1. Requirements for wire selection for RS-485 bus communication:
Use 2-core shielded twisted pair cable,
Copper, wire diameter 0.5~0.75 square millimeters,
Impedance 38～88 ohms/km,
Capacitive reactance 30～50nF/km,
2-core shielded twisted pair with a lay length of 20 mm (if the line distance does not exceed 500 meters,
The wire standard can be appropriately lowered, but it must be a twisted pair wire.
2. System bus:
It consists of two or more devices that are physically connected to each other. A maximum of 128 bus devices are allowed to be connected. Without a repeater, the bus length is no more than 1200 meters.
3. The system bus should not have branching. If branching is unavoidable, the following three requirements must be met:
● The branch length is not more than 10 meters;
● The total length of the bus shall not exceed 800 meters;
● The total number of devices on this branch line shall not exceed 50.
● All communication signal lines should be kept as far away from interference sources as possible. Signal lines should be routed in weak-current wells and should not be run in parallel with strong-current (such as 220-volt residential power) or radio frequency signal lines (such as CATV, large-signal audio cables). If they are to be run in parallel, the distance should be greater than 0.5 meters.
● The joints of all circuits must be connected by welding or screwing, and must be waterproof and moisture-proof. For example, the joints can be welded and then tightly wrapped with waterproof tape or sealed with epoxy resin.
4. Signal common ground:
● All devices on the same network segment must have a unified signal ground to avoid common-mode interference.
● When centralized power supply is used, the DC negative poles of all power supplies on the same network segment (including the power supplies of communication equipment) are directly connected together to form a common signal ground. At this time, the signal ground is the DC power ground.
● When a single household has independent power supply, connect the ground (black wire) pins of all bus devices on the same network segment together to form a common signal ground.
5. Recommended selection of bus wires
| Wiring distance | Signal line | Power cord |
| Within 200 meters | 2*0.3mm2, twisted pair, sheathed cable | 2*0.5mm2, parallel, sheathed wire |
| 200-500 | 2*0.5mm2, twisted pair, sheathed cable | 2*0.75mm2, parallel, sheathed wire |
| 500-1200 | 2*0.75mm2, shielded twisted pair, sheathed cable | 2*1.0mm2, parallel, sheathed wire |
| Over 1200 meters | 2*1.0mm2, shielded twisted pair, sheathed cable Add a repeater at 1200 | 2*1.5mm2, above, parallel, sheathed wire |
485 wiring precautions:
1. 485 communication lines must use shielded twisted pair cables, preferably with multiple strands for backup, and the total length should not exceed 1200 meters.
2. Keep the wiring as far away from high-voltage wires as possible, and do not run it in parallel with the power lines, let alone bundle them together.
3. The 485 bus must be a hand-in-hand bus structure, and star connection and fork connection must be strictly prohibited.
4. If there are more than 30 controllers or the line length is greater than 500 meters, a 485 repeater must be used.
5. AC-powered equipment and chassis must be truly grounded and well grounded.
6. Use shielded wire to connect the GND grounds of all 485 devices.
7. Connect a 120-ohm terminal resistor to the 485+ and 485- of the last 485 device.
