If SPIF is set, reading the received data or initiating a new data transfer automatically clears the SPIF bit. Then reading the data that was received (by reading the SPDR) or initiating a new data transfer (by writing to the SPDR) automatically clears the SPIF flag. The SPIF is set when a data transfer is complete, and is cleared by a read of the SPSR status register, followed by a read or write to the SPDR data register. The MODF bit is cleared by a read of the SPSR followed by a write to the SPCR. WCOL is cleared by a read to the SPSR followed by a read or write to the SPDR. After a data transfer is initiated by writing to the SPDR data register, the processor may poll the SPSR status register until the SPIF flag is set. Grounds between buildings may vary by a small voltage, but with very low impedance and hence the possibility of catastrophic currents - enough to melt signal cables, PCB traces, and transceiver devices. To further reduce the effects of interference, a low Baud rate of 2400 (or 1200) Baud is used. The total length of your network is not strictly governed by the baud rate.

Pre-coded device drivers configure the SPI for a standard data format, and it is easy to customize a data format and baud rate for your application. RS-485 standard conformant drivers provide a differential output of a minimum 1.5 V across a 54-Ω load, whereas standard conformant receivers detect a differential input down to 200 mV. 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. Texas Instruments (Technical report). RS485 is also commonly used for building automation as the simple wiring configuration and lengthy cable length are ideal for joining remote devices. The RS485 communication interface allows the slave unit (i.e. control module) to be interrogated and some options programmed by a remote computer. If a slave device has already stored a byte into its SPDR register, that byte will be exchanged with the master’s byte. However, if you’re looking to experiment, the latter will provide you with more versatility.

You can find out more about the differences between RS485 and RS232 here. It is like the Universal Serial Bus (USB) or ethernet that we can find in many of our modern computers. Now, why don’t companies just use USB or ethernet to transmit data between their devices? It has anti-ultraviolet and anti-rain features as well, making it suitable for long-term outdoor use. By adding another 2 wires, making it a 4 wires system, it allows data transmission in both directions to and fro devices at the same time, also known as full-duplex. Serial communication also has a deterministic behaviour to avoid collisions of data packets, making it more reliable for a linkage system with many devices. Another major advantage is that there can be more than one slave in the connection. However, in a full-duplex setup, they are limited to a master and slave communication where slaves cannot communicate with each other. In this setup both the TX and RX share a single pair of wires, therefore it is limited to half-duplex data transmission as the data cannot be transmitted simultaneously to and fro each device.

Most Modbus implementations use RS485 due to the allowance of longer distances, higher speeds and multiple devices on a single network. The interface can be used to support analog to digital and digital to analog converters, networks of many computers controlled by a single master, or networks of devices controlled by several coordinated masters. The data rates, often termed baud, of RS-485 can vary from a rapid 10 Mbps (megabits per second) to a sluggish 9.6 kbps (kilobits per second). In this way, data can be sent back and forth across an RS-485 network. RS485, CAN bus to build network with both of the signal/power in two wires (total bus can provide 960 watt power and 12 watt in every terminal). PTZ and camera zoom Could be controlled by RS485 and RJ45 network. RS485 is used more industrially where many devices need to be interconnected together for a system. Ultimately, it can be thought of in a way that serial communication is more made for this usage as compared to the common USB and ethernet. Compared to serial communication, USB and ethernet are significantly more complex and expensive. There are many standards in serial communication and RS485, also known as TIA-485 (-A) or EIA-485, is just one of them.

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