What is the Function of RS 485 Connection? > 자유게시판

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Table 9-6 Serial Cable Connections. Table 9-6 shows the connection diagram for a standard 9-pin serial cable. Given the availability of ready-made communications cables, it is not necessary to study or understand the following descriptions of cable connections. 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. In summary, the code provided for implementing the second serial port is very flexible and can be used to support dual concurrent communications ports. 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! The maximum sustainable baud rate on the secondary serial port is 4800 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. Each of the two channels on the UART Wildcard can be configured for RS232, RS422, or RS485. Each of the two channels on the UART Wildcard implements two 16-character FIFOs, one for outgoing characters and one for incoming characters. This function cannot accept incoming data; consult its glossary entry for details. Function prototypes for this function and other versatile serial I/O routines are defined in the COMM.H header file, and are described in detail in the Control-C Glossary. In this case, cable connections may be made to Serial 1 on either the 10-pin Serial Communications Header or the Serial 1 Connector. Otherwise a full 6 wire cable must be used. The default serial routines used by the onboard kernel assume that full duplex communications are available, so you cannot use the RS485 protocol to program the controller. Done that establishes the RS485 receive mode is coded such that it waits until all queued (pending) characters have been transmitted before the driver chip is taken out of transmit mode.

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. 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 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. These devices allow you to view and modify the parameters of heat controllers, connected to the RS485 port via the communication protocol and the NCP to monitor the parameters of heat meters (Mbus), connected to RS232 port via a converter RS232/Mbus (M-bus master). The RS485 system used for Modbus communication provides a main cable (Bus or backbone), to which all the devices have to be connected with branches (also known as stubs) that are as short as possible. These detailed signal descriptions and cable diagrams are presented to provide complete information for those who have special communications requirements and for those who wish to make their own application-specific communications cables.

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, and after the compilation is done, enter Mosaic Terminal by clicking on the terminal icon and use the "Send File" menu item to send GETSTART.DLF to the QScreen Controller. Now select the "Comm" 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 do this now, remember to move the QScreen Controller’s serial connector back to Serial Port 1, rs485 cable and to change the terminal’s baud rate back to 19200 baud using the "Comm" item under the terminal’s "Settings" menu. The secondary serial port is implemented by a software UART that controls two pins on PortA. Before running the program, let’s switch to the secondary serial port. On the other hand, the secondary serial port (Serial2) is implemented using hardware pins PA3 (input) and PA4 (output), and is controlled by the associated interrupts IC4/OC5 and OC4, respectively.