Comparison of Application Design of LVDS and RS422/RS485
In the company's products, some clock transmissions use LVDS level, and some use RS422 level.
In fact, the RS422 level is also a differential form, its level amplitude is larger than that of LVDS, and its anti-interference ability is stronger than that of LVDS. In the RS422 level specification, the maximum rate supported is 10Mbps (transmission 15 meters). When the clock or data is lower than 10Mbps, but the anti-interference requirements are stricter, the advantages of using RS422 are more obvious than LVDS. This is not difficult to find in the company's products. For example, in switches, the RS422 method is used to transmit clocks between boards at different backplane layers. In some systems, the synchronous communication between boards on different layers of the rack, when the speed is not high, uses RS422 level interface for data and clock. Compared with RS485, in the RS422 circuit, there can only be one transmitter and up to 10 receivers.
The RS485 level is also a differential form, and its level amplitude is larger than that of RS422, which can be compatible with the RS422 interface. The maximum rate supported is 10Mbps (transmit 15 meters). RS485 has stronger anti-interference ability, and supports multiple transmitters (32) and multiple receivers (32) to be connected together. Compared with RS422, RS485 is more suitable for multi-point communication design with long distance and poor environmental conditions. For example, a centralized monitoring unit monitors multiple power boards of each switch placed in the equipment room at the same time to form distributed monitoring.
Select RS422 interface for synchronous serial communication design, both clock transmission and data transmission, generally only in the system, such as a single board with the slots on the backplane farther apart; or in the same rack , the boards of different backplane layers are connected by twisted pair cables. If the slots are on the same backplane and the slots are not far apart, it is completely possible to design the communication as an LVDS interface.
In different systems and different racks, synchronous serial communication is generally not designed, mainly from the perspective of reliability. Different systems, different racks, the data rate is not high, it is more appropriate to design asynchronous serial communication in the form of RS485.
The application of LVDS in the company's veneer design
Compared with TTL level, the interference between LVDS level signals is significantly smaller than the interference between TTL signals.
■ Most of the HW lines going in and out of the socket on the single board use LVDS level to send and receive. Such as the HW line of each switch fabric board of the switch.
■ High-speed clock signal, more suitable for receiving and sending at LVDS level. For example, each clock board outputs the clock externally.
■ The communication connection and media service signal between boards are also suitable for designing with LVDS interface. For example, in the ADSL system, there are 32 LVDS connections between the CORE board and the ATUC board.
