1 Overview
The MAX3483, MAX3485, MAX3486, MAX3488, MAX3490, and MAX3491 are 3.3V, low-power transceivers for RS-485 and RS-422 communication. Each device has a driver and a receiver. The MAX3483 and MAX3488 have limited slew rate drivers to reduce EMI and reflectance caused by improper termination matching cables, enabling error-free data transmission up to 250kbps. The driver slew rate of the MAX3486 is partially limited, and it can achieve a transmission rate of up to 2.5Mbps. MAX3485, MAX3490 and MAX3491 can achieve a transmission rate of up to 10Mbps.
The driver has a short-circuit current limit, and the driver output can be placed in a high-impedance state through a thermal shutdown circuit to prevent excessive power loss. The receiver input has fail-safe characteristics, when the input is open, it can ensure a logic high output.
Using MAX3488, MAX3490 and MAX3491 can achieve full-duplex communication, while MAX3483, MAX3485 and MAX3486 are designed for half-duplex applications.
This article introduces MAX3485
2 Chip Introduction
2.1 Main features
Half-duplex
rate: 10Mbps
slew rate limit: NO
receiving permission control: YES
off current: 2 nA
pin count: 8
2.2 Pin configuration
According to the above diagram and table:
DE and RO are enable pins. When DE is low and RE is low, it is receiving; when DE is high and RE is high, it is sending;
RO and DI are data pins. RO is receiving, DI is sending;
Therefore, we often connect DE and RE directly and use an IO port to control (see 3.2 circuit implementation).
3 Application
3.1 Application scenarios
Industrial control local area network
Integrated service digital network
Low-power RS-485 / RS-422 transceiver (Several projects I have done are this function)
Packet switching technology
Telecom
Transceiver for EMI sensitive applications
3.2 Circuit realization
485 is a 2-wire type. Two 485 interface devices can be connected via two wires A and B (that is, at least two 485 chips). The connection method is shown in the following figure:
When we use MAX3485, we generally use the following circuit:
We can see from the picture above: RO is directly connected to TTL_level UART_RX (or analog serial port RX), DI is directly connected to TTL_level UART_TX (or analog serial port TX), R34 is 1K.
3.3 Software implementation
In software implementation, we need to make the control pin high when sending data, and make the control pin low when receiving data. When driving at the bottom layer (such as using pin control to use STM8), we can clearly know the start and end positions, so we only need to add IO control codes in relevant places, but when using the code of others' package numbers ( For example, when using Lianshengde wifi chip), we need to find out when to receive and when to send, mainly when the sending is finished. In general, we will find if there is an interruption after sending, or whether there is a change in semaphore (Lianshengde wifi module uses the semaphore number).
