The diagram below shows potentials of the A (blue) and B (red) pins of an RS-485 line during transmission of one byte (0xD3, least significant bit first) of data using an asynchronous start-stop method. The byte-sized messages are transmitted and received via the MOSI (master out/slave in) and MISO (master in/slave out) pins. The RS485 connections are not brought out to the Serial 1 Connector. These signals may alternatively be redirected to the digital inputs and outputs used by the second serial port if hardware handshaking is required. 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. 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 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. 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. For example, at 4800 baud (bits per second), each bit lasts about 200 microseconds (µs), and if communications are full duplex (e.g., if the QScreen Controller echoes each incoming character), then there is a serial interrupt every 100 µs or so. The maximum sustainable baud rate on the secondary serial port is 4800 baud. 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. The remaining "inactive" slaves may actively receive, or listen to, data on the communications line, but only one slave at a time can transmit a message.

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 Silence() routine searches the incoming serial characters for a pre-determined keyword (for example, the ascii "name" of this particular slave). When the network master wants to talk to this particular slave, it outputs the slave’s ascii name onto the serial bus. The master and slave could even exchange ascii QED-Forth commands. The QED-Forth kernel includes pre-coded drivers that configure and control the SPI for maximum speed data transfers. QED-Forth includes three built-in routines to facilitate control of the RS485 transceiver. 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. The secondary serial port is implemented by a software UART that controls two pins on PortA. In this case, cable connections must be made to Serial 1 at pins 5 and 6 of the 10-pin Serial Header or pins 7 and 8 on the 24-pin Field Header. In summary, the code provided for implementing the second serial port is very flexible and can be used to support dual concurrent communications ports.

Typing a carriage return at the terminal should now produce the familiar "ok" response via the Serial2 port. 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 EIA has officially disbanded and the standard is now maintained by the TIA as TIA-485, but engineers and applications guides continue to use the RS-485 designation. We’ll use code from the GETSTART.C program. Because all of the serial I/O routines on the QScreen Controller are revectorable, it is very easy to change the serial port in use without modifying any high level code. This chapter describes those drivers, and presents code that makes it easy to configure the SPI for different data transfer rates and formats. The BufferToSPI() function implements fast data transfer from a specified buffer in the controller’s memory to an SPI device. SD card with 74HC4050 SPI buffering, MAX3232 for RS232 on Serial 2, battery bacled DS1307 with 32.786 KHz crystal. Before running the program, let’s switch to the secondary serial port. A jumper, J3, configures the primary serial port for either RS232 or RS485 operation.

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