I am currently working on a digital input card which will feature quadrature encoder inputs. To my (basic) knowledge, quad encoders use two pulse trains that are 90 degrees out of phase with each other and sometimes an index output for reference position. This gives 3 output signals (possibly six if they are differential outputs)

Now, this is the schematic for the quadrature inputs on our card. We are supporting both single ended and differential outputs. My question is the following:

Has anyone ever used quad encoders with 4 output pulse trains? What are these I and S differential signals shown on the schematics? I cannot find any quad encoder that outputs these signals...

Quadrature encoder inputs

Thank you very much for any info.

  • \$\begingroup\$ Do you have any documentation on this encoder? Part number? I stands for "Index" - it gives a pulse once per revolution - used for accumulated error correction and/or homing/position calibration.. \$\endgroup\$
    – Eugene Sh.
    Feb 19 '19 at 15:28
  • \$\begingroup\$ It is the boards documentation which needs to be consulted \$\endgroup\$ Feb 19 '19 at 15:30
  • \$\begingroup\$ Hmmm 4 axes, I can count three. \$\endgroup\$
    – Andy aka
    Feb 19 '19 at 18:04
  • \$\begingroup\$ "There is no doc, this is currently in design and i do not have access to the person who first made this schematic." - this is a problem for your organization to solve. Stack Exchange sites are for questions which can be specifically answered as a matter of public facts. Perhaps it's time that you dive into understanding the implementation of the board; someone in your company certainly needs to. \$\endgroup\$ Feb 19 '19 at 19:32
  • \$\begingroup\$ You might want to use another term than "axis" for your question since it's not an axis but something more akin to a channel or signal pair. \$\endgroup\$
    – DKNguyen
    Feb 19 '19 at 21:38

I just found this (TMS320x2833x, 2823x Enhanced Quadrature Encoder Pulse (eQEP) Module from TI). Probably not the same component that you use, but similar functionality.

eQEPI: Index or Zero Marker The eQEP encoder uses an index signal to assign an absolute start position from which position information is incrementally encoded using quadrature pulses. This pin is connected to the index output of the eQEP encoder to optionally reset the position counter for each revolution. This signal can be used to initialize or latch the position counter on the occurrence of a desired event on the index pin.

QEPS: Strobe Input This general-purpose strobe signal can initialize or latch the position counter on the occurrence of a desired event on the strobe pin. This signal is typically connected to a sensor or limit switch to notify that the motor has reached a defined position.


"I" is the index reference for absolute positioning.

  • \$\begingroup\$ "S" might be some kind of speed which I have never seen an encoder calculate on its own. \$\endgroup\$
    – DKNguyen
    Feb 19 '19 at 15:27
  • \$\begingroup\$ There is no doc, this is currently in design and i do not have access to the person who first made this schematic. He/She has not documented the design either. I guess I for index makes sense but then i sill have to find out what S stands for. The card is intended to be a universal input card so i tought someone might have come across this scheme before. \$\endgroup\$ Feb 19 '19 at 15:32
  • \$\begingroup\$ Could be some synchronization pulse. Can you show the whole schematic? Where is it routed to? \$\endgroup\$
    – Eugene Sh.
    Feb 19 '19 at 15:35
  • \$\begingroup\$ Pure speculation: Perhaps "S" is some sort of "stop" or limit signal. "I" and "S" together might delimit the operational range of the device. \$\endgroup\$
    – Dave Tweed
    Feb 19 '19 at 15:47
  • \$\begingroup\$ More pure speculation: index and sector. I worked on a product that used a geared-down encoder with a precise index pulse, on an axis that had a flag and photointerruptor that provided a sector pulse. But this is all guesswork -- snake and sulphur are probably out, as are sane and somnolent. But there's still a lot of 's' words that could apply. \$\endgroup\$
    – TimWescott
    Feb 19 '19 at 16:13

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