The General Knowledge Of Rs485; Basic Characteristic Of Rs485; Rs485 Transmitting Distance; Connect Mode And Terminal Impedance - ATN TASC 336-7 Operator's Manual

4.2. THE GENERAL KNOWLEDGE OF RS485

4.2.1. BASIC CHARACTERISTIC OF RS485

RS485 is a half duplex communication bus whose impedance is
120Ω. Its carrying capacity is different because the different con-
nection interface, IC is 32-128 actual load (including master equip-
ment and slaves equipments).

4.2.2. RS485 TRANSMITTING DISTANCE

When 0.56 mm (24AWG) twisted pair line is used as the commu-
nication electric cable, there are different transmission distance
according to different Baud rate setting. Maximum transmission
distance and corresponding.
Baud rate
2400 BPS
9600 BPS
The max transmission distance would be shortened in conditions
as: when this product uses a slim communication electric cable
or the thermal speed dome is used in the environment with strong
electromagnetic interference or there are many equipments con-
nected to control bus, vice versa.

4.2.3. CONNECT MODE AND TERMINAL IMPEDANCE

The RS485 requires daisy-chain connection mode, and two 120Ω
terminal impedances should be connected. The mode was showed
as figure 4.1. Simplified connection mode shows in figure 4.2, make
sure that the distance of D can't be more than 23 feet (7 meters).
Maximum
distance
1800 m
800 m
FIGURE 4.1
Baud rate
4800 BPS
19200 BPS
Maximum
distance
1200 m
600 m
4-3