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. RS232 referenced ground to decipher the data it was transmitting. 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). This module is powered by the RS232 port from the connecting computer. Ensure SSCOM is added to the firewall, otherwise the firewall may block TCP connections, or test by temporarily disabling all firewalls on the computer. Characters may be lost if the transmit driver is turned off while pending characters are still being transmitted. 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.

This detects the presence of more than one master on the SPI bus. The RS498 bus was working correctly, and we could read values from the device. If you are using the QScreen as a master device, each external SPI device will require a separate select line (/SS). The RS232 protocol specifies the use of two separate grounds, a signal ground and a protective (or "chassis") ground. So what is a RS232 to RS485 converter? If your application requires RS485, use the primary serial port (serial1) for RS485 communications, and use the secondary serial port (Serial 2) to program and debug your application code using the RS232 protocol. 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. The DWOM bit determines whether Port D needs pull-up resistors; it should be set to 0. The MSTR bit determines whether the device is a master or slave. To connect an RS485 device or RS485 network to any PC or controller, the PC or the controller should provide RS485 interface also.

If a slave device has already stored a byte into its SPDR register, that byte will be exchanged with the master’s byte. 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. Is there an easy way to log data to a PC from multiple instruments on an RS485 bus? We have built sophisticated instruments using the QScreen Controller that operate very reliably using multiple interrupts in addition to the software UART. 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. 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.

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. 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 primary serial port, Serial1, is supported by the 68HC11's on-chip hardware UART (sometimes called a USART), and does not require interrupts to work properly. 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. A single master can broadcast commands to all the slaves, and can direct commands to an individual slave using its unique address. For those of you interested in the details, here’s how it works: The low-level serial driver routines named Key(), rs485 cable AskKey() and Emit() are revectorable routines that can be redirected to use either of the serial ports. 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.

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