Parity is a method of error checking that is supported by some communications equipment. When supported, the ‘sending’ operating system adds an extra bit (the Parity bit) to each byte of data that it sends to the ‘receiving’ operating system.
Each byte of data contains:
- A Start bit
- Data bits
- Parity bit
- Stop bits.
Before the data is sent from the PC running ClearSCADA, the PC's operating system sets the Parity bit. The setting that is applied varies according to the Parity configuration setting of the channel (which has to match the configuration supported by the communications equipment):
- None—Parity checking is not used.
- Even—The Parity bit is set to 1 if the number of 1s in the Data Bits is an odd number (so that the total number of 1s, including the Parity bit, form an even number). For more information, see Even .
- Odd—The Parity bit is set to 1 if the number of 1s in the Data Bits is an even number (so that the total number of 1s, including the Parity bit, form an odd number). For more information, see Odd .
- Mark—Sets the Parity bit to 1. For more information, see Mark .
- Space—Sets the Parity bit to 0. For more information, see Space .
When the operating system has set the Parity bit, it transmits the data via the relevant Port (defined in the channel's configuration). When the intermediate device that relays the message receives the data, it performs a calculation to determine whether the setting for the Parity bit is consistent with its configuration. If it is not consistent, the data is rejected and a parity error is generated.
The Parity bit check allows the communications equipment to determine whether the received data is valid, and varies according to the Parity configuration of the channel and communications equipment.