This way, the lines will be biased to known voltages and nodes will not interpret the noise from undriven lines as actual data; without biasing resistors, the data lines float in such a way that electrical noise sensitivity is greatest when all device stations are silent or unpowered. 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. 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. There are surface-mount resistor pads on the Docking Panel to bring out the RS485 signals to the DB9 Serial 1 Connector. All of the serial ports are supported by pre-coded C-language software drivers that make it easy to exchange data. Configuration and data accessibility were a snap: the FieldLogger's user-friendly configurator software was accessible by Ethernet, USB or RS485, and also provided means for online monitoring, downloading of all logged data, and quick exporting to spreadsheets and other formats.

These protocols are summarized on this page, but for more information regarding their data formats and their use for simplex or multi-drop serial lines, consult Understanding Serial Communications (but keep in mind that that page is directed to the use of the UART Wildcard, so it uses different driver functions). In fact, a single driver chip on the UART Wildcard is used to implement both RS422 and RS485 communications for a given serial channel. In a computer system, SCSI-2 and SCSI-3 may use this specification to implement the physical layer for data transmission between a controller and a disk drive. I used male-female terminated wires & crimp housings to make this little jumper cable to drive the board with SPI transfers. This ability to exchange messages means that the SPI is capable of full duplex communication. Of the processor’s three synchronous SPI (Serial Peripheral Interface) ports, two are available for inter-processor communications on multi-processor systems, and the third is brought out to the Wildcard expansion bus. 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). 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, multi-drop system.

The Serial1 and Serial2 ports have identical communications capabilities, although more of the Serial1 signals (both RS232 and RS485) are made available on the Docking Panels headers and connectors. 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. RS232’s greatest benefit is its universality; most personal computers can use this protocol to send and receive serial data. The RS232 protocol specifies the use of two separate grounds, a signal ground and a protective (or chassis) ground. Two asynchronous communications ports named Serial1 and Serial2 can each be configured for RS232 or RS485 protocols. The PDQ Single Board Computer (SBC) has two asynchronous serial communications ports named Serial1 and Serial2. The RS485 data direction of Serial1 and Serial2 are controlled by the PJ0 and PJ1 processor pins, respectively. All of the RS232 signals start with the / (slash, rs485 cable pronounced not) character to indicate that the signals on the serial cable are logically inverted.

Any number of characters can be sent, and the transmitter will automatically re-trigger with each new character (or in many cases a "bit-oriented" timing scheme is used in conjunction with network biasing for fully automatic operation, including any Baud rate and/or any communications specification, eg. 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. To solve the "data collision" problem often present in multi-drop networks hardware units (converters, repeaters, micro-processor controls) can be constructed to remain in a receive mode until they are ready to transmit 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. After connecting as for point B, in the Settings it is possible to define a name for the device, and if the reading periodicity is less than 250 minutes, you can set an interval of hours in which the reading is not carried out / transmitted (for example you can disable the night). 1) connect etc. as for B. and go to Settings. 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.