EM1 / HEDS Reference Guide

The EM1 and HEDS products are transmissive optical encoder modules. These modules are designed to detect rotary or linear position when used together with a codewheel or linear strip. The EM1 and HEDS modules consist of a lensed LED source and a monolithic detector IC enclosed in a small polymer package. These modules use phased array detector technology to provide superior performance and greater tolerances over traditional aperture mask type encoders.

Both the EM1 and HEDS module provide digital quadrature outputs. The EM1 comes standard with a third index channel output on all resolutions. The HEDS is available with a third index channel output on some resolutions.

The EM1 and HEDS transmissive optical encoder module are powered from a single +5VDC power supply. Additional power supply voltages for the EM1 will be available in the near future. The EM1 single-ended outputs are capable of sinking or sourcing 8mA each.

The resolution of the modules and encoder disks or linear strips must match. Two mounting holes are provided to accept 4-40 machine screws. Both the EM1 and HEDS have identical mounting and pin-out configurations.

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


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 the Agilent HEDS-9000, HEDS-9100, HEDS-9200, HEDS-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 all 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. Non-index codewheels are interchangable between the EM1 and HEDS modules. 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 and special resolutions.

Disk Compatibility

 1" Disks2" Disks
CPRNon-indexWith IndexNon-indexWith Index
32   EM1-1-32    
50 HEDS-9100-S00 HEDS-9140-S00    
64       EM1-1-32
96 HEDS-9100-C00 HEDS-9140-C00    
100 HEDS-9100-C00 HEDS-9140-C00 HEDS-9100-S00 HEDS-9140-S00
110 HEDS-9100-C00      
120 HEDS-9100-C00      
192 HEDS-9100-E00 HEDS-9140-E00    
200 HEDS-9100-E00 HEDS-9140-E00 HEDS-9100-C00 HEDS-9140-C00
250 HEDS-9100-E00 HEDS-9140-F00    
256 HEDS-9100-E00 HEDS-9140-F00    
360 HEDS-9100-G00 HEDS-9140-G00    
400 HEDS-9100-H00 HEDS-9140-H00 HEDS-9100-E00 HEDS-9140-E00
500 HEDS-9100-A00 HEDS-9140-A00 HEDS-9000-A00 HEDS-9140-F00
512 HEDS-9100-I00 HEDS-9140-I00 HEDS-9000-A00  
540 HEDS-9100-I00      
720   EM1-1-720    
900   EM1-1-900    
1000 HEDS-9100-B00 EM1-1-1000 HEDS-9000-B00 HEDS-9040-B00
1016 HEDS-9100-J00      
1024 HEDS-9100-J00 EM1-1-1024 HEDS-9000-J00 HEDS-9040-J00
1250   EM1-1-1250    
1800       EM1-2-1800
2000     HEDS-9000-T00 HEDS-9040-T00
2048     HEDS-9000-U00 HEDS-9040-T00
2500       EM1-2-2500

Linear Strip Compatibility

CPRNon-indexWith Index
120   EM1-0-120
125   EM1-0-127
127   EM1-0-127
150   EM1-0-150
180 HEDS-9200-Q00  
200   EM1-0-200
250   EM1-0-250
300 HEDS-9200-300  
360 HEDS-9200-360  
500   EM1-0-500

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