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MX4428 MXP Engineering / Technical Manual Document: LT0273
Analog Loop Design Considerations
Page 4-2 24 March 2006 Issue 1.5
4.1 ANALOGUE LOOP CONFIGURATION SELECTION
4.1.1 LINES & LOOPS
The interface between the MXP and its addressable devices requires two wires.
The MXP has two lines (“left” and “right”) which are designed to be connected in a loop.
The LOOP configuration is generally preferred and indeed will often be mandatory for
compliance with standards as discussed below.
However the MXP can be used to connect to multiple lines in a star configuration. Dual line
mode is not supported.
4.1.2 LOOP FAULT TOLERANCE
Standards require that a line/loop fault condition cause minimal disruption to the system's
ability to detect and transmit alarms to the Fire Panel. The MXP achieves this in the
following way .....
The MXP has access to each device from both ends of the loop. The loop is normally
sourced from “left” and monitored at the “right” terminals. Disappearance of 40V power at the
“right” end, due to an open circuit FAULT on either the + or – wires, can be detected
(causing a FAULT event to be sent to the MX4428 Master) and corrected by switching the
Line driver onto the “right” terminals. The LOOP mode is therefore inherently fault tolerant to
any one open circuit on any one of the 2 wires.
However, a short circuit on the loop will, in general, cause the MXP to loose communication
with all devices. Thus it is recommended that isolator bases be used to minimise the loss
due to a short.
When designing fire alarm systems, the designer should be aware of any local statutory
requirements, as well as those of AS1670.1 and NZS4512.
4.1.3 AS1670.1 DESIGN REQUIREMENTS
Australian Standard AS1670.1 sections 3.1 and 3.2 require the analogue loop to comply with
the following:
• The maximum number of actuating devices (i.e. detectors and input modules) in an
alarm zone shall not exceed 40.
• A single short circuit shall not disable more than 40 devices connected to the MXP
loop/line. This means than if more than 40 devices are to be connected to an MXP, short
circuit isolators must be used. The count of 40 includes conventional detectors
connected to a DIM800 or other ancillary input device
4.1.4 NZS4512 DESIGN REQUIREMENTS
• Isolator Bases must be fitted between zones (or on the first device either side of a zone
boundary) so that a single short circuit or break will affect no more than one zone.
• In many cases the tones produced by the 814SB sounder base are not acceptable.
Refer to NZS4512 for detailed requirements.