Consult their Glossary Entries for Details

A 2-wire synchronous IIC (Inter-IC) bus provides multi-drop signaling at rates up to 100 Kbaud. The WattNode® BACnet and Modbus® meters communicate using RS-485 (TIA/EIA-485-A) half-duplex, differential signaling at 9600, 19200, 38400, and 76,800 baud. 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. So take advantage of that and let your systems MCUs implement heart beat handshaking directly though the RS485 communications layer using appropriately designed small packets. 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. If an existing standard has some value to you, then you might have a look at PoE. A PoE device needs to accept both variations in either polarity (4 possibilities) to be fully standards-compliant, in addition to the signalling that a standards-compliant source requires to enable the power on that port. The SCK pin’s synchronous clock signal has configurable phase, polarity and baud rate so that it can interface to a variety of synchronous serial devices. The QScreen allows the details of the synchronous communications protocol to be customized for compatibility with a variety of peripherals.

RS232 has numerous handshaking lines (primarily used with modems), and also specifies a communications protocol. The dual communications channels also provide an easy way to link systems that communicate using different serial protocols. In fact, a single driver chip on the UART Wildcard is used to implement both RS422 and RS485 communications for a given serial channel. RS485 is a half duplex version of RS422 that is capable of multi-drop operation. RS485 multi-drop networks are daisy-chained networks with a single cable connecting multiple devices. You can use the QScreen’s RS485 link to create such a multi-drop serial network. Done that establishes the RS485 receive mode is coded such that it waits until all queued (pending) characters have been transmitted before the driver chip is taken out of transmit mode. We will send some data from the transmitter side over the cable from the Nano which is converted to RS485 signals via MAX RS485 Module working in Master Mode. A mode fault occurs when the SPI senses that a multimaster conflict (MC68HC11F1 Technical Data Manual, p.8-7) exists on the network as explained above in connection with the /SS input.

The secondary serial port is connected similarly except that the onboard connection of /RTS to /CTS, and /DSR to /DTR are permanent. They are often used for serial connections going to a terminal server in datacentres, for example. 2 pairs otherwise. RS485 each direction, for example. One uses the data lines for 10/100 Ethernet, so that there are still 2 unused pairs in that cable. The other uses those unused pairs. This is often used by switches and other networking gear that uses the data lines anyway. A logic-low start bit marks the start of a character, followed by 5 to 8 data bits per character. You might make it work anyway, with a bit of stuffing at the protocol level, or it might "just work" with sparse communication and a receiver that interprets 0V differential as idle. However this contradicts the facts that an idle mark state is a logical one and the termination polarization puts B at a higher voltage in Profibus guidelines.

In that case, rs485 cable 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. So I would not run low level MCU GPIO level signals over the cable wires. Table 9-6 Serial Cable Connections. By default, the RS485 connections are not brought out to the Docking Panel’s DB-9 Serial1 Connector, although custom placement of zero-ohm surface-mount resistors on the Docking Panel can route the RS485 signals to the DB-9. The Serial1 and Serial2 ports have identical communications capabilities, although more of the Serial1 signals (both RS232 and RS485) are made available on the Docking Panels headers and connectors. Although the RS232 protocol specifies functions for as many as 25 pins, each communications channel requires only three for simple serial interfaces: TxD1 (transmit data), RxD1 (receive data), and DGND (digital ground). It does not specify or recommend any data protocol. When PE is cleared (equal to zero), the most-significant bit of each transmitted character will be a data bit. Once the data has been exchanged, a flag bit in the SPSR status register is set to indicate that the transfer is complete. Sometimes you can read from some register that has a known value.