The maximum cable length depends on the baud rate and the type of cable used. As a general rule, the MAX485EPA can support cable lengths up to 4000 feet (1219 meters) at a baud rate of 9600bps, and up to 1000 feet (305 meters) at a baud rate of 115200bps.
To ensure reliable communication in noisy environments, use twisted-pair cables, keep the cable length as short as possible, and use a common-mode choke or ferrite bead to filter out electromagnetic interference (EMI). Additionally, consider using a shielded cable and connecting the shield to the GND pin of the MAX485EPA.
The maximum number of transceivers that can be connected to a single RS-485 bus is 32, as specified in the RS-485 standard. However, the actual number of devices that can be connected may be limited by the bus loading, cable length, and other system parameters.
To prevent signal reflections, a termination resistor (typically 120 ohms) should be connected between the differential lines (A and B) at each end of the bus. This ensures that the signal is properly absorbed and does not reflect back into the bus.
The power-up sequence for the MAX485EPA is as follows: 1) Apply VCC, 2) Apply DE (driver enable) and RE (receiver enable) signals, and 3) Apply the RS-485 bus signals. This sequence ensures that the device is properly initialized and ready for operation.
MAX485EPA datasheet
by Maxim Integrated Products
