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. Each of the two channels on the UART Wildcard implements two 16-character FIFOs, one for outgoing characters and one for incoming characters. The UART Wildcard provides two simultaneous communications links, each configurable as RS232, RS485 or RS422. 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 QScreen Controller combines an embedded computer based on the 68HC11 microcontroller with a touch panel and LCD (liquid crystal display) graphic user interface (GUI) that is ideal for instrument control and automation. 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. Even though the MOSI pin is not connected to anything, the master initiates a transmission using a "dummy" byte. A data transfer is initiated by a master device when it stores a message byte into its SPDR register.

A write collision occurs when a byte is written to the SPI data register, SPDR, while data is being exchanged. A logic-low start bit marks the start of a character, followed by 5 to 8 data bits per character. This bit should be set only after all other SPI configuration is complete. You will be able to determine the actual configuration based on the documentation that accompanied the device. Regardless of the network, however, there are only four signals used: SCK provides a synchronized clock, MOSI and MISO signals are used for data transmission and reception, and /SS configures the QScreen as a master or slave device. To ensure that no two devices drive the network at the same time, it is necessary that each slave device be able to disable its own RS485 data transmitter. The RS485 network is wrongly referred to as a "2 wires plus shield" network.

Note that the local and the remote must share a common ground, so a minimum of 5 wires are required for full duplex RS422 communications: two transmit wires, two receive wires, and a common ground. The RS232 driver and receiver use separate conductors on the serial cables, enabling full duplex communications. Because a single pair of conductors is used for both transmission and reception, RS485 is useful for multi-drop applications in which a master communicates with multiple slave serial devices, or nodes. The master can instruct a single slave to go into transmit mode, and then the master can put itself into receive mode, thereby allowing the master to retrieve data from the slave. A single master can broadcast commands to all the slaves, and can direct commands to an individual slave using its unique address. The master is in charge of designating which receiver is on at any one time. To avoid contention on the RS485 bus, the application software must assure that only one transmitter is enabled at a time. TSB-89A, Application Guidelines for TIA/EIA-485-A does not recommend using star topology.

Similarly, when establishing the transmit mode, the application software is responsible for ensuring that the RS485 receiver is not disabled until all expected characters have been received. The RS422 protocol has similarities with RS485 in that both conduct data transmission using differential signals. RS485 uses the same differential signaling scheme as RS422, and hence has the same superior signal-to-noise characteristics and range described above. These characteristics make RS-485 useful in industrial control systems and similar applications. The physical characteristics of an RS485 cable include twisted pair conductors, typically two or four, which help reduce electromagnetic interference and improve signal integrity. QED-Forth includes three built-in routines to facilitate control of the RS485 transceiver. The QScreen Controller controls the RS485 transceiver with bit 5 of Port D 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. The PDQ Board controls the Serial1 and Serial2 RS485 transceivers with bits PJ0 and PJ1, respectively, of PORTJ of the processor. 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.