The recommended arrangement for connecting devices to an RS-485 network is to use a series on nodes connected along a line or bus with terminating resistors used at either end of the bus to reduce reflections. The RS485 Pi contains protection against voltage spikes in the form of a TVS Diode and two 10-Ω, Pulse-Proof Thick-Film Resistors. In that case, when using very long cables you can improve noise immunity and assure a valid idle level when the transceiver is not active by installing bias resistors. Resistive termination - If the PDQ Board is at the end of the RS485 cable you can terminate the cable by installing jumper caps at both jumper locations, "Term" and "RTerm". RS485 Pi at the top of the stack due to the height of the DB9 connector. The SN65HVD72 IC converts the 3.3V UART signals to RS-485 voltages allowing communication with RS-485 compatible devices over a DB9 serial cable or twisted pair cable. The technique was invented decades ago, and allowed you to connect to a few dozen MODBUS devices over simple twisted pair copper wire, over a thousand feet distance or more. It's such a successful technique that it has not been supplanted by new modern communication technologies like TCP/IP.
There are many different types of serial communication protocols like I2C and SPI which can be easily implemented with Arduino and today we are going to look at another most commonly used protocol called RS485 which is very commonly used in high noise industrial environments to transfer the data over a long distance. The RS485 system used for Modbus communication provides a main cable (Bus or backbone), to which all the devices have to be connected with branches (also known as stubs) that are as short as possible. The branches must be no longer than 1200 m! The RS-485 port on the RS485 Pi can be accessed through the male DB9 socket or the solder points on the PCB. The RS-485 port can be accessed through the DB9 port or the solder points on the PCB. The SPI can transfer data much more rapidly than an asynchronous serial link - its maximum rate is 2 Megabits/second. The actual baud rate produced differs from that requested by a small error owing to rounding of an internal divisor. 1 running at the prior established baud rate (typically 115200 baud). Running MODBUS/RTU over an RS485 network is pretty cool, in an old-school way.
There are surface-mount resistor pads on the Docking Panel to bring out the RS485 signals to the DB9 Serial 1 Connector. End of line termination resistor (120 ohm 1/4 watt) to be installed at both ends of network. The last line says the device name, in this case /dev/ttyUSB1. This can vary greatly depending upon the device. Please note that you can only use one RS485 Pi board on a Raspberry Pi. This will be required if the RS485 Pi is used as an end node within an RS485 network. Bus Probes can be configured and tested using our web-based test app (you will need to connect the probes to your PC or Android smartphone via our Web485 board). A 120-Ω resistor is included with the RS485 Pi which will need to be fitted if the board is connected to the end of the RS-485 bus. Finally, if this means that enabling RS485 mode is not required in this configuration, can the FTDI cable be bypassed entirely by using Raspberry Pi's built-in UARTs combined with a chip like ZT485EEN?
Equipment using the RS-485 standard can be used over long distances in noisy environments. So I would not run low level MCU GPIO level signals over the cable wires. Remember that the /SS is active low so to select a device you need to set the pin low; otherwise the pin should idle high. Please contact Mosaic Industries if you need this custom configuration. You need to use the latest Raspbian Jessie release and disable the built in Bluetooth to use the RS485 Pi on the Raspberry Pi 3, see our KB page for the config changes needed. The shield works as a "drain" for any noise that could be picked up by the RS485 network and "drain" it to ground. It gets away most of the time with this configuration, with the shield draining noise and providing the ground reference at the same time. As it is a half-duplex IC hence it cannot both transmit and receive at the same time. But it does not work ALL the time. Each serial port can be configured for the RS232 or RS485 protocol, and runs at standard baud rates up to 115,200 bits per second.