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! You can operate the port at any baud rate up to 4800 baud; just specify the rate you want before the BAUD2 command. 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. This chapter describes those drivers, and presents code that makes it easy to configure the SPI for different data transfer rates and formats. These steps greatly reduce the chance that the communicating devices might be damaged by contention on the SPI bus. In this manner, data can be exchanged between the master and each slave on the bus. The RS485 protocol uses differential data signals for improved noise immunity; thus RS485 can communicate over greater distances than RS232. Many RS422 devices are compatible with RS232.

Characters may be lost if the transmit driver is turned off while pending characters are still being transmitted. 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. Slave devices use the master in/slave out pin, MISO, for transmitting, and the master out/slave in pin, MOSI, for receiving data. It is a half duplex protocol, meaning that only one party at a time may transmit data. It will indicate when each one starts and ends. If you put the wrong firmware on a module, it will brick it. There are surface mount resistor pads on the QScreen that will allow you to bring out the secondary serial port to the Field Header on pins 5-6 or 7-8 as shown with the parentheses in Table 11-3. Pads are also available to bring out the RS485 signals to the DB9 Serial 1 Connector. The secondary serial port is implemented by a software UART that controls two pins on PortA. The ComBoard RTS input line controls the driver enable line so the device only talks when it is its turn. In fact, a single driver chip on the UART Wildcard is used to implement both RS422 and RS485 communications for a given serial channel.

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. A jumper, J3, configures the primary serial port for either RS232 or RS485 operation. The secondary serial port is connected similarly except that the onboard connection of RTS to CTS, and DSR to DTR are permanent. The default serial routines used by the onboard kernel assume that full duplex communications are available, so you cannot use the RS485 protocol to program the controller. We recommend that you keep the faster Serial1 port as the default serial link as you work through the exercises in this book. It is supported by virtually all personal computers, and is the default protocol for both of the QScreen Controller’s serial ports. Likewise, the terminal’s transmit signal TxD is connected to the QScreen Controller’s receive signal RxD1. 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 arrows in the diagram point to pins configured as inputs, and originate from output pins.

Table 9-6 shows the connection diagram for a standard 9-pin serial cable. You can use the QScreen’s RS485 link to create such a multi-drop serial network. Because the software UART is interrupt based, competing interrupts that prevent timely servicing of the Serial2 interrupts can cause communications errors on the secondary serial channel. The Serial 1 port can be configured for either RS232 or RS485 communications at up to 19200 baud. Move the serial cable from the "Serial Port 1" connector to the "Serial Port 2" connector on the QScreen. The RS485 connections are not brought out to the Serial 1 Connector. In this case, cable connections may be made to Serial 1 on either the 10-pin Serial Communications Header or the Serial 1 Connector. Function prototypes for this function and other versatile serial I/O routines are defined in the COMM.H header file, and are described in detail in the Control-C Glossary. The pinout of the QScreen’s Serial Header (H5), QScreen’s Field Header (H3), and the Serial Connectors are shown in the following tables. It is easy to switch between different connectors and interface types. Before running the program, let’s switch to the secondary serial port.

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