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By default, the RS485 connections are not brought out to the Docking Panel’s DB-9 Serial1 Connector. By default, the RS485 connections are not brought out to the Docking Panel’s DB-9 Serial1 Connector, although custom placement of zero-ohm surface-mount resistors on the Docking Panel can route the RS485 signals to the DB-9. They should generally not be needed, except if you use long cables, multiple RS485 devices, and resistive termination. RS485 multi-drop networks are daisy-chained networks with a single cable connecting multiple devices. Thus RS485 is the standard protocol of choice when multi-drop communications are required. The default serial routines used to download programs to the operating system assume that full duplex communications are available, so you cannot use the RS485 protocol to program the controller. Two RS485 transceivers are present on the PDQ Board, one for each channel. The PDQ Board, however, does not implement hardware handshaking. 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. Many terminals and PCs, however, do rely on hardware handshaking to determine when the other party (in this case the PDQ Board) is ready to accept data.

By connecting pairs of these handshaking signals together, the terminal or PC can be made to think that the PDQ Board is always ready to send and receive data. The RS422 driver and receiver use separate differential conductor pairs on the serial cables, enabling full duplex communications. The master is in charge of designating which receiver is on at any one time. So long as the error between the actual baud rate and that specified is less than 1.5% (or the error between transmitter and receiver is less than 3%) there should be no communication errors. The end devices are responsible for terminating the cable so that there are no reflections from the cable ends. 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. The PT bit, with mask 0x01, determines whether even parity or odd parity is used if parity bit generation is enabled. The QScreen Controller controls the RS485 transceiver with bit 5 of Port D of the processor. As described below, the RS485Transmit() function controls the RS485 data direction of each serial port.

There are surface mount resistor pads on the QScreen that will allow you to bring out the secondary serial port to the Field Header on pins 5-6 or 7-8 as shown with the parentheses in Table 11-3. Pads are also available to bring out the RS485 signals to the DB9 Serial 1 Connector. There are surface-mount resistor pads on the Docking Panel to bring out the RS485 signals to the DB9 Serial 1 Connector. The Serial 1 port can be configured for either RS232 or RS485 communications at up to 19200 baud. TXsoil can be configured and read with a smartphone or PC, with just a few clicks. RS232’s greatest benefit is its universality; most personal computers can use this protocol to send and receive serial data. Slave devices use the master in/slave out pin, MISO, for transmitting, and the master out/slave in pin, MOSI, for receiving data. Two devices are at the ends of the cable, while others are connected somewhere in between. 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.

Serial 2 is implemented by a software UART in the controller’s QED-Forth Kernel that uses two of the processor’s PortA I/O pins to generate a serial communications channel. RS232 uses inverse logic; that is, a positive bit at the 68HC11 UART is inverted by the onboard RS232 driver chip and appears as a negative signal on the serial cable. RS232 uses inverse logic; that is, rs485 cable 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. 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). Likewise, the terminal’s transmit signal /TxD is connected to the PDQ Board’s receive signal /RxD1. The communications is asynchronous because no synchronizing clock signal is transmitted along with the data. 0), and expect valid data to be present on rising clock edges. A data transfer is initiated by a master device when it stores a message byte into its SPDR register. 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.