Having a second serial port is also handy for system debugging. As described below, the RS485Transmit() function controls the RS485 data direction of each serial port. 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. 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. 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. The default baud rate after a factory cleanup is 115200 baud. The Serial 1 and Serial2 ports can be configured for either RS-232 or RS-485 communications at standard baud rates up to 115200 bits per second. 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.

For this reason, frame-level cyclic redundancy checks are much more widely used for validating data from serial links, network connections and storage media. 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. The master and slave can then exchange data. To use a QScreen as a slave in a multi-drop network, simply define a word, (named Silence(void), for example) that when executed calls RS485Receive() to wait for any pending character transmission to complete, then disable the transmitter, and then execute a routine such as Key() to listen to the communications on the serial bus. The master initiates a communications request to a "slave node" by addressing that unit. A true multi-point network consists of multiple drivers and receivers connected on a single bus, where any node can transmit or receive data. Typically one device (node) is addressed by the host computer and a response is received from that device.

In some cases, however, a sophisticated network may have device groups on a network that use different clock configurations. Note that the data is changed by the transmitting device one half clock cycle before it is valid. 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). Independent channels are established for two-way (full-duplex) communications. Since both channels can operate simultaneously and independently, serial debugging can be performed while the application program is communicating via its primary channel. Two RS485 transceivers are present on the PDQ Board, one for each channel. The secondary channel is very useful for debugging application programs that communicate with other computers or I/O via the primary channel. The standard C serial I/O routines such as printf(), scanf(), putchar(), and getchar() give you high level access to the serial ports. Each serial port can be configured for the RS232 or RS485 protocol, and runs at standard baud rates up to 115,200 bits per second.

Modem to modem lines often use 1200, 4800, 9600, 14400, 28800, 33600, and 56000 baud. In general if you are not connected to a modem the handshaking lines can present a lot of problems if not disabled in software or accounted for in the hardware (loop-back or pulled-up). Each of the two channels on the UART Wildcard can be configured for RS232, RS422, or RS485. In a finished instrument, either or both channels can be used to communicate with other serial devices, or with other computers and/or terminals using RS232 or RS485. RS485 is another protocol supported by the primary serial port on the QScreen Controller. RS485 is another protocol supported by the primary serial port on the PDQ Board. This page describes the serial ports and how to use them for instrument control and automation applications. The issue also exists in programmable logic controller applications. There are a variety of ways the MOSI, MISO, SCK and /SS pins on your QScreen Controller can be connected. 2) the GSM version also allows you to indicate a possible PIN (leave blank if there is no PIN) and the operator's APN (possibly you can search for it on Google).

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