According to transmission line theory, only when the output impedance matches the load will the signal be in a traveling wave state (ie, normal communication state).
In reality, the transmission line is a non-ideal transmission line. In order to achieve impedance matching, a resistor (terminal resistor) is connected in parallel to the end of the line to increase the load to reduce signal reflection and passively achieve impedance matching.
The output impedance of RS485 is about 120Ω, but the actual characteristic impedance of ordinary cables is only 60~80Ω. Therefore, a special cable with a characteristic impedance of 120Ω (Patent No.: 2010 2 0559128.9) is more conducive to communication. The general recommendations are as follows:
Unarmored twisted pair shielded cable STP-120Ω (for RS485 & CAN) one pair 20 AWG, cable outer diameter is about 7.7mm, blue sheath. Suitable for indoor, pipeline and general industrial environments. When in use, one end of the shielding layer should be grounded!
Unarmored twisted pair shielded cable STP-120Ω (for RS485 & CAN) one pair 18 AWG, cable outer diameter is about 8.2mm, gray sheath. Suitable for indoor, pipeline and general industrial environments. When in use, one end of the shielding layer should be grounded!
Armored twisted pair shielded cable ASTP-120Ω (for RS485 & CAN) one pair 18 AWG, cable outer diameter is about 12.3mm, black sheath. It can be used in places with serious interference, frequent rodent infestations, and places with lightning protection and explosion-proof requirements. When in use, the armor layer should be grounded at both ends, and the innermost layer must be shielded and grounded at one end!