This signal synchronizes the exchange of bytes between the QScreen and its peripherals. Differential data transmission (balanced differential signal) offers superior performance in most applications. The RS422 driver and receiver use separate differential conductor pairs on the serial cables, enabling full duplex communications. RS422 (differential) was designed for greater distances and higher Baud rates than RS232. When communicating at high data rates, or over long distances in real world environments, single-ended methods are often inadequate. Rs485 is low impedance balanced , so twisting the pair helps with noise immunity as well as reducing the attenuation effect of higher baud rates due to the parallel capacitance of the wires in the cable jacket over the long lengths. There are surface-mount resistor pads on the Docking Panel to bring out the RS485 signals to the DB9 Serial 1 Connector. RS232 uses inverse logic; that is, a positive bit at the HCS12 UART is inverted by the onboard RS232 driver chip and appears as a negative signal on the serial cable.

We can gain insight into the operation of the RS232 protocol by examining the signal connections used for the primary serial port in the above table. Each UART (sometimes referred to as a "USART") controls the serial-to-parallel and parallel-to-serial conversion and performs all of the timing functions necessary for one asynchronous serial communications link. As described below, the RS485Transmit() function controls the RS485 data direction of each serial port. Most computers conform to IBM PC AT-compatible RS232 interfaces which use 9-pin D-Type connectors, consequently the PDQ Board brings out its serial ports to two female 9-pin D-Type connectors on the Docking Panel. These 9-pin standard DB-9 serial connectors are located on the back of the Docking Panel. The PDQ Board’s transmit data signal /TxD1 (pin 2 on the 9-pin serial connector) is connected to the terminal’s receive data signal /RxD (pin 2 on its 9-pin connector). The RS232 protocol specifies the use of two separate grounds, a signal ground and a protective (or chassis) ground.

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). RS485 meets the requirements for a truly multi-point communications network, and the standard specifies up to 32 drivers and 32 receivers on a single (2-wire) bus. A true multi-point network consists of multiple drivers and receivers connected on a single bus, where any node can transmit or receive data. With the introduction of "automatic" repeaters and high-impedance drivers / receivers this "limitation" can be extended to hundreds (or even thousands) of nodes on a network. Even though the MOSI pin is not connected to anything, the master initiates a transmission using a "dummy" byte. Single master systems (many other communications schemes are available) offer a straight forward and simple means of avoiding "data collisions" in a typical 2-wire, half-duplex, rs485 cable multi-drop system. The master initiates a communications request to a "slave node" by addressing that unit. RTS (Request to send) does have some utility in certain applications. If your computer does not have an RS232 serial port, low cost USB-to-RS232 serial cables are available; contact Mosaic Industries for details.

To provide a convenient means of attaching two grounds to the serial cable, there are several pins (labeled DGND) on the communications connector that are connected to the controller’s ground plane. In this case, cable connections may be made to Serial 2 on either the 10-pin PDQ Board Serial Communications Header, or the Docking Panel’s 10-pin right-angle Serial Header, or the Docking Panel’s Serial2 DB-9 Connector. The data exchange format may be a line of ascii text. Rather, it relies on software handshaking via transmission of XON/XOFF characters (ascii 0x11 and 0x13, respectively) to coordinate data transfer and ensure that information is not lost when one of the communicating parties is busy. Unlike the standard RS232 protocol, RS485 allows many communicating parties to share the same 3-wire communications cable. RS485 is another protocol supported by the primary serial port on the PDQ Board. Serial1 port the default startup serial link. The Serial1 and Serial2 ports are is supported by the HCS12's dual on-chip hardware UARTs, and do not require interrupts to work properly. The PDQ Single Board Computer (SBC) has two asynchronous serial communications ports named Serial1 and Serial2.