At the end of a received character, the service routine takes about 45 µs. A logic-low start bit marks the start of a character, followed by 5 to 8 data bits per character. For example, at 4800 baud (bits per second), each bit lasts about 200 microseconds (µs), and if communications are full duplex (e.g., if the QScreen Controller echoes each incoming character), then there is a serial interrupt every 100 µs or so. There are different sets of standard baud rates in use depending on the application. 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. The pinout of the QScreen’s Serial Header (H5), QScreen’s Field Header (H3), and the Serial Connectors are shown in the following tables. If you are using the QScreen as a slave device and require the /SS signal for your external SPI hardware, configure one of the Port A pins on the Field Header as an input pin. The SCK (serial clock) pin is a configurable synchronous data clock output.

Remember that the /SS is active low so to select a device you need to set the pin low; otherwise the pin should idle high. 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. 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. If the programmer has enabled the local interrupt mask for the SPI, an interrupt is recognized at this point. 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 serial output at the UART idles at the logic high (mark) level.

Each RS422 driver transmits a differential pair of output signals at 0 and 5 volts. 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. In RS485 mode, the RS422 transmit and receive pairs are shorted together with a pair of onboard jumpers as explained in the UART Wildcard Hardware: RS485 Jumpers section below. 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 has two serial communications ports: a primary serial port called Serial 1 that supports both RS232 and RS485 protocols, and a secondary serial port called Serial 2 that supports RS232. Using the primary serial port is easy. The RS422 driver and receiver use separate differential conductor pairs on the serial cables, enabling full duplex communications. The RS232 protocol specifies the use of two separate grounds, a signal ground and a protective (or "chassis") ground.

Most modems communicate using RS232 and a set of hardware handshaking signals used to regulate data flow. A modem (modulator/demodulator) provides a way of encoding digital data as a set of audio signals that can be sent over a telephone line. The dual communications channels also provide an easy way to link systems that communicate using different serial protocols. The protocols are described in turn. 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, rs485 cable and a common ground. Note that the local and the remote must share a common ground, so a minimum of 3 wires are required for full duplex RS232 communications: a transmit wire, a receive wire, and a common ground. Both the local and remote UARTs must be configured for the same communications parameters. Both the local and remote UARTs must be configured for the same baud rate. The resulting signal levels on the interface cable connect the local and remote in a manner specified by a standard protocol. Pre-coded device drivers configure the SPI for a standard data format, and it is easy to customize a data format and baud rate for your application.