1. The onboard MAX485 chip is a low power consumption and limited slew rate transceiver for RS-485 communication
2. The board is connected to 5.08 (mm) spacing 2P terminal, which is convenient for RS-485 communication wiring
3. All the pins of the chip have been led out, which can be controlled and operated by the single-chip microcomputer
4. Working voltage: 5V
 
The MAX485 TTL to RS-485 Interface Module allows MCUs to use the RS-485 differential signaling for robust long distance serial communications.
 
KEY FEATURES OF MAX485 TTL TO RS-485 INTERFACE MODULE:
Use MAX485 Interface chip
Uses differential signaling for noise immunity
Distances up to 1200 meters
Speeds up to 2.5Mbit/Sec
Multi-drop supports up to 32 devices on same bus
Red power LED
5V operation
 
RS-485 provides for robust serial communications over long distances of up to 1200 meters (4000′) or in electrically noisy environments and is commonly used in industrial environments.  It supports up to 2.5MBit/Sec data rates, but as distance goes up, the maximum data rate that can be supported comes down.
You can think of RS-485 as RS232 on steroids.  The data starts out as typical TTL level serial as far as the microcontroller is concerned while the RS-485 module takes care of converting the electrical signals between TTL and the differential signaling used by RS-485.
Multi-Drop Support for Multiple Devices
 
A significant benefit of RS-485 is that it supports multiple devices (up to 32) on the same cable, commonly referred to as ‘multi-drop’.
This works by passing the bus through each device where it picks off the signal as it passes through as shown below.
These devices are typically setup in a Master / Slave configuration with one Master and one or more Slave devices.  Since they all share the same bus, to avoid conflict the Slave devices only talk when they are asked for something by the Master such as requesting a temperature reading.
 
Differential Signaling
The RS-485 uses differential signaling and requires only 2 wires and a common ground.
Differential signals operate by putting the signal on 1 wire and the inverse of the signal on the other wire.  This improves the signals noise immunity and the ability to recover the signal at the far end of the cable as noise tends to couple into both lines equally and therefore cancels out at the receiving end.
 
Wiring RS-485
These two differential data lines are labeled as A & B.  On the module, these are available on the screw terminal block as well as the two center pins on that end of the module.
When connecting the modules together, the wiring is straight through, so A on one end should be connected to A on the other end and B connects to B.
The wires should ideally be twisted pair.  Using twisted pair becomes more important for longer runs or where there is a lot of electrical noise.  For simple breadboard testing or other short runs, it is not necessary.  A common ground is needed, but this can often be provided by the earth ground at each end for shorter runs.  Network cable is often used for connecting RS-485 as it provides twisted pair, plus it can provide a ground wire as well.
 
Pull-up Resistors
The module provides four 10K pull-up resistors on the data lines.
There are two 20K resistors on the A/B differential lines.  These pull the lines to a known state when data is not being transmitted.
Finally there is a single 120 ohm resistor (R7).  This resistor goes between the A/B differential lines on each end of the cable to prevent reflections.  If using in a multi-drop configuration, the modules on the two ends of the line should keep these resistors.  Modules in the middle of the line should have these resistors removed to prevent loading the lines too heavily as shown in the pic above.  This requirement can often be ignored when the number of devices is small.
 
Module Connections
The module has two 4-pin headers on the assembly.  The headers are spaced 1.6″ apart, so if using with solderless breadboards, it is necessary to bridge two different breadboards.
1 x 4 Header (Data side)
RO  =  Receiver Output.  Connects to a serial RX pin on the microcontroller
RE  =  Receiver Enable.  Active LOW.  Connects to a digital output pin on a microcontroller. Drive LOW to enable receiver, HIGH to enable Driver
DE  =   Driver Enable.  Active  HIGH.  Typically jumpered to RE Pin.
DI  =   Driver Input.  Connects to serial TX pin on the microcontroller
1 x 4 Header (Output side)
VCC =  5V
B      =  Data ‘B’ Inverted Line. Common with the B
A      =  Data ‘A’ Non-Inverted Line.  Connects to A on far end module
GND =  Ground
1 x 2 Screw Terminal Block (Output side)
B      = Data ‘B’ Inverted Line. Connects to B on far end module
A      = Data ‘A’ Non-Inverted Line.  Connects to A on far end module