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In the most common multi-drop RS-485 protocol, one computer is designated as a "master" and the rest of the computers or devices on the serial bus are designated as "slaves". RS-485 signals are used in a wide range of computer and automation systems. RS-485, also known as TIA-485(-A) or EIA-485, is a standard, originally introduced in 1983, defining the electrical characteristics of drivers and receivers for use in serial communications systems. RS-485 supports inexpensive local networks and multidrop communications links, using the same differential signaling over twisted pair as RS-422. RS-485, like RS-422, can be made full-duplex by using four wires. In these distributed processing networks, a variety of machines and instruments work locally, but communicate and share data or resources with one another globally using a single serial link. 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. It provides a convenient means of connecting the QScreen Controller to a variety of peripheral devices, including analog to digital and digital to analog converters, real time clocks, and other computers which use high speed communication. By polling the Port A pin or by setting up an interrupt service routine, you can configure the QScreen to ignore the SCK input when /SS is high and keep MISO in a high-impedance state so that it does not interfere with the SPI bus.

Moreover, if Serial2 is running full duplex at 4800 baud, any other interrupt service routine that takes longer than 100 µs is likely to cause a problem. SPIE is a local interrupt mask that allows an interrupt to be recognized when an SPI data transfer has completed, or if a write collision or mode fault is detected. If the /SS pin of the master is an input and if a low input level is detected, the processor sets the MODF bit in the SPI status register a "mode fault" condition. 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. If SPIF is set, reading the received data or initiating a new data transfer automatically clears the SPIF bit. InitRS485() configures Port D to ensure that bit 5 is an output. The maximum sustainable baud rate on the secondary serial port is 4800 baud.

You can implement the slave select lines by configuring Port A pins as outputs. To use a QScreen as a slave in a multi-drop network, simply define a word, (named Silence(void), for example) that when executed calls RS485Receive() to wait for any pending character transmission to complete, then disable the transmitter, and then execute a routine such as Key() to listen to the communications on the serial bus. DH 485 is a proprietary communications protocol used by Allen-Bradley in their line of industrial control units. This section describes the QED-Forth routines that control the RS485 transceiver, and presents some ideas that may prove useful in designing a multi-drop data exchange protocol. In this section we will consider the most general and simple configurations. Section 4 defines the electrical characteristics of the generator (transmitter or driver), receiver, transceiver, and system. RS-485 only specifies the electrical characteristics of the generator and the receiver: the physical layer.

The equipment located along a set of RS-485 wires are interchangeably called nodes, stations or devices. Ideally, the two ends of the cable will have a termination resistor connected across the two wires. The two values provide a sufficient margin for a reliable data transmission even under severe signal degradation across the cable and connectors. For the QScreen, /SS is not used for SPI communication because it is used to control the direction of the RS485 transceiver; you can use any digital I/O line as a /SS signal. This function properly configures the directions of the SPI I/O pins, and configures the data transfer such that data is valid on the falling trailing edge of the clock, with the clock idling in the low state. The /SS (active-low slave select) is typically used to enable data transfers by slave devices when it is active low. The master and slave could even exchange ascii QED-Forth commands. When the network master wants to talk to this particular slave, it outputs the slave’s ascii name onto the serial bus.

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