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In the digital transmission system, there are two types of digital transmission series, one is called "Plesiochronous Digital Hierarchy" (PDH), also called PDH, PDH (Plesiochronous Digital Hierarchy) is a small capacity optical transceiver, PDH. Optical transceivers are generally paired applications. They are also called point-to-point applications. The PDH optical transceiver capacity is generally 4E1, 8E1, 16E1; the other is called Synchronous Digital Hierarchy (SDH).
Light path problem:
In the security monitoring and control project, most of the optical cables are laid by users themselves, generally G652 single-mode optical fibers. Because the system coverage is generally not large, the optical link loss of the equipment with standard (≤20KM) equipment is very rich. Therefore, the optical transceiver has no excessive requirements for optical path loss, but users often encounter no image, image jitter, and image quality. Poor issues, when the majority of problems are at the fiber pigtails, jumpers or adapters at the ends of the optical path, and rarely with the trunk light path.
Common problems include:
1. Incorrect insertion of fiber optic connector;
2. The fiber optic connector core (ceramic tube) is contaminated.
The solution is:
1. Reinsert the active connector or swap the fiber jumper;
2. Wipe the plug with 99.9% absolute ethanol and plug the core;
3. Use a multimeter to check the camera video cable to determine whether there is a video signal.
Data interface:
In order to meet the needs of security monitoring, the system equipment (matrix, hard recorder, decoder) provides RS-485 data interface. The advantage of this format of data interface is long transmission distance, strong load capacity, and can form four. Line full-duplex communication bus, any two devices on the line can achieve two-way communication, and four-wire RS-422 bus can only achieve two-way communication between the master and slave, but not between the slaves. Its disadvantage is that there is an enabler, in a three-state form, which brings instability to the communication and even "dead". If there is no communication (runaway), look for the cause from the following aspects:
1. Check whether there is any control signal. Use a multimeter to exchange 10V file controller (matrix, hard record, etc.) to output RS-485 port and see if it has control signal output.
2. It is judged whether the optical transceiver RS-485 interface is normal, if the UA-B voltage is zero, it is regarded as abnormal.
The pan/tilt can't be controlled by turns. This phenomenon is caused by two reasons: a) The RS-485 port A+ and B- are connected reversely; b) The system impedance is seriously mismatched.
Hazards and Countermeasures
1. Generation of Transient Interference: Transient interference is caused by large inductive loads, such as the switching of motors, transformers, relays, and other devices, and in the process of lightning, it often invades the optical transceiver by electrostatic induction.
2. The hazard of transient interference: Because it interferes with high frequency, short duration, large interference amplitude (hundreds of kilovolts), it can burn out the key parts of the optical transceiver RS-485 interface chip, main chip, etc. The traces, especially during the thunderstorm season in summer, have a great impact on this destructive force, causing users, businesses and manufacturers to be very troublesome.
3. Countermeasures: Although the optical transceiver manufacturers use various protection methods, such as bypass method (self-healing diode), absorption method (bidirectional suppression diode, etc.), isolation method (optocoupler isolation), but still can not completely eliminate the transient Damage caused by interference, RS-485 interface is frequently damaged, causing great pressure on users and manufacturers.