It is a half duplex protocol, meaning that only one party at a time may transmit data. Many terminals and PCs, however, do rely on hardware handshaking to determine when the other party (in this case the QScreen Controller) is ready to accept 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 9 6. The transmit and receive data signals carry the messages being communicated between the QScreen Controller and the PC or terminal. The Serial 1 port can be configured for either RS232 or RS485 communications at up to 19200 baud. 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. 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.
A jumper, J3, configures the primary serial port for either RS232 or RS485 operation. 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. For those of you interested in the details, here’s how it works: The low-level serial driver routines named Key(), AskKey() and Emit() are revectorable routines that can be redirected to use either of the serial ports. You can use it to communicate with other devices. RS232’s greatest benefit is its universality; practically all personal computers can use this protocol to send and receive serial data. It controls the serial-to-parallel and parallel-to-serial conversion and performs all of the timing functions necessary for asynchronous serial communications. Given the availability of ready-made communications cables, it is not necessary to study or understand the following descriptions of cable connections. In this case, cable connections may be made to Serial 1 on either the 10-pin Serial Communications Header or the Serial 1 Connector. Although the RS232 protocol specifies functions for as many as 25 pins, each communications channel requires only three for simple serial interfaces: TxD1 (transmit data), RxD1 (receive data), and DGND (digital ground).
RS232 allows both communicating parties to transmit and receive data at the same time; this is referred to as full duplex communications. Rather, it relies on software handshaking via transmission of XON/XOFF characters to coordinate data transfer and ensure that information is not lost when one of the communicating parties is busy. The RS232 protocol provides for four handshaking signals called ready to send (RTS), clear to send (CTS), data set ready (DSR), and data terminal ready (DTR) to coordinate the transfer of information. RS232 is by far the most common protocol. Its simplest implementation requires only three wires: one to transmit serial data, a second to receive serial data, and a third to provide a common ground reference. Table 9-6 shows the connection diagram for a standard 9-pin serial cable. 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 dual communications channels also provide an easy way to link systems that communicate using different serial protocols. Since both channels can operate simultaneously and independently, debugging can be performed while the application program is communicating via its primary channel.
While these signals provide a data path, they do not provide hardware handshaking that allows the two communicating parties to let each other know when they are ready to send or receive data. The RS232 protocol specifies the use of two separate grounds, a signal ground and a protective (or “chassis”) ground. Their main difference boils down to one having only support for RS485, while the other one supports RS232 and TTL as well. • Video Input: aviation connector,digital component signal access.NVR and PTZ camera connect by special aviation cable, support 1 channel 1080P IP PTZ camera input and 4 channel AHD input or analog input. The interface can be used to support analog to digital and digital to analog converters, networks of many computers controlled by a single master, or networks of devices controlled by several coordinated masters. 2. Support IP and analog dual signal output. Pin 3 of PortA is the Serial2 input, and pin 4 of PortA is the Serial2 output.
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