1, the basic characteristics of RS485 bus
According to the RS485 industrial bus standard, the RS485 industrial bus is a half-duplex communication bus with a characteristic impedance of 120Ω, and its maximum load capacity is 32 payloads (including the master device and the controlled setting).
2, RS485 bus transmission distance
When using a 0.56mm (24AWG) twisted pair cable as the communication cable, the theoretical maximum transmission distance is as follows according to the baud rate:
Baud rate
Maximum distance
2400BPS
1800m
4800BPS
1200m
9600BPS
800m
When the use of a finer communication cable, or when the product is used in a strong electromagnetic interference environment, or more devices are connected to the bus, the maximum transmission distance is shortened; otherwise, the maximum distance is extended.
3, connection and termination resistance
1) The RS485 industrial bus standard requires a daisy-chain connection between the devices. Both terminals must have a 120Ω terminating resistor (as shown in Figure 1). The simplified connection can use the wiring method of Figure 2, but the “D†segment distance cannot More than 7 meters.
Figure 1
Figure II
2) Connection method of ball machine terminal 120Ω matching resistor
The ball machine terminal 120Ω matching resistance can be connected through the dial switch on the dome chassis, as shown in Figure 3. When the dome camera is shipped from the factory, the default 120Ω matching resistor is not connected. You can switch the 120Ω matching resistor into the line by turning ON the 10th bit of the DIP switch. Conversely, if you do not connect a 120Ω matching resistor, turn off bit 10 to OFF.
Figure 3
4. Problems in practical application
In practice, users often use a star connection method. At this time, the termination resistor must be connected to the two devices that are farthest from the line (as shown in Figure 4, 1# and 15# devices), but because this connection method does not meet the RS485 industry The requirements for the use of standards, so when the distance between the various equipment lines is far, it is easy to produce signal reflection, anti-interference ability and other issues, resulting in decreased reliability of the control signal. At this point, the phenomenon that occurs is that the dome camera is not controlled at all, or it cannot stop by itself.
Figure 4
For this case, it is recommended to add an RS485 distributor. This product can effectively convert the star connection to meet the connection method specified in the RS485 industry standard, thus avoiding problems and improving the communication reliability, as shown in Figure 5.
Figure five
5, RS485 bus common faults
Symptom Possible Cause Solution
The ball function self-test but can not control 1, the host, ball machine address, baud rate does not match; 1, change the host or dome machine address, baud rate, make it consistent
2, RS485 bus +, - polarity reversed; 2, exchange RS485 +, - wiring polarity;
3, loose wiring; 3, fastening wiring;
4. Interruption in the RS485 line; 4. Replace the RS485 line.
Ball function control but not smooth 1, RS485 line bad contact; 1, reconnect the RS485 line;
2, a broken RS485 line; 2, replace the RS485 line;
3, host, dome distance is too far; 3, install terminal matching resistor;
4, the ball machine and take too much. 4, install RS485 distributor.
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