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Grounds between buildings may vary by a small voltage, but with very low impedance and hence the possibility of catastrophic currents - enough to melt signal cables, PCB traces, and transceiver devices. Each RS232 driver uses inverting logic and implements a single-ended bipolar output voltage (that is, one signal that swings above and below ground). The RS485 protocol uses differential data signals for improved noise immunity; thus RS485 can communicate over greater distances than RS232. Thus in Table 9 6 , RTS1 is connected to CTS1, and DSR1 is connected to DTR1 and DCD1 onboard the QScreen Controller using zero ohm shorting resistors. Using the primary Serial1 port is easy. If your application requires communicating with a device that expects to receive a parity bit, the generation of a parity bit and selection of even or odd parity, and whether there are seven or eight data bits in each byte, is performed by setting or clearing bits in the configuration registers SCI0CR1 for Serial1 and SCI1CR1 for Serial2.

This allows for basic error detection, in that if noise on the transmission line causes one bit to be received incorrectly, either received as a '0' when transmitted as a '1' or vice-versa, the error would be detected due to the count of '1' bits in the byte being odd when it is expected to be even, or vice-versa depending on the parity checking settings. If it doesn’t, confirm that the terminal’s baud rate is correct by selecting the Comm item in the Settings menu of the Mosaic Terminal program, and click on 115200 baud. Once the project is open, click Build→ Build, and after the compilation is done, enter the Mosaic Terminal by clicking Tools→ Mosaic Terminal and use the Send File menu item to send GETSTART.DLF to the PDQ Board. RS232’s greatest benefit is its universality; practically all personal computers can use this protocol to send and receive serial data. Data translation between different machines can be performed with ease, and applications that communicate via the one serial port can be debugged using the other serial channel. 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. The PT bit, with mask 0x01, determines whether even parity or odd parity is used if parity bit generation is enabled. The PE bit, with mask 0x02, determines whether the most-significant bit in each byte is used as a parity bit. If two bits are received incorrectly, the error will go unnoticed by parity checking. For seven data bits with a parity bit, M would be cleared (equal to zero), and PE would be set in order to make the most-significant bit of a normal eight-bit byte be used by the serial port as a parity bit. The PDQ Board's two serial ports support limited use of generating a parity bit. Two devices are at the ends of the cable, while others are connected somewhere in between. The end devices are responsible for terminating the cable so that there are no reflections from the cable ends. RC termination - In some applications requiring low power you may not want to load the line with 120 Ω resistors at each end.

The GROUND line serves as a common voltage reference for the master and slave. That places a 120 Ω resistor across the RS485 differential line at the driver chip. The terminal’s serial receiver chip re-inverts the signal to its positive sense. Because all of the serial I/O routines on the PDQ Board are revectorable, it is very easy to change the serial port in use without modifying any high level code. In that case, when using very long cables you can improve noise immunity and assure a valid idle level when the transceiver is not active by installing bias resistors. Bias termination - Using resistive termination decreases noise immunity, particularly if the cable is loaded with many devices. Check out also our article about BIAS and TERMINATION resistors. They should generally not be needed, except if you use long cables, multiple RS485 devices, and resistive termination. Our USB development kits use GasLab® software for setup, calibration, data logging, and real-time data analysis.

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