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In each case, your QVGA Controller will be communicating at 19200 baud, and this baud rate will remain in effect until another BAUD1.AT.STARTUP command is executed, or until you invoke the Special Cleanup Mode as described earlier. 19200 baud, select the "Comm" item under the "Settings" menu and click on 19200 in the "Baud Rate" selection box. Now select the "Communications" item in the "Settings" menu of the Terminal program, and click on 1200 baud (or whatever baud rate you selected in the command above). If you have not yet compiled the GETSTART program and you want to do the exercises here, open GETSTART.C in your TextPad editor, click on the Make Tool (the Make icon), and after the compilation is done, enter the Terminal Program by clicking on the terminal icon and use the "Send Text File" menu item to send GETSTART.DLF to the QVGA Controller. Since both channels can operate simultaneously and independently, debugging can be performed while the application program is communicating via its primary channel. Modbus devices communicate using a Master-Slave technique where only one device (the Master) can initiate transactions (AKA queries). The only difference between the master and slave devices is that the master initiates the transmission.

The transmitter and receiver devices are connected to the same wire pairs, and the ground wire is connected to the signal ground on both devices. Rather, the UART deduces the correct time to sample the incoming signal based on the start and stop bits in the signal itself. For example, at 4800 baud (bits per second), each bit lasts about 200 microseconds (µs), and if communications are full duplex (e.g., if the QVGA Controller echoes each incoming character), then there is a serial interrupt every 100 µs or so. The advantage of such a multi-drop RS-232 network is that the communications are full duplex, with each communicating party capable of simultaneous transmission and reception of data. To implement this multi-drop scheme, each slave keeps its RS-232 transmitter silent until it is addressed by the master and is given permission to transmit. The QVGA Controller, however, does not implement hardware handshaking. The primary serial port, Serial1, is supported by the 68HC11's on-chip hardware UART, and does not require interrupts to work properly.

We have built sophisticated instruments using the QVGA Controller that operate very reliably using multiple interrupts in addition to the software UART. However, note that the ReadWatch() and SetWatch() functions disable interrupts for about one millisecond (msec.), and the functions that write to EEPROM disable interrupts for 20 msec. The QScreen Controller’s kernel software contains a complete set of high level driver routines for the Serial2 port, and these functions are summarized in the Control-C Glossary. The QVGA Controller’s kernel software contains a complete set of high level driver routines for the Serial2 port, and these functions are summarized in the Control-C Glossary. The software routines, RS232Transmit() and RS232Silent() control the dual RS232 transmitters on the board. This allows the processor that is master to control the input /SS pins of other CPU’s, for example. If the 68HC11 is initialized as a master (by setting the MSTR bit in the SPCR control register as explained below) then bit 5 of the Port D data direction register (DDRD) determines whether /SS is an input or an output. If bit 5 of DDRD is 0, then /SS is an input. If bit 5 of DDRD is 1, then /SS is a general purpose output that operates independently of the SPI.

Otherwise, dip switch 5 should be OFF so that it frees bit 3 of PORTA for use as general-purpose I/O. Its use of differential signaling makes it resistant to noise, and its support for up to 32 devices on a single network makes it suitable for a wide range of applications. To properly operate the network each slave computer executes RS232Silent at startup; thus all of the slave transmitters remain silent individually addressed by the master. This allows standard point-to-point full duplex communications, as well as a multi-drop configuration with one master (a single QVGA Controller or a desktop computer) and multiple QVGA Controller slaves. This makes RS485 more suitable for applications with multiple devices requiring communication. RS485 is made for serial communication with high speeds, support for long distances, and support for multiple devices (slaves). That’s why common communication protocols like BACnet MS/TP and Modbus typically recommend baud rates between 9.6 - 115.2 kbps. The main difference between these two protocols is that RS485 is designed for use in multidrop networks, while RS422 is designed for point-to-point communication. Most newer consumer electronics comes with an USB, Firewire or other high-speed communication port, however a lot of industrial and commercial equipment, both old and new, is made only with a RS485 or RS422 serial communications port.

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