Most communications equipment supports a feature called Flow Control that allows the equipment to stop receiving data for an amount of time. The purpose of this is to allow the communications equipment to temporarily suspend incoming messages so that it has time to process the data it has received.
Flow Control works by allowing the 'receiving' communications equipment to indicate when it is ready to receive new data. The operating system can only send the message if the 'receiving' communications equipment is ready for new data.
If the 'receiving' communications equipment is busy processing data that it has already received, it will use Flow Control to inform the 'sending' equipment that it cannot accept any new data. Any messages that are sent during this time are stored in the operating system’s transmission buffer, where they will remain until the 'receiving' communications equipment is ready to receive new data again. At this point, the data in the transmission buffer will be sent to the 'receiving' communications equipment. If the transmission buffer becomes full, any new messages that are sent will be disregarded, and after the defined number of retries, will fail.
During this process, the Flow Control setting is used to determine:
- Which application raises the RTS line that is used by the 'sending' communications equipment to transmit the messages. The RTS line can be raised by the operating system.
- Whether the 'receiving' communications equipment is ready to receive new data
- Whether the 'sending' communications equipment is sending data (or is about to send data).
As with many channel connection settings, the Flow Control setting of each channel has to match the Flow Control configuration of your communications device. The possible Flow Control options are:
- None (if Flow Control is not required). The ‘sending’ communications equipment or modem assumes that the ‘receiving’ equipment or modem can receive data at all times.
- Software (XON/XOFF)
- Hardware (RTS/CTS)
- RTS Toggle
- Manual.
If you choose Manual, you can also define the RTS/CTS Delay settings:
- Pre-message RTS Delay—The amount of time (in milliseconds) that separates the assertion of the RTS signal and the transmission of data.
- Abort Pre-message Delay if CTS Raised—This setting only applies if an advanced driver channel is set to have a Connection Type of Serial(see Configure a Channel’s Connection Type).
Select this check box if ClearSCADA is to abort the Pre-Message RTS Delay when the 'receiving' communications equipment asserts the Clear to Send (CTS) signal.
Clear the check box if a Pre-message RTS Delay is required at all times (even when a CTS signal has been sent).
- Post-message RTS Delay—This setting only applies if an advanced driver channel is set to have a Connection Type of Serial(see Configure a Channel’s Connection Type).
The Post-message RTS Delay is the amount of time (in milliseconds) that ClearSCADA waits between sending the message data and instructing the operating system to lower the RTS signal.
The Post-message RTS Delay should match the amount of time it takes for your 'sending' communications equipment to complete the data receival process.
When configuring the RTS Delays for a channel, you need to have a good understanding of how long it takes your communications equipment to prepare for sending data. The RTS Delay settings you configure for the channel need to match the actual 'warm up' times of your communications equipment.
The Pre-message RTS Delay should match the amount of time it takes for your 'receiving' communications equipment to prepare for incoming data.