Unlike the standard RS232 protocol, RS485 allows many communicating parties to share the same 3-wire communications cable. Note that the local and the remote must share a common ground, so a minimum of 3 wires are required for half duplex RS485 communications: a pair of transceive wires and a common ground. The RS232 driver and receiver use separate conductors on the serial cables, enabling full duplex communications. Without termination resistors, reflections of fast driver edges can cause multiple data edges that can cause data corruption. The output of that switcher could be anything you want, including multiple outputs at different voltages. There is a huge benefit of the RS-485 electrical layer protocol that allows for long distance communications in the presence of multiple volts of common mode voltage differences between the two ends. 5 should work in your situation because of the voltage you will be running (5-10V) and your signal is not high frequency (above 500Mhz) in the first place.

In order to avoid signal reflections, a 120 Ohm termination resistance must be fitted on each end of the main cable. In this case, cable connections must be made to Serial 1 at pins 5 and 6 of the 10-pin Serial Header or pins 7 and 8 on the 24-pin Field Header. If on the other hand each terminal can accept only a single cable, a proper branch must be created using three auxiliary terminals for each instrument to be connected. In a finished instrument, either or both channels can be used to communicate with other serial devices, or with other computers and/or terminals using RS232 or RS485. The secondary channel is very useful for debugging application programs that communicate with other computers or I/O via the primary channel. A jumper, J3, configures the primary serial port for either RS232 or RS485 operation. If your application requires RS485, you can use the secondary serial port (serial2) to program and debug your application code using the RS232 protocol, and use the primary serial port (Serial1) for RS485 communications. Depends what you mean by a serial cable - Fibre, Ethernet, RS232, RS485, RS422, USB, HDMI, SMPTE292 are all examples of serial communications media.

The cable to be used is a shielded twisted pair (telephone type). Systems of this type (4-wire, half-duplex) are often constructed to avoid "data collision" (bus contention) problems on a multi-drop network (more about solving this problem on a two-wire network in a moment). Suggestions are often made to deal with practical problems that might be encountered in a typical network. While these signals provide a data path, they do not provide hardware handshaking that allows the two communicating parties to let each other know when they are ready to send or receive data. Owing to hardware constraints, if modem handshaking is needed on UART channel 1, then channel 1 must be configured for RS232, and channel 2 cannot be configured for RS232 communications. This distance does not include the branches (which must nevertheless be short). Longer branches could cause signal reflections and generate disturbances and consequent errors in the reception of data. The main cable must be no longer than 700 m! Using a repeater, the main cable is divided into different segments, each of which can be up to 700 m in length and connect 32 devices (this number includes the repeaters).

In order to increase the extent of the Modbus network, repeaters can be used; and signal amplifying and regenerating devices provided with two communication ports that transfer to each what they receive from the other. The maximum number of devices that can be connected to a main cable is 32, including the Master. If one or the other end fail to see the heartbeat packets come or get acknowledgement of them then know that your cable is either disconnected or has broken wires. The only way this would not work is if the secondary device only takes "write only to device" transmissions with no possibility of any acknowledgement or return result. Typing a carriage return at the terminal should now produce the familiar "ok" response via the Serial2 port. Any required SPI output signals must be configured as outputs, either by calling InitSPI() or by setting the appropriate bits in the Port D data direction register DDRD. Rather, the UART deduces the correct time to sample the incoming signal based on the start and stop bits in the signal itself. I would highly discourage the use of extra signal pins in the connectors for "cable connected" checking that do not meet the same specs as the RS485 signalling signals.

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