Generally speaking, the vast majority of bus gas detectors use 4-core shielded wires, 2 power lines, 2 signal lines, and the transmission distance is relatively long, about 1-2Km; The gas detector of the split wire system adopts three core wires, 2 power lines, 1 signal line, the negative pole of the power supply and the signal line are common, and the transmission distance is relatively close, within 1Km.
Choose bus system or split system, and confirm it according to the actual working conditions.
1. The number of gas detectors to be accessed.
If the number of probes is more than 8, it is recommended to choose the bus system. This can reduce the difficulty of wiring and wiring costs, all probes are counted into the host through a cable, and the theoretical maximum access to 256 gas detectors; The number of gas detectors is less than eight, and the breakout system can be selected, and each probe is connected to the main machine by a separate cable.
2. Distance.
In the actual working conditions, because the installation position of each probe and the distance of the probe from far to near, when the distance is relatively far, it is recommended to use the bus system, the general transmission distance of the bus system is 1-2Km, only need to lay a main line cable, all probes can be connected to the main line; In addition, the transmission distance of RS485 can be increased by adding repeaters, and the distance is theoretically not limited. The breakout system is used for short distances.
3. Signal processing needs.
In many cases, the test data of a gas detector needs to be processed multiple times. 4-20mA is a standard industrial signal, which can be connected to various PLC and DCS systems, but when it is necessary to save data and display data twice, RS485 signals are required.
The following is a summary of the advantages and disadvantages of the bus system and the branch system:
Advantages of bus system:
The signal is uniform, and the probability of failure is low. The bus control system does not have this inconvenient factor at all, and transmits data in the same form on the data line, which enhances the reliability of the data.
Simple wiring and small workload. The obvious advantages of the bus system are that the wiring volume is small, the wiring is simple, and the cost is small. Four-bus system, two signal lines, two power lines, simple and convenient wiring.
Disadvantages of bus system:
Signal delay. The data is brushed out one by one, especially when the number of probes is large.
Power supply problems. All probes are centrally powered by the host, and when the number of probes is large, the power supply capacity of the host is insufficient, and local power supply is required.
Advantages of the split line system:
Good data synchronization, no limit on power supply. Compared with the bus system, each gas detector in the split line system communicates with the controller separately, and the on-site situation can be transmitted to the control part in a timely manner, so that the monitoring personnel can make timely and effective judgments, and the peripheral control equipment can make corresponding control actions in time and effectively to avoid dangerous accidents.
Disadvantages of the split line system:
Complex wiring. High signal interference. There is a lot of wiring, large workload, complicated wiring, complex installation and construction, and high material costs.
Install the gas alarm, select the corresponding cable specifications according to the installation method, split wire type and bus type.
Split-wire installation generally adopts 3×1mm2 three-core cable shield, and it is recommended to use 500×3.1mm5 cable shield wire for more than 2 meters.
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