Specifications

Section 2: System Description
2-21
The monitoring points connected to the MSM must have dry electrical contacts, either normally
open (N/O) or normally closed (N/C), which indicate that the monitoring device is in either the
active or the inactive state.
Monitoring device connections to the MSM are by means of the terminal block mounted on the
face of the MSM. The monitoring device inputs are wired by twisted pair, non-shielded cable, with
a 36K-ohm resistor. The resistor is connected in series for a normally closed (N/C) contact or
parallel for a normally open (N/O) contact (see Figure 2-9). A 36K-ohm resistor must also be
installed across any unused contact pairs
When normally open monitoring contacts are activated, the contacts close, shorting the 36K
resistor. Conversely, when normally closed monitoring contacts are activated, the contacts will
open, causing an open circuit A resistance sensing circuit in the MSM senses 36K ohms as the
normal condition.
Whether the normal state is open or closed, the MSM will send a signal to the ACU indicating
an abnormal condition for a monitor point. This signal consists of serial digital pulses that are
decoded by the ACU during the normal polling sequence of the reader ports.
The ACU can only recognize specific monitoring contacts if they are defined with the MONITOR
command during database set up. Each contact pair has a unique address in the form X.Y, where
X is the sensor port to which the MSM is connected and Y is the pair number, 1 through 4.
NOTE:
If a door switch and a request-to-exit device for a given door are connected to the monitoring
device contacts of the same MSM, the REX device must be connected to a contact pair with
a lower number than the door switch. Otherwise, normal actuation of the REX device can
cause a DOOR FORCED OPEN report to be generated and can cause an alarm device to
be set off. The REX device is usually connected to contact pair 1 and the door switch to
contact pair 2.