We know that the main components of an anti-theft alarm system are alarm motherboard, front-end detector and alarm sending device (networked alarm communication and on-site acousto-optic alarm).
The front-end detector also includes passive infrared, infrared plus microwave dual identification, infrared correlation, infrared guardrail, manual alarm, fire detection, glass breaking and so on. It is suitable for different environments according to different functions.
The front detector is the sensor of the alarm system, and the alarm system detects the external alarm through the front detector. The connection and signal transmission between the front-end detector and the alarm host is, in the final analysis, a process of sending and receiving switching signals. The so-called switching signal is a process of open circuit and short circuit of electrical circuit.
Take the common alarm system as an example. When the system is powered on and working normally, if the detector is powered off or triggered by an alarm, the relay in the detector will act to change the contact from the closed state to the open state. When the alarm host detects this change of the corresponding port in the protected area, it will take corresponding actions (including ignoring, alarming, signal output, etc.). ) displays the current state settings.
In fact, the working principles of alarm host and alarm detector are very simple, but why can't many people configure and use them correctly in practice? In fact, some key basic knowledge is not well understood. Today, let's talk about wired defense zones and the connection between wired detectors and alarm systems.
Basic knowledge is essential: what is terminal resistance?
EOL resistor: EOL (end of line) circuit, which is characterized by the connection resistance of the circuit terminal. A short circuit to ground will trigger the action of circuit contacts. For example, when the system is deployed, an open circuit or short circuit will trigger an alarm. The scientific name is the line end resistance, and the specific specifications of the line end resistance. Different brands and models of alarm host have their own specifications, and different manufacturers have different resistance values. Commonly used are1kΩ, 2.2KΩ, 4.7KΩ, 5.6KΩ and 6.8KΩ, which are installed on various detectors, that is, at the end of the line.
Normally closed quantity is connected in series in the circuit, and normally open quantity is connected in parallel in the circuit. When giving an alarm, the host will detect the change of resistance value. In other words, as long as the resistance output by the detector to the host is not the nominal resistance value (for example, 4.7KΩ), it will give an alarm. The task is to prevent damage. Cutting wires or short circuits will call the police.
It is clear that there are two ways of line end resistance: one is series connection and the other is parallel connection.
Common connection modes in alarm host:
Normally closed loop (NC): short circuit is normal, and open circuit gives an alarm. The disadvantage of this circuit is that if someone short-circuits the line, the probe will lose its function. The alarm host cannot recognize that it is a man-made short circuit.
Normally open circuit (none): short circuit alarm, open circuit is normal. The disadvantage of this circuit is that if someone breaks the circuit (cuts off the signal line), the probe will lose its function. The alarm host cannot identify the man-made road section.
EOL: Short circuit is normal, open circuit alarm. The advantage of this circuit is that if someone opens the circuit (short circuit and open circuit), the alarm host can give an alarm. Short circuit alarm, open circuit fault, and normal resistance value (such as 4.7KΩ). The advantage of this circuit form is that it has different responses to short circuit and open circuit, and it is especially suitable for smoke detectors and panic button. If the mouse bites a section or breaks a section because it helps something, the alarm host thinks that the circuit is faulty.
Therefore, in practical application, simply connecting normally closed and normally open loops without detecting the line end resistance has great alarm safety hazards.
The meaning of alarm host AUX, Z-, Z+:
Unlike wireless detectors that use batteries for power supply, wired detectors usually use DC voltage for power supply, and AUX of the alarm host supplies power for the detector power output to the wired detectors. Usually, the voltage values output by the alarm host AUX are DC 12V and DC 14V.
Z- stands for the negative pole of the wired sector, usually Z- ground (GND), and Z+ stands for the positive pole of the wired sector, usually with a certain voltage.
Common connections of alarm detectors:
The front-end detector generally has six lead ports: power supply+(generally labeled as+), power supply-(generally labeled as-), ALARM signal normally closed output (generally labeled as NC or ALARM), alarm signal male terminal (generally labeled as C or alarm) and two signal disconnection output ports (generally labeled as T or TAMPER). We can make different connections through different lines and resistors.
1, without anti-disassembly wiring
If the tamper-proof function of the detector is not enabled, the alarm system cannot sense whether the detector is damaged. Such wiring does not need to set a separate tamper-proof area or tamper-proof setting in the alarm host, and the signal line of the detector only needs four cores. Its connection mode is the simplest and most reliable, but its security is poor. In this connection mode, the alarm host can only sense whether the detector is triggered by the alarm, but can't detect other things, such as the opening of the box cover and the damage of the line (the alarm system still thinks that the detector works normally when the line is short-circuited, but it thinks that the line is cut or the detector alarms when the power is off).
2. Independent wiring in tamper-proof area
The tamper-proof signal of the detector is specially connected to the special tamper-proof area of the alarm host. This connection mode is reliable and simple, and the anti-disassembly area is programmed separately by the alarm host to realize the anti-disassembly of equipment and lines. Because extra wires are needed to transmit tamper-proof signals, the wire selection of the detector must exceed six cores. In this connection mode, when the detector box cover is opened, the line is cut off or the detector is powered off, whether the alarm system is on alert or not, it will trigger the anti-disassembly area corresponding to the alarm host to give an alarm that the equipment is disassembled. However, this mode will not trigger the alarm when the detector anti-disassembly interface or line is short-circuited, which has certain limitations.
3. Single-wire terminal resistance wiring
This connection method has the basic equipment anti-disassembly identification, and there is no need to set a separate anti-disassembly area in the alarm host. The signal line of the detector only needs four cores, and the protection area corresponding to the detector only needs to be set as a single-wire protection area. In this connection mode, the alarm host can judge what kind of alarm occurs by outputting different resistance values to the detector signal line in different States.
4. Two-wire end resistance
This connection method has the strongest identification for the disassembly prevention of equipment, and there is no need to set a separate disassembly prevention area on the alarm host. The signal line of the detector only needs four cores, and the corresponding area of the detector only needs to be set as a two-wire terminal resistance area. In this connection mode, the alarm host can judge what kind of alarm occurs by outputting different resistance values to the detector signal line in different States.
Common connection methods of wired infrared crosstalk detectors;
(Take CK238 as an example to illustrate the connection diagram)