RS485Receive() clears bit PD5 to place the transceiver in receive mode, and RS485Transmit() sets bit PD5 to place the transceiver in transmit mode. To use a QScreen as a slave in a multi-drop network, simply define a word, (named Silence(void), for example) that when executed calls RS485Receive() to wait for any pending character transmission to complete, then disable the transmitter, and then execute a routine such as Key() to listen to the communications on the serial bus. 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, and to change the terminal’s baud rate back to 19200 baud using the "Comm" item under the terminal’s "Settings" menu. 19200 baud, select the "Comm" item under the "Settings" menu and click on 19200 in the "Baud Rate" selection box. 1 running at the prior established baud rate (typically 19200 baud). When the keyword name is received by the Silence() routine running in the slave, the slave QScreen Controller executes RS485Transmit() to send an acknowledgment to the master (which should now be listening to the serial bus to accept the acknowledgment). 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. When the exchange is complete, the slave can again execute the Silence() routine to disable its transmitter and begin listening for its name. 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 slave devices are connected to the master device, and they respond to the master’s requests for data. The master and slave can then exchange data. The master and slave could even exchange ascii QED-Forth commands. The data exchange format may be a line of ascii text. What are the data transmission rates in serial communication protocols? Both of these protocols are used in applications such as data acquisition, automation, what is rs485 cable and robotics. With such an implementation of a RS485 network it is necessary that there is error detection implemented in the higher level protocol to detect the data corruption and resend the information at a later time. 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.

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. The secondary serial port is implemented by a software UART that controls two pins on PortA. 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. Most computers conform to IBM PC AT-compatible RS232 interfaces which use 9-pin D-Type connectors, consequently the QScreen Controller brings out its serial ports to two female 9-pin D-Type connectors. It is supported by virtually all personal computers, and is the default protocol for both of the QScreen Controller’s serial ports. We have built sophisticated instruments using the QScreen Controller that operate very reliably using multiple interrupts in addition to the software UART. If your computer does not have an RS-232 serial port, low cost USB-to-RS-232 serial cables are available; contact Mosaic Industries for details. The signal will eventually stabilize (until the next bit transition) so the slower the bit rate the more time the signal will have to stabilize before it is sampled (usually by a UART).