When ClearSCADA passes data to the operating system, it provides the operating system with information about the port configuration. The operating system is responsible for sending the data with the correct start, stop, and parity bits.
The operating system sends:
- A Start bit, consisting of 1 bit
- Data bits, consisting of up to 8 bits
- Stop bits, consisting of 1 bit, 1.5 bits, or 2 bits
- A parity bit (only if parity is in use), consisting of 1 bit (see Parity).
Usually, each byte is 10 bits or 11 bits in size, depending on whether the parity bit is used. The Start and Parity bits are always 1 bit in size, but the Data Bits and Stop Bits can be configured to be larger in size.
Your communications equipment supports specific sizes for Data Bits and Stop Bits (most equipment supports 7 or 8 Data Bits and 1 Stop Bit). For the operating system to send messages containing bytes of the correct size, you need to configure the channels in your database to have Data Bits and Stop Bits settings that match those of your equipment.
Example:
Communications equipment is connected to a PC running ClearSCADA via a serial connection. A channel is configured to represent this connection.
The communications equipment supports Data Bits of 8 bits and Stop Bits of 1 bit and does not support a Parity bit. This means that it can only successfully receive messages that have bytes consisting of:
- 1 Start bit
- 8 Data Bits
- 1 Stop Bit
So that ClearSCADA sends messages with data of 10 bits (where the Start bit is 1 bit, the Data bits are 8 bits, and the Stop bit is 1 bit), the channel is configured to have these settings:
- Data Bits—8 Bits
- Stop Bits—1 bit.