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This connection may be used to limit the common-mode signal that can be impressed on the receiver inputs. These signals may alternatively be redirected to the digital inputs and outputs used by the second serial port if hardware handshaking is required. 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. It is essentially a form of serial communication. It provides a convenient means of connecting the QScreen Controller to a variety of peripheral devices, including analog to digital and digital to analog converters, real time clocks, and other computers which use high speed communication. If you are using the QScreen as a master device, each external SPI device will require a separate select line (/SS). 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. 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. We have built sophisticated instruments using the QVGA Controller that operate very reliably using multiple interrupts in addition to the software UART. TSB-89A, Application Guidelines for TIA/EIA-485-A does not recommend using star topology. In addition there is no good way to add terminations resistors at the ends of a "star" network. Star and ring topologies are not recommended because of signal reflections or excessively low or high termination impedance. The recommended arrangement of the wires is as a connected series of point-to-point () nodes, a line or bus, Ideally, the two ends of the cable will have a termination resistor connected across the two wires. The combination of the cable impedances and/or termination resistors will load the network and can make communications unreliable. The primary and secondary serial communications ports are accessible through the QVGA Controller's 10 pin, dual row Communications Connector (H14) and through the individual Serial 1 and Serial 2 connectors.

The QScreen 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). 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). A serial communications cable is supplied with QVGA Starter Kits. By connecting pairs of these handshaking signals together, the terminal or PC can be made to think that the QVGA Controller is always ready to send and receive data. We can gain insight into the operation of the RS232 protocol by examining the signal connections used for the primary serial port in Table 11 5. The transmit and receive data signals carry the messages being communicated between the QVGA Controller and the PC or terminal. An RS485 transceiver is present on the QVGA Controller, and its data direction is controlled by pin 4 of port PPC of the peripheral interface adapter (PIA). The QVGA Controller, however, does not implement hardware handshaking. In a computer system, SCSI-2 and SCSI-3 may use this specification to implement the physical layer for data transmission between a controller and a disk drive.

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 offers a unique addition to full duplex RS232: it can place its RS232 transmitter into a high impedance silent mode under software control. 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), what is rs485 cable then there is a serial interrupt every 100 µs or so. These standard 9-pin serial connectors are located on the top edge of the QVGA Controller. From the QVGA Controller’s point of view, these three signals (TxD, RxD, and ground) are the only connections required to perform serial communications. The pinout of the Communications Connector (H14) is shown in Table 11 2, and the following diagrams show how these signals map to the two serial communications connectors. Thus in Table 11 5 , RTS1 is connected to CTS1, and DSR1 is connected to DTR1 onboard the QVGA Controller using zero ohm shorting resistors. By using repeaters very large RS-485 networks can be formed.