ELM401
Design Considerations
There are a few details to consider when using the
ELM401 to interface to a rotary encoder. The first is
the fact that the signals available from the encoder are
usually only dry contacts closing and opening. You will
need to provide pullup resistors for these signals in
order to use them in a circuit.
the outputs are in a tristate condition. During this time,
the outputs will sit at the level they were at before
power up (0V) due to stray capacitance having
discharged through the protection diodes. If you
require that the outputs be at a high level as soon as
possible after powerup, you may want to install a
pullup resistor (of about 10 KW) on each output to
charge the capacitance quickly.
The size of the pullup resistor is chosen based on
the encoder specifications. The main concern is the
maximum current carrying capacity, which sets a lower
limit for the pullup resistance. An upper limit for the
resistance is set by the minimum current required for
contact wetting. If you do not provide enough current
through mechanical contacts when they are closed,
they will tend to go open with time. A maximum current
specification is usually in the range of 1 to 10 mA,
while the minimum wetting current would be in the
range of 1 mA. This means that with a 5V supply, a
pullup resistor of 5 to 10 KWis typically required.
After the initial 20 msec period, the ELM401 sets
all pins to their quiescent levels, but does not change
any outputs for an additional 50 msec. This ensures
that the external circuits have had adequate time to
initialize, before being presented with signals to
process.
The second concern is the use of capacitors on
the ‘A’ and ‘B’ signal lines. Many encoder circuits show
these as a way to provide some pre-filtering of the
signal. That is fine, as long as you realize that the
ELM401 inputs are CMOS and do not have Schmitt
trigger waveshaping. This means that you should keep
the rate of change of the input signal as high as
possible to avoid problems (we usually try to maintain
at least 1V/µsec). Typically, with a 5V supply, a 10 KW
pullup, and TTL thresholds, this means capacitor
values of no more than about 330 pF, while with a
2.0V supply, the limit would be about 100 pF.
One other issue to consider is that during the initial
circuit startup, there is a period (of about 20 msec)
when the ELM401 is being held in a reset state, and
ELM401DSB
Elm Electronics – Circuits for the Hobbyist
www.elmelectronics.com
7 of 8
ELM [ ELM ELECTRONICS ]
