Contemporary Control Systems BAScontrol20 Installation Guide - Manual (Page 11)

TD100700-0IA
11
Binary Inputs BI1–BI4
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.
Configuration is only required to set the BACnet
properties. 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 1.2 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 7 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 7 V
high-threshold, the “off” state is sensed. To return to the “on” state, this
voltage must fall below 3 V. The four-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