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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 Control-C Glossary contains a list of functions that temporarily disable interrupts, and the glossary entries give further information regarding how long interrupts are disabled. The UART Wildcard implements these optional RS232 modem handshaking signals on channel 1. The handshaking signals can be disabled and/or ignored by applications that do not need them. The RS485 protocol uses differential data signals for improved noise immunity; thus RS485 can communicate over greater distances than RS232. The SCK (serial clock) pin is a configurable synchronous data clock output. 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. This provides a wide range of interface possibilities.

Rather, it relies on software handshaking via transmission of XON/XOFF characters to coordinate data transfer and ensure that information is not lost when one of the communicating parties is busy. The Silence() routine searches the incoming serial characters for a pre-determined keyword (for example, the ascii "name" of this particular slave). 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. The QScreen Controller controls the RS485 transceiver with bit 5 of Port D of the processor. It is supported by virtually all personal computers, and is the default protocol for both of the QScreen Controller’s serial ports. To provide a convenient means of attaching two grounds to the serial cable, there are several pins (labeled GND) on the communications connector that are connected to the controller’s ground plane. The primary and secondary serial communications ports are accessible through the QScreen's 10 pin, dual row Serial Header (H5) which is typically not installed, the 24 pin, dual row Field Header (H3), and through the individual DB-9 Serial 1 and Serial 2 connectors. In this case, cable connections may be made to Serial 1 on either the 10-pin Serial Communications Header or the Serial 1 Connector.

We may already have the solution. It is a half duplex protocol, meaning that only one party at a time may transmit data. The data transfer that is in process when the write collision occurs is completed. A write collision occurs when a byte is written to the SPI data register, SPDR, while data is being exchanged. The master can instruct a single slave to go into transmit mode, and then the master can put itself into receive mode, thereby allowing the master to retrieve data from the slave. In fact, a single driver chip on the UART Wildcard is used to implement both RS422 and RS485 communications for a given serial channel. For RS485 operation: Install the jumper shunt onto J3. RS485 drivers : Up to 1200 m. 1200 is the baud rate that you choose; you can specify any standard baud rate up to 4800 baud. The Serial 2 port is dedicated to RS232 communications at up to 4800 baud.

Each RS232 driver uses inverting logic and implements a single-ended bipolar output voltage (that is, one signal that swings above and below ground). Serial 2 is implemented by a software UART in the controller’s QED-Forth Kernel that uses two of the processor’s PortA I/O pins to generate a serial communications channel. From the QScreen Controller’s point of view, these three signals (TxD, RxD, and ground) are the only connections required to perform serial communications. The BufferToSPI() function implements fast data transfer from a specified buffer in the controller’s memory to an SPI device. RS232’s greatest benefit is its universality; practically all personal computers can use this protocol to send and receive serial data. There are several protocols that govern the format of exchanged data, with the RS232 protocol used primarily by personal computers, and the RS485 protocol used in industrial control systems. RS485 is another protocol supported by the primary serial port on the QScreen Controller. Move the serial cable from the "Serial Port 1" connector to the "Serial Port 2" connector on the QScreen. Yes, it would work for that too, but 24 awg cable with plenum grade jacket is the standard.

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