 8mA outputs

See more info below

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EM1 Product Description

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The EM1 is a transmissive optical encoder module designed to be an improved replacement for the HEDS-9000 series encoder module. This module is designed to detect rotary or linear position when used together with an encoder disk or linear strip. The EM1 consists of a lensed LED source and a monolithic detector IC enclosed in a small polymer package. The EM1 uses phased array detector technology to provide superior performance and greater tolerances over traditional aperture mask-type encoders.

The EM1 provides digital A & B quadrature outputs with an optional third output index channel. Each EM1 module is resolution-specific and is matched to the resolution of an encoder disk or linear strip. The EM1 module now supports all standard resolutions offered by the HEDS-9000 series encoder module and additional resolutions. The EM1 operates with a single 5V supply and provides single-ended outputs capable of sinking and sourcing 8mA. An internal 0.1 µF decoupling capacitor is designed into the EM1 to provide enhanced noise immunity over the HEDS-9000 series encoder modules.

For open collector and higher voltage applications, add the PC3 cable driver, or for differential cable driver outputs, add the PC4 cable driver. Encoder disks, linear strips, quadrature decoder chips, counter chips, computer interface boards, mating connectors, and cables are also available.

Product Specifications

View here or download the specifications


32 EM1-1-32-N          
50 EM1-1-50-N EM1-1-50-I        
64     EM1-1-32-N      
96 EM1-1-100-N  EM1-1-100-I        
100 EM1-1-100-N EM1-1-100-I EM1-1-50-N EM1-1-50-I     
120 EM1-1-100-N       EM1-0-120-N EM1-0-120-I
125         EM1-0-127-N EM1-0-127-I
127         EM1-0-127-N EM1-0-127-I
192 EM1-1-200-N EM1-1-200-I        
200 EM1-1-200-N EM1-1-200-I EM1-1-100-N  EM1-1-100-I  EM1-0-200-N EM1-0-200-I
250 EM1-1-250-N  EM1-1-250-I      EM1-0-250-N EM1-0-250-I
256 EM1-1-250-N EM1-1-250-I        
300         EM1-0-300-N EM1-0-300-I
360 EM1-1-360-N EM1-1-360-I        
400 EM1-1-400-N EM1-1-400-I EM1-1-200-N EM1-1-200-I     
500 EM1-1-500-N EM1-1-500-I EM1-2-500-N EM1-2-500-I EM1-0-500-N EM1-0-500-I
512 EM1-1-512-N EM1-1-512-I EM1-2-500-N EM1-2-500-I    
720 EM1-1-720-N EM1-1-720-I        
800 EM1-1-800-N EM1-1-800-I   EM1-1-400-I    
900 EM1-1-900-N EM1-1-900-I        
1000 EM1-1-1000-N EM1-1-1000-I EM1-2-1000-N EM1-2-1000-I    
1024 EM1-1-1024-N EM1-1-1024-I EM1-2-1024-N EM1-2-1024-I    
1250 EM1-1-1250-N EM1-1-1250-I        
1800     EM1-2-1800-N EM1-2-1800-I    
2000     EM1-2-2000-N EM1-2-2000-I    
2048     EM1-2-2048-N EM1-2-2048-I    
2500     EM1-2-2500-N EM1-2-2500-I    

For more information, see the EM1/HEDS Compatibility Guide.

Other resolutions may be available upon request.


Temperature -40 100                         C  
A/B Output Frequency 0 300 kHz  
Disk RPM 0 (18 x 10^6) / CPR RPM  
Linear Strip Speed 0 (3 x 10^5) / LPI inches/sec.  
Disk/Linear Strip Radial Position Tolerance ±.005      inch  


