SN75HVD10: Spec for RS422 application

Hello Nanfang,

The RS-485 and RS-422 specifications are very similar and the RS-485 receiver and driver electrical characteristics are specified such that they cover the RS-422 requirements. Therefore compliant RS-485 drivers and receivers can be used in most RS-422 applications.

Here is a basic comparison of the RS-485 and RS-422 standards:

1. Bus Topology
- RS-422: Multi-drop
- RS-485: Multi-Point

2. Number of Drivers
- RS-422: One
- RS-485: Many

3. Number of Receivers
- RS-422: 10
- RS-485: Minimum 32, up to 256

4. Differential Output Voltage
- RS-422: 2V across 100 ohms
- RS-485: 1.5V across 54 ohms

5. Driver Output Common-Mode Range
- RS-422: Unspecified
- RS-485: -7V to +12V

6. Driver Short Circuit Current *
- RS-422: 150uA
- RS-485: 250uA

* In RS-422, driver short circuit current is specified from each A and B output to ground. In RS-485, driver short circuit current is specified from A to B, B to A, and from each A and B output to -7V to +12V.

7. Minimum Receiver Input Impedance
- RS-422: 4k ohms
- RS-485: 12k ohms

8. Receiver Input Common-Mode Range
- RS-422: -7V to +7V
- RS-485: -7V to +12V



This application note provided a detailed comparison and discussion of the differences between the RS-485 and RS-422 specifications. ( www.ti.com/.../slla070d.pdf )

Regards,
Jonathan