Figure 11 1 shows a typical multi-drop communications network. Table 11 5 shows the connection diagram for a standard 9-pin serial cable. A serial cable accomplishs this connection. In this case, cable connections must be made to Serial 1 at pins 5 and 6 of the 10-pin Serial Header or pins 7 and 8 on the 24-pin Field Header. A three-post jumper located between the third socket and H6 configures the primary serial port for either RS232 or RS485 operation. For RS485 operation: Install the jumper shunt across the two pins closest to the J3 silk screen. For RS232 operation: Install the jumper shunt across the two pins closest to the crystal (the default configuration). To provide a convenient means of attaching two grounds to the serial cable, there are several pins (labeled DGND) on the communications connector that are connected to the controller’s ground plane. Chassis and signal grounds are connected together to the digital ground (DGND) signal. The QVGA Controller’s transmit data signal TxD1 (pin 2 on the 9-pin serial connector) is connected to the terminal’s receive data signal RxD (pin 2 on its 9-pin connector).
Likewise, the terminal’s transmit signal TxD is connected to the QVGA Controller’s receive signal RxD1. If you do this now, remember to move the QVGA Controller’s serial connector back to Serial Port 1, and to change the terminal’s baud rate back to 9600 baud using the “Communications” item under the terminal’s “Settings” menu. 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. Select the firmware for the appropriate module — Wherever you have downloaded it to.
We have built sophisticated instruments using the QVGA Controller that operate very reliably using multiple interrupts in addition to the software UART. The primary serial port, Serial1, is supported by the 68HC11’s on-chip hardware UART (sometimes called a USART), and does not require interrupts to work properly. 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. 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. 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. The software routines, RS232Transmit() and RS232Silent() control the dual RS232 transmitters on the board. When a slave is addressed, it executes RS232Transmit() at which point full duplex (two-way) communications commences between the master and the selected slave. When the network master wants to talk to this particular slave, what is rs485 cable it outputs the slave’s ascii name onto the serial bus. When the communication is complete the slave silences its transmitter so that the master can talk to a different slave on the network.
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. Many terminals and PCs, however, do rely on hardware handshaking to determine when the other party (in this case the QVGA Controller) is ready to accept data. The primary serial port, Serial1, is supported by the 68HC11’s on-chip hardware UART, and does not require interrupts to work properly. If more than one slave tried to drive the transmit line simultaneously, their serial drivers would fight with each other for control of the bus. All devices, or units, on the link generate an electrical load to produce or detect this voltage, increasing with the addition of more devices and units. RS-485 signals manifest as a differential voltage across the communication link. Connecting a standard full duplex link RS232 between two computers is the same as with a standard RS232 link, with the TxD (transmitter output) of each computer connected to the RxD (receiver input) of the other computer.