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1. What kind of communication line RS-485
should adapt to? How many units of equipments
can be mounted on one line?
Have to make used of RVSP Shielded Twisted Pair cable. Reason
has to do with the line distance and number of equipment to
be connected as mention on the below table. Using shielded
twisted pair will help to reduce and eliminate distributed
capacity between 2 RS-485 lines and surrounding common mode
interference.
| Communication Distance |
No. of Equipments |
Communication Line Specification |
| 1-400m |
1 - 32 |
0.5mm² |
| 400-800m |
1 - 16 |
0.5mm² |
| 400-800m |
17 - 32 |
0.75mm² |
| 800-1200m |
1 - 8 |
0.5mm² |
| 800-1200m |
9 - 21 |
0.75mm² |
| 800-1200m |
22 - 32 |
1.0mm² |
Most engineering contractor uses Cat 5 or Cat 5E cable as RS-485 communication line which is consider not appropriate. This is due to
1. Ordinary network cable does not shield, therefore not able to prevent common mode interference.
2. Diameter on network cable is too small only 0.2mm² which
will reduce the transmission distance and number
of connected devices.
3. Network cable uses single-line copper wire which easily breaks compare to multi-line.
2. Why need to Ground the wire?
In order for RS-485 transceiver to work correctly, the common mode voltage should be in between -7V ~ +12V. Otherwise communication will be disrupted; serious case will even damage the communication connector. Common mode interference will increase the common mode voltage. One way to eliminate common mode interference is to ground the shielded layer of RS-485 communication line.
3. Can electronic lock set and controller
or reader sharing the same source of power supply?
Cannot. When electronic lock is in idle stage, the SKPS voltage ripple
is at 40 - 50 mV; the voltage ripple will goes up
to 100mV-300mV when it is activated; ripple will go through
the earth into controller and reader, leading heat to communication
chip and CPU and causes instability in communication; serious
case will damage the chip. Moreover, at the moment of turning
the electronic lock power on/off, the internal wire wrap will
tend to charge and discharge and produce a pulse of up to
850 mA. If the 2 ends of the electronic lock is not parallel
diode, the ripple signal will be liable to flow into controller
and reader. Smartkey has recommended the use of one SKPS
power supply to connect to a controller and all
readers connected to it; and the electronic locks connected
use separate SKPS power supply each.
4. How to lay RS-485 cable
Keep the communication line as far away as possible from
high-voltage wires, not parallel with the power cord, and
cannot be tied together.
5. Why RS-485 cable uses daisy chain
configuration rather than star topology to connect equipment?
Star topology will have reflection signals, thus affecting
RS-485 communication. Distance between main terminal to
destination devices should be kept short; no longer than
5m. If termination is not properly done at each branch line,
there will be reflection signals which will cause strong
interference to communication, therefore have to eliminate
this from happen.
There are 2 places that use RS-485 line within Smartkey
Access Controller. One is from PC to controller and the
other one is from controller to its connected RS_485 readers.
6. Can there be a contact point between
equipment on RS-485 connection?
Use same cable within the same network system to minimize
the contact point on the line. Make sure the contact point
is properly attached, tightly wrapped, avoid loose and oxidation.
Ensure that a single, continuous signal path as the main
bus line.
7. What is common mode interference
and differential mode interference? How to eliminate the
communications interference?
RS-485 communication line is composed of dual twisted pair cable. It uses voltage difference approach to transmit signal, which is known as differential voltage transmission.
Differential mode interference transmit signal between the two transmission lines known as symmetry interference. Eliminate the differential mode interference in the circuit is to partially increase resistance value, and a twisted pair;
Common mode interference transmit signal between transmission line and ground know as asymmetry interference. Elimination such interference includes...
1. Use shielded twisted pair wire and ground the wire.
2. At strong electric field consider using galvanized pipe for shielding.
3. When wiring, stay away from high voltage line, and do not lay high voltage line and signal lines together.
4. Do not share the same power supply as electronic door lock.
5. Use linear regulator power supply or high-quality switching power supply (Ripple interference <50mV)
8. Under what circumstances RS-485 communications
need to increase terminal resistance?
Under normal situation, terminal resistance is not required. Unless, RS-485 transmission signal is beyond 300m, terminal resistance is needed at the start and the end of the line. This is especially when small amount of devices is mounted on the RS-485 bus line. When large number of devices (> 22 devices) is mounted on the bus line, terminal resistance is not necessary as it will lower RS-485 bus line load capacity. When terminal resistance is required, just set it via controller jumper, if the other end is going to PC, short-circuit brown and white wire at the RS-485 converter.
9. How to extend RS-485 communication
distance?
Standard length of RS-485 network is 1.2 km, 32 nodes.
If it goes beyond this limit, have to make use of RS-485
hub to extend the distance or nodes
Using RS-485 hub, a large-scale network can be separated into several network segment. RS-485 hub act as a "bridge" on the RS-485 network and each network segment will still following the standard rules or 1.2km long and 32 nodes.
Diagram using RS-485 Hub to extend network distance£º
Use RS-485 hub to solve RS-485 divergence problem, as shown on diagram£º
Using RS-485 Hub to build star topology RS-485 network
RS-485 repeater hub is the concept of the RS-485 extension, besides solving the multi-divergence problem; it also solved the network isolation problem, e.g. network down cause of short-circuit; does affect the rest of the network hence greatly enhancing the security and stability.
From the development of daisy chain method to star topology
network using RS-485 hub, we can now understand the benefits
of start topology network and RS-485 hub as one of the expansion
point.
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