  • Specifications apply over the entire operating temperature range.
  • Typical values are specified at Vcc = 5.0V and 25C.
Supply Voltage 4.5 5.0 5.5 V Ripple < 100 mVpp
Supply Current, EM1-0- (linear strip)   27 33 mA LPI < 300, no load
  54 65 mA LPI ≥ 300, no load
Supply Current, EM1-1- (1" disk)   27 33 mA CPR < 500, no load 
  54 65 mA CPR ≥ 500, no load
Supply Current, EM1-2- (2" disk)   27 33 mA CPR < 1000, no load 
  54  65 mA CPR ≥ 1000, no load 
Low-level Output     0.5 V IOL = 8mA max.
  0.05   V No load
High-level Output 2.0     V IOH = -8mA max.
  4.8   V No load
Output Current Per Channel -8   8 mA  
Load Capacitance      100  pF  
Output Rise Time   110   nS  
Output Fall Time   100   nS  
Electrostatic Discharge     ± 4 kV IEC 61000-4-2



  • Specifications apply over the entire operating temperature range.
  • Values are for the worst error over full rotation.
  • Refer to the timing diagram below.
Symmetry X, Y 150 180 210 °e
Quadrature  Z 60 90 120 °e
Index Pulse Width Po 60 90 120 °e
Ch. I Rise After Ch. B or Ch. A Fall t1 50 100 200 ns
Ch. I Fall After Ch. B or Ch. A Rise t2 -10 15 25 ns


CPR: The number of Cycles (C) of the A or B outputs Per Revolution.

Cycle Error: An indication of cycle uniformity. The difference between an observed shaft angle which gives rise to one electrical cycle, and the nominal angular increment of 1/CPR of a revolution.

Index (I): The index output goes high once per revolution, coincident with the low states of channels A and B, nominally 1/4 of one cycle (90 °e).

LPI: Lines Per Inch. The number of Cycles (C) of the A or B output per inch of linear strip movement.

One Shaft Rotation: 360 mechanical degrees.

One Electrical Degree (°e): 1/360th of one cycle.

One Cycle (C): 360 electrical degrees (°e). Each cycle can be decoded into 1 or 4 states, referred to as X1 or X4 resolution multiplication.

PPR: The number of resolvable Positions Per Revolution of the encoder disk with x4 quadrature decoding.

Quadrature (Z): The phase lag or lead between channels A and B in electrical degrees, nominally 90 °e.

Symmetry: A measure of the relationship between (X) and (Y) in electrical degrees, nominally 180°e.


Mounting Screws 3.5-4 in-lbs


US Digital is the designer and manufacturer of the EM1 transmissive optical encoder module. The design of the EM1 provides electrical and mechanical compatibility with HEDS-9000HEDS-9100HEDS-9200HEDS-9040, and HEDS-9140 series modules.

The process of switching from the HEDS to the EM1 module should not require any mechanical or electrical changes. Simply use the EM1 and matching codewheel in place of the HEDS module and codewheel. The EM1 has a built-in index channel available on most resolutions, for both rotary disks and linear strips. The EM1 uses a US Digital designed codewheel with 2 tracks rather than 3 tracks for index versions. The EM1 offers improved output drive capability and will source and sink 8mA at TTL levels. 

Physically, the EM1 has no external wire loops which can interfere when mounting. The connector pins are 0.051" shorter than HEDS modules, while still providing .30" insertion depth. US Digital's EM1 offers custom resolutions.


Pin Description
1 Ground
2 Index
3 A channel
4 +5VDC power
5 B channel


Title Date Description Download
EM1 LED Die - PCN 1016 2/7/2013 As part of US Digital's continual assurance of supply strategy, we have qualified additional sources for our LED die used in our EM1 encoder module, which in turn impacts all of the following products:

EM1, E2, E3, E5, E6, H1, H15, H3, H5, H6, HB5M, HB6M, HD25, PE, S1, S2, S5, S6, T5 and T6

The device specification will remain the same, i.e. there will be no change to form, fit or function of the product(s) as specified by US Digital. The appropriate quality and reliability testing has been performed on representative products to ensure normal parametric distribution, consistent with US Digital's quality and reliability standards.
EM1 & EM2 Update - PCN 4199 1/14/2014 Based on our continuous process improvement program, US Digital is changing the current marking method for our EM1 and EM2 encoder modules to a serialization method. This change will allow for each module to have a unique code; the current marking method is based on a date code system that includes all encoder modules produced within a specific week / year. The serialization system will be based on a hexadecimal system. Download

Additional Information

Product Notes

  • US Digital® warrants its products against defects in materials and workmanship for two years. See complete warranty for details.



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