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RS485 is another protocol supported by the primary serial port on the QVGA Controller. 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. A three-post jumper located between the third socket and H6 configures the primary serial port for either RS232 or RS485 operation. The SCK pin clocks the serial A/D’s CLK input which causes the A/D’s conversion result to be transferred to the master via the MISO line. If you are using the QScreen as a slave device and require the /SS signal for your external SPI hardware, configure one of the Port A pins on the Field Header as an input pin. 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.

Table 11 5 shows the connection diagram for a standard 9-pin serial cable. 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. From the QVGA Controller’s point of view, these three signals (TxD, RxD, and ground) are the only connections required to perform serial communications. In the alternative, ground one end of the shield and connect the other end to ground through a bi-directional transient protector (from a few volts to a few hundred volts depending on the situation). 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 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. Ideally, the two ends of the cable will have a termination resistor connected across the two wires.

I will know the price of the (TWO) wire RS485. The termination also includes pull up and pull down resistors to establish bias for each data wire for the case when the lines are not being driven by any device. The aforementioned SenseCAP Sensor Hub at its stock has only 4 ports to connect RS485 sensors despite it being able to support up to 32 different RS485 sensors. It’s important to note that all devices on the network need to use the same logic for interpreting voltage differences as bits (i.e., whether A being at a higher voltage than B represents a ‘1’ or a ‘0’).In a network with multiple devices, each device needs to have a unique address so that it knows when to listen and when to ignore the communications on the line. RS-485 cables typically have two twisted pairs of wires for differential signaling, allowing for greater immunity to electromagnetic interference. RS485 provides a means of interconnecting multiple devices in a series using a twisted pair of wires to facilitate data exchange. This allows you to configure full duplex multi-drop networks in which a single master can sequentially address one of many slaves and start a full-duplex exchange of data.

The RS485 network must be designed as one line with multiple drops, not as a star or ring topology. 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. It is possible to connect several RS485 circuits in parallel if the distances are below about 200 feet per leg @ 9600bps. At greater distances and higher data rates, the cable impedances add up and load the network. This means that it can transmit data up to 4,000 feet without needing any signal amplification or repeaters. It provides a convenient means of connecting the QVGA 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. RS232 could only handle one at a time. 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.

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