Common Issues with Optical Terminals in Security Surveillance

In security surveillance projects, fiber optic technology is increasingly used to transmit images, audio, data and other information. The key equipment of fiber optic transmission systems – digital video optical terminals – is favored by a wide range of users.

I. Regarding Optical Link Issues:

In security surveillance projects, the optical cables are mostly laid by the users themselves, typically G652 single-mode fiber. Since the system coverage area is generally not large, the optical link loss margin of standard equipment (≤20KM) is quite sufficient. Therefore, optical terminals do not have excessive requirements for optical link loss. However, users often encounter problems such as no image, image flickering, or poor image quality. In most cases, these issues originate from the pigtails, patch cords, or adapters at both ends of the optical link, and are rarely related to the main optical cable.

Common problems include: 1. The fiber optic connector is not inserted correctly; 2. The ferrule (ceramic tube) of the fiber optic connector is contaminated. Solutions are: 1. Re-insert the connector or replace the fiber patch cord; 2. Use 99.9% anhydrous ethanol to wipe the plug and the socket ferrule; 3. Use a multimeter to check the camera video cable to determine whether a video signal is present.

After the above treatments, the problems are generally resolved. If conditions permit, an optical fiber tester (OTDR) or optical power meter can be used to test the optical link loss. Some users, due to constraints, rent or borrow existing optical cables from telecommunications operators. If it is a LAN, it is mostly multimode fiber. In this case, it is necessary to determine the year of manufacture of the fiber. If it is a product before 2005, only multimode optical terminals can be used, and the transmission distance is very limited – for example, 4 to 8 channels of video transmission generally cannot exceed 500 to 1000 meters. Multimode fiber produced in the last two years can also operate at the 1300nm wavelength. IDM series single-mode optical terminals can also transmit over it, with a transmission distance of up to 3 to 4KM. It should be emphasized that: currently, for optical modules in digital optical terminals, multimode modules are more expensive than single-mode ones, with fewer suppliers, and multimode fiber is also much more expensive than single-mode fiber. Therefore, the use of multimode fiber transmission is generally not recommended.

II. Regarding Data Interfaces:

To meet the needs of security surveillance, various system devices (matrix switchers, DVRs, decoders) provide RS-485 data interfaces. The advantages of this data interface format are long transmission distance, strong load capacity, and the ability to form a four-wire full-duplex communication bus, where any two devices on the bus can achieve two-way communication. In contrast, a four-wire RS-422 bus can only achieve two-way communication between the master and slave devices, not between slave devices. Its disadvantage is that it has an enable terminal in a tri-state form, which can cause communication instability or even "lock-up". If communication failure (loss of control) occurs, the causes should be investigated from the following aspects:

1. Check whether there is a control signal – Use a multimeter on the AC 10V range to measure the RS-485 port output of the controller (matrix switcher, DVR, etc.) to see whether a control signal is being output.

2. Determine whether the RS-485 interface of the optical terminal is normal – If the UA-B voltage is zero, it is considered abnormal.

3. If the system operates normally but occasionally loses control, it is because the system is in a critical state. It is necessary to increase the load capacity of the controller (such as connecting a code expander), improve system impedance matching, and improve material quality. After the above measures, the system can work stably for a long time.

If the PTZ rotates randomly and cannot be controlled, this phenomenon is caused by two reasons: a) The RS-485 port A+ and B- are reversed; b) The system impedance is severely mismatched.

III. Regarding Switching Signals

Switching signals are pulse trains at TTL levels. They can control alarm lights, alarm bells, relays, and other devices. The load capacity of the switching interface is measured by the current it can control. For example, the switching load capacity of the EW series optical terminals is ≤1.5A.

1. The switching interface of the IDM series optical terminals supports normally open buttons. However, with the wiring method shown below, both normally open and normally closed forms are supported:

2. Switching interfaces cannot be directly connected in parallel. If necessary, they can only be connected through a distribution circuit.

3. Some customers use the RS-485 bus to transmit switching signals. Based on our practical experience, this approach is not advisable, as it often results in system lock-up after a period of operation (e.g., 3 to 4 days). Defects in the manufacturing of the switching-to-RS-485 converter may be the root cause.

IV. Regarding the Hazards of Transient Interference and Countermeasures

1. Generation of transient interference: Transient interference is generated by the switching of large inductive loads, such as motors, transformers, relays, and other equipment, as well as during lightning events. It often invades the optical terminal in the form of electrostatic induction.

2. Hazards of transient interference: Due to its high interference frequency, short duration, and large amplitude (hundreds to thousands of volts), it can burn out the RS-485 interface chip, main chip, and other critical components of the optical terminal without leaving any trace. Especially during the summer thunderstorm season, this destructive force is highly impactful, causing significant headaches for users, vendors, and manufacturers alike.

3. Countermeasures: Although optical terminal manufacturers employ various protection methods, such as bypass methods (self-recovery diodes), absorption methods (bidirectional suppression diodes, etc.), and isolation methods (opto-coupler isolation), they still cannot completely eliminate the damage caused by transient interference. Frequent RS-485 interface failures place great pressure on both users and manufacturers.