The Hidden Mystery Behind Rs485 Cable > 자유게시판

The Serial1 and Serial2 ports have identical communications capabilities, although more of the Serial1 signals (both RS232 and RS485) are made available on the Docking Panels headers and connectors. It may be used to control video surveillance systems or to interconnect security control panels and devices such as access control card readers. Circuits may be terminated on screw terminals, D-subminiature connectors, or other types of connectors. The pinout of the PDQ Board’s Communications Header (H2), Docking Panel’s Communications Header (H1), and the Docking Panel’s Communications DB-9 Connectors are shown in the following tables. The mating 10-pin connectors that join the H6 header of the PDQ Board to the H4 header of the Docking Panel are typically not accessed directly, and are not discussed in detail here. It is NOT necessary to introduce long delays in a network to avoid "data collisions." Because delays are NOT required, networks can be constructed, that will utilize the data communications bandwidth with up to 100% through put.

From the QScreen Controller’s point of view, these three signals (TxD, RxD, and ground) are the only connections required to perform serial communications. Because all of the serial I/O routines on the PDQ Board are revectorable, it is very easy to change the serial port in use without modifying any high level code. 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. A 2-wire synchronous IIC (Inter-IC) bus provides multi-drop signaling at rates up to 100 Kbaud. The Serial 1 and Serial2 ports can be configured for either RS-232 or RS-485 communications at standard baud rates up to 115200 bits per second. They translate the bit-by-bit data on the serial cable into bytes of data that can be interpreted by the operating system or by your application program. In a finished instrument, either or both channels can be used to communicate with other serial devices, or with other computers and/or terminals using RS232 or RS485. Having a second serial port is also handy for system debugging. 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.

If your application requires use of the secondary serial port as well as other interrupt routines, the key is to keep the interrupt service routines short and fast. The following table summarizes the available serial channels. 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. Since both channels can operate simultaneously and independently, serial debugging can be performed while the application program is communicating via its primary channel. 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). Two RS485 transceivers are present on the PDQ Board, one for each channel. The PDQ Board, however, does not implement hardware handshaking.

The Serial ports are implemented by the dual on-chip hardware UARTs (Universal Asynchronous Receiver/Transmitters) on the Freescale 9S12 (HCS12) microcontroller. As described below, the RS485Transmit() function controls the RS485 data direction of each serial port. Unlike the standard RS232 protocol, RS485 allows many communicating parties to share the same 3-wire communications cable. In fact, the program works the same as it did before, but now it is using the secondary serial port instead of the primary port - and you didn’t even have to recompile the code! In general, all devices on a network should use the same phase, polarity, and baud rate clock signal. The RS232 protocol specifies the use of two separate grounds, a signal ground and a protective (or chassis) ground. We can gain insight into the operation of the RS232 protocol by examining the signal connections used for the primary serial port in the above table.

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