RS232 to RS485 Converter > 자유게시판

The secondary serial port is implemented by a software UART that controls two pins on PortA. Because the software UART is interrupt based, competing interrupts that prevent timely servicing of the Serial2 interrupts can cause communications errors on the secondary serial channel. Also, several non-serial interrupts can stack up; if they have higher priority than the serial interrupts, they will be serviced before the Serial2 interrupt routine, and again a serial input or output bit may be lost. 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. These signals may alternatively be redirected to the digital inputs and outputs used by the second serial port if hardware handshaking is required. A jumper, J3, configures the primary serial port for either RS232 or RS485 operation. 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.

Like RS232, the data bits are transmitted in reverse order, with the least significant bit transmitted first. The data bits are also transmitted in reverse order, with the least significant bit transmitted first, after a start bit. In some circumstances a one-way data flow may suffice. In this case, cable connections may be made to Serial 1 on either the 10-pin Serial Communications Header or the Serial 1 Connector. The pinout of the QScreen’s Serial Header (H5), QScreen’s Field Header (H3), and the Serial Connectors are shown in the following tables. The primary and secondary serial communications ports are accessible through the QScreen's 10 pin, dual row Serial Header (H5) which is typically not installed, the 24 pin, dual row Field Header (H3), and through the individual DB-9 Serial 1 and Serial 2 connectors. You might also consider operating the secondary serial port at a lower baud rate to relax the timing constraints. 1 running at the prior established baud rate (typically 19200 baud). If you do this now, remember to move the QScreen Controller’s serial connector back to Serial Port 1, and to change the terminal’s baud rate back to 19200 baud using the "Comm" item under the terminal’s "Settings" menu.

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). You can operate the port at any baud rate up to 4800 baud; just specify the rate you want before the BAUD2 command. In summary, the code provided for implementing the second serial port is very flexible and can be used to support dual concurrent communications ports. Move the serial cable from the "Serial Port 1" connector to the "Serial Port 2" connector on the QScreen. By connecting pairs of these handshaking signals together, the terminal or PC can be made to think that the QScreen Controller is always ready to send and receive data. The RS485 protocol uses differential data signals for improved noise immunity; thus RS485 can communicate over greater distances than RS232.

From openHAB, you can only see TCP "not working" and cannot even tell which area to look more closely at. 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 secondary serial port is connected similarly except that the onboard connection of RTS to CTS, and DSR to DTR are permanent. 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. We recommend that you keep the faster Serial1 port as the default serial link as you work through the exercises in this book. 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. The QScreen Controller’s kernel software contains a complete set of high level driver routines for the Serial2 port, and these functions are summarized in the Control-C Glossary. If you are running Serial2 at 4800 baud, the rest of your application must be able to function properly using the remaining portion of the CPU time.

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