The two lowest order bits in the SPCR control register, named SPR1 and SPR0, determine the data exchange frequency expressed in bits per second; this frequency is also known as the baud rate. Compatible with all formats, data/baud rates, and bits per character. 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. RS232 uses inverse logic; that is, a positive bit at the 68HC11 UART is inverted by the onboard RS232 driver chip and appears as a negative signal on the serial cable. In fact, a single driver chip on the UART Wildcard is used to implement both RS422 and RS485 communications for a given serial channel. Each RS422 driver transmits a differential pair of output signals at 0 and 5 volts. Because a single pair of conductors is used for both transmission and reception, RS485 is useful for multi-drop applications in which a master communicates with multiple slave serial devices, or nodes.

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. The QScreen Controller, however, does not implement hardware handshaking. The Serial 1 port is implemented with the 68HC11's on-chip hardware UART (Universal Asynchronous Receiver/Transmitter). In fact, the program works the same as it did before, but now it is using the secondary serial port instead of the primary port -- and you didn’t even have to recompile the code! 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 resulting signal levels on the interface cable connect the local and remote in a manner specified by a standard protocol. 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 secondary channel is very useful for debugging application programs that communicate with other computers or I/O via the primary channel.

We can gain insight into the operation of the RS232 protocol by examining the signal connections used for the primary serial port in Table 9 6. The transmit and receive data signals carry the messages being communicated between the QScreen Controller and the PC or terminal. The data transfer that is in process when the write collision occurs is completed. If SPIF is set, reading the received data or initiating a new data transfer automatically clears the SPIF bit. 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. To avoid contention on the RS485 bus, the application software must assure that only one transmitter is enabled at a time. Owing to hardware constraints, if modem handshaking is needed on UART channel 1, then channel 1 must be configured for RS232, and channel 2 cannot be configured for RS232 communications. These signals may alternatively be redirected to the digital inputs and outputs used by the second serial port if hardware handshaking is required. 12 volts and outputs logic level (0 or 5 volt) signals to the UART circuitry.

A break sequence forces the serial output to a logic low (space) at the UART. Each RS232 driver uses inverting logic and implements a single-ended bipolar output voltage (that is, one signal that swings above and below ground). Pin 3 of PortA is the Serial2 input, and pin 4 of PortA is the Serial2 output. The maximum Serial2 communications rate is 4800 baud. The QED-Forth kernel includes pre-coded drivers that configure and control the SPI for maximum speed data transfers. In this manner, data can be exchanged between the master and each slave on the bus. When the /SS input goes low, the slave (or QScreen in this case) transfers data in response to the SCK clock input that is initiated by the master. The DWOM bit determines whether Port D needs pull-up resistors; it should be set to 0. The MSTR bit determines whether the device is a master or slave. At any given time, only the master and a single "active" slave communicate. 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.

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