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RS-485 is used in basically connecting the temp sensor or thermostat in the room to the master controller. In other words, each local UART on the wildcard can both send data to and receive data from a remote UART on the other end of a connecting serial cable. The terminal program communicates with the PDQ Board via this serial port. The USE.SERIAL2 command means that the operating system’s terminal interface now communicates via Serial2. To provide a convenient means of attaching two grounds to the serial cable, there are several pins (labeled GND) on the communications connector that are connected to the controller’s ground plane. The BufferToSPI() function implements fast data transfer from a specified buffer in the controller’s memory to an SPI device. The device that initiates a data transfer is the master, and all other devices on the network are slaves. 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.

When PE is set (equal to one), the most-significant bit in each byte transmitted will be a parity bit that is either set or cleared by the serial port automatically in order to achieve even or odd parity. A UART is a Universal Asynchronous Receiver/Transmitter that converts parallel data from the host processor (any Mosaic controller) into a serial data stream. The PDQ Board controls the Serial1 and Serial2 RS485 transceivers with bits PJ0 and PJ1, respectively, rs485 cable of PORTJ of the processor. 1 or 2 to specify Serial1 or Serial2, respectively, and clears the appropriate PORTJ bit to place the transceiver in receive mode. RS485Init() configures PORTJ to ensure that bits 0 and 1 are outputs, and disables both RS485 transmitters, leaving the Serial1 and Serial2 RS485 channels in receive mode. The BAUD routine described at the start of this subsection configures the baud rate of each of the Serial1 and Serial2 channels. The default baud rate for both Serial1 and Serial2 is 115200 baud after a factory cleanup. We assume that you are now communicating with the PDQ Board via the default Serial1 port at the standard 115200 baud rate.

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 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). This is an extra single bit appended to the end of each byte or character transmitted, which is set or cleared as necessary to ensure that the total number of '1' bits in the byte is always odd or even. So, for eight data bits with a parity bit, M would be set (equal to one) in order to add an extra bit to each byte transmitted, and PE would be set in order to make that extra bit be used as a parity bit.

For seven data bits with a parity bit, M would be cleared (equal to zero), and PE would be set in order to make the most-significant bit of a normal eight-bit byte be used by the serial port as a parity bit. The words port and channel are used interchangeably to refer to a serial communications link. Data translation between different machines can be performed with ease, and applications that communicate via the one serial port can be debugged using the other serial channel. 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 PT bit, with mask 0x01, determines whether even parity or odd parity is used if parity bit generation is enabled. The master and slave could even exchange ascii QED-Forth operating system commands. When the exchange is complete, the slave can again execute the Silence() routine to disable its transmitter and begin listening for its name. This section describes the QED-Forth routines that control the RS485 transceiver, and presents some ideas that may prove useful in designing a multi-drop data exchange protocol.