Contemporary Control Systems BAScontrol20 User Manual - Manual (Page 21)

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4.8 Binary Inputs
To sense the action of a push-button or relay, the contacts must have no applied
energy, and be rated for low-voltage, low-current switching. The BASC20 provides
the energy to be sensed. With a web browser, access the Main Screen, click the title
link of any channel UI1–UI8. Set the Channel Type to Binary Input and the Units to
NO_UNITS. As shown in
Figure 10 , connect the contacts between points A and C.
For common mechanical contacts, polarity is not an issue. The open-circuit voltage
is 12 VDC and the short-circuit current is 0.5 mA.
For solid-state switch sensing, we recommend that an attached solid-state device
have an opto-isolated open-collector NPN transistor output stage with a collector-
emitter output voltage (Vce) of at least 30 V. Output sinking current should be
greater than 5 mA. The collector-emitter saturation voltage should be less than 0.2
V when sinking 2 mA. The emitter must be connected to point C and the collector to
point A (the more positive point). The BASC20 sets the low-threshold to 3 V and the
high-threshold to 6 V. When a contact is made or the solid-state switch is on (resulting
in a saturated output), the voltage at point A is close to zero volts. The corresponding
LED for that channel will be on. If the contact is opened or the solid-state switch is
turned off, the voltage at point C quickly rises towards 12 V. Once the voltage
passes the 6 V high-
threshold, the “off” state is sensed. To return to the “on” state,
this voltage must fall below 3 V. The three-volt difference is called hysteresis. There
is no need to add an external pull-up resistor when using a contact closure input.
Contact closure inputs are sampled every 10 ms and for a change of state to be
recognized, the input state must be stable for two consecutive samples. Therefore,
contact closure response is 20 ms.
Figure 10 — Binary Input Connection