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! If PT is cleared, then all transmitted bytes with a parity bit will have an even number of total '1' bits. If your application requires communicating with a device that expects to receive a parity bit, the generation of a parity bit and selection of even or odd parity, and whether there are seven or eight data bits in each byte, is performed by setting or clearing bits in the configuration registers SCI0CR1 for Serial1 and SCI1CR1 for Serial2. The DECIMAL command makes sure that numbers are interpreted in the decimal base. The USE.SERIAL2 command means that the operating system’s terminal interface now communicates via Serial2. When the keyword name is received by the Silence() routine running in the slave, the slave PDQ Board 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).

The above parity settings will also determine how incoming data is interpreted (whether the most significant bit is considered a parity bit or part of the data being transmitted, and how many bits total to expect in each byte). The M bit, with mask 0x10, determines whether eight or nine bits total are transmitted with each byte, regardless of whether or not the most-significant bit is a parity bit. In the most common multi-drop RS485 protocol, one computer is designated as a master and the rest of the computers or devices on the serial bus are designated as slaves. Bias termination - Using resistive termination decreases noise immunity, particularly if the cable is loaded with many devices. In that case, when using very long cables you can improve noise immunity and assure a valid idle level when the transceiver is not active by installing bias resistors. At any given time, only the master and a single active slave communicate. Given a properly wired network and a properly configured SPCR control register, a master device may transmit a message by simply storing the byte to the SPDR data register.

A data transfer is initiated by a master device when it stores a message byte into its SPDR register. The master and slave could even exchange ascii QED-Forth operating system commands. The primary channel’s UART translates the bit-by-bit data on the serial cable into bytes of data that can be interpreted by the QED-Forth Kernel or by your application program. The terminal program communicates with the PDQ Board via this serial port. The PDQ Board controls the Serial1 and Serial2 RS485 transceivers with bits PJ0 and PJ1, respectively, of PORTJ of the processor. We assume that you are now communicating with the PDQ Board via the default Serial1 port at the standard 115200 baud rate. The default baud rate for both Serial1 and Serial2 is 115200 baud after a factory cleanup. 1 running at the prior established baud rate (typically 115200 baud). If it doesn’t, confirm that the terminal’s baud rate is correct by selecting the Comm item in the Settings menu of the Mosaic Terminal program, and click on 115200 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.

Because we chose the default baud rate (which the terminal is presumably already set for), you can simply move the serial cable from the Serial Port 1 connector to the Serial Port 2 connector on the Docking Panel to complete the change to the new port. The Serial2 channel is always configured for RS232 communications, and can sustain baud rates up to 4800 baud. Each of the two UARTs on the wildcard is capable of full-duplex communications, meaning that both transmission and reception can occur simultaneously (although the RS485 protocol is half duplex as explained below). Note that the local and the remote must share a common ground, so a minimum of 3 wires are required for half duplex RS485 communications: a pair of transceive wires and a common ground. For this reason, frame-level cyclic redundancy checks are much more widely used for validating data from serial links, network connections and storage media. RS422 This standard gives a much higher data rate than RS232, but it uses differential transmission techniques. The UART Wildcard provides two simultaneous communications links, each configurable as RS232, rs485 cable RS485 or RS422. The InitSPI() function provides a convenient way to initialize the SPI as the master at a 2MHz baud rate.