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Document: LT0273 MX4428 MXP Engineering / Technical Manual
MXP Technical Description
Issue 1.5 24 March 2006 Page 7-7
In response to DISCON IN– going low, the microprocessor outputs a 10 msec pulse to
DISCON OUT+, which applies “0V” to RL3 pin 16 through D5 and U8 pin 2, and “+24V”
through Q11 to RL3 pin 1, thereby setting the relay contacts to their open state. When
DISCON IN+ goes low the microprocessor outputs a 10 msec pulse to CONOUT, which
applies “0V” to RL3 pin 1 through D6 and U8 pin 1, and “+24V” through Q10 to RL3 pin 16,
thereby re-setting the relay contacts to the closed state.
The DISCON IN– signal is configured as an interrupt signal to the CPU. This allows a very
fast response to it going low and ensures the relay is opened immediately. This is necessary
as a short anywhere on the responder loop will often result in the responder power
collapsing completely and the relay must be opened while sufficient charge remains in
capacitors C64, C65, C66, and C59-C62.
When neither DISCON OUT+ nor CONOUT is high, no power is applied to the coil of RL3.
7.2.4.3 MX4428 Communications Circuitry
The duplicated MX4428 communications channels are implemented using serial ports 1 and
2 of the 68302 CPU. The two transmit lines are isolated with optocouplers and buffered with
Darlington drivers. The receive circuits are protected with series resistors and shunt
diodes/capacitors, digitised with comparators, and isolated with optocouplers.
Passing messages around the loop is done entirely with software. For details of the MX4428
responder protocol and loop operation, refer to “F4000 Technical Manual - Appendix C,
Responder Communication Protocol”.
7.2.5 ANALOGUE LOOP INTERFACE
The Analog Loop (also known as MXP loop or NOSEx loop) is a two-wire circuit with the
MXP being the master and up to 200 addressable devices which are slaves.
The MXP supplies the loop power (36 - 39V dc) which powers the addressable devices and
sounder outputs. The loop + voltage is modulated with a 4V p-p dual frequency sinewave in
order to transmit data using the power wire.
Both the MXP and the addressable devices transmit and receive in the same way. The
addressable devices normally only transmit immediately after they have been polled by the
MXP, however in special circumstances they can transmit interrupt messages when they
have something urgent to send.
An example of a message on the loop is shown in
Figure 7.2.