Only one active master may control the network at a time; however, the device that assumes the role of master may change according to an appropriate protocol. Regardless of the network, however, there are only four signals used: SCK provides a synchronized clock, MOSI and MISO signals are used for data transmission and reception, and /SS configures the QScreen as a master or slave device. InitRS485() configures Port D to ensure that bit 5 is an output. Each device has a communication port with two terminals, which are indicated for the sake of convenience as A and B. In these two terminals the communication cable is connected so that all the devices that take part in the communication are connected in parallel. The two jacks are wired in parallel by circuit traces on the circuit board. The device that initiates a data transfer is the master, and all other devices on the network are slaves. It may be that only the byte sent from the master to the slave is meaningful; nevertheless, each device simultaneously transmits and receives one byte. It receives bytes sent by a slave device via the "master in/slave out" pin, MISO. The byte-sized messages are transmitted and received via the MOSI (master out/slave in) and MISO (master in/slave out) pins.

Configured as a master device, the QScreen transmits bytes via the "master out/slave in" pin, MOSI. As the master transmits a byte to an active slave (that is, a slave with its /SS input active low), the master receives a byte from the slave. The status of a device as master or slave determines how the various pins must be configured. If you are using the QScreen as a slave device and require the /SS signal for your external SPI hardware, configure one of the Port A pins on the Field Header as an input pin. Hardware is interfaced to the SPI via three PORTD pins named SCK, MOSI, and MISO brought out to pins 7, 8, and 10 on the Wildcard Port Header (see Appendix B). Port to modem communications usually use 300, 1200, 2400, 4800, 9600, 19200, 38400, 57600 and 115200 baud. 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 signals are used in a wide range of computer and automation systems. If you are running Serial2 at 4800 baud, the rest of your application must be able to function properly using the remaining portion of the CPU time.

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. In all other cases, half duplex and full duplex will work equally well. Also, several non-serial interrupts can stack up; if they have higher priority than the serial interrupts, they will be serviced before the Serial2 interrupt routine, and again a serial input or output bit may be lost. The primary serial channel can operate at standard speeds up to 19200 baud and can be configured for either RS232 (the default) or RS485 operation. The Serial2 channel is always configured for RS232 communications, and can sustain baud rates up to 4800 baud. The maximum sustainable baud rate on the secondary serial port is 4800 baud. The SPI can transfer data much more rapidly than an asynchronous serial link - its maximum rate is 2 Megabits/second. The QED-Forth kernel includes pre-coded drivers that configure and control the SPI for maximum speed data transfers.

QED-Forth includes three built-in routines to facilitate control of the RS485 transceiver. When it is low, the transceiver is in receive mode. 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. 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. RS485Receive() clears bit PD5 to place the transceiver in receive mode, and RS485Transmit() sets bit PD5 to place the transceiver in transmit mode. When this bit is high, the transceiver is in transmit mode. The QScreen Controller controls the RS485 transceiver with bit 5 of Port D of the processor. If your application requires use of the secondary serial port as well as other interrupt routines, the key is to keep the interrupt service routines short and fast.

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