Impedance Matching and Termination Resistors
For RS-422/485 serial communications, when an electrical signal travels through two different resistance junctions in a transmission line, the impedance mismatch will sometimes cause signal reflection. Signal reflection causes signal distortion, which in turn will contribute communication errors. The solution to this problem is to establish the same impedance at the line ends as in the line itself by terminating them with resistors.
Industio CI-134 already installs the resistors on board for your convenience, indicated as follows. The value of the termination resistor should equal the characteristic impedance of the transmission line. Resistors should be added near the receiving side.
CI-134 | | | | RS-422/485 Device |
2 | TxD+(B) | | | RxD+(B) |
| |
1 | TxD-(A) | | | RxD-(A) |
|
3 | RxD+(B) | | | TxD+(B) |
|
4 | RxD-(A) | | | TxD-(A) |
|
7 | RTS+(B) | | | CTS+(B) |
| |
6 | RTS-(A) | | | CTS-(A) |
| |
8 | CTS+(B) | | | RTS+(B) |
| |
9 | CTS-(A) | | | RTS-(A) |
| |
Note: 
Stands for termination resistor near the receiving side
CAP Jumper
Interface Switch
Data Mode Switch
Termination Resistor Jumper for Port 1
Termination Resistor Jumper for Port 2 Termination Resistor Jumper for Port 4
Termination Resistor Jumper for Port 3
JP4
JP3
Industio CI-134 Series Board
5-4Industio CI-134 Series User’s Manual
