RS232 (RTS) Keyed

RS232 RTS Keyed communications are primarily used for multidrop serial DNP3 communications, but can also be used for the Port Functions listed below.

• DNP3
• COM User (Function Block)
• Modbus
• Terminal Server.

Collision avoidance

SCADAPack x70 devices use collision avoidance mechanisms for DNP3 communications on RS232 (RTS) Keyed interfaces, and can control data transmission on an external device using the RS232 CTS and RTS control lines. To implement collision avoidance, the device:

• Waits the Response Delay time before checking for an active Clear to Send (CTS) signal. If the signal is active, no data is transmitted and the device waits for CTS to become inactive.
• When the channel is available, a timer is started and the device waits a period of time that is based on the time specified for the Fixed Delay, and a random value between 0 and the Maximum Random Delay.
• After this period, the device checks CTS again. If CTS is still active, the device waits until CTS is inactive, and starts the timer again after it becomes inactive.
• The device waits the Fixed Delay time and a random value between 0 and the Maximum Random Delay. Once the device detects that CTS is inactive, it transmits the message.

If the Fixed Delay and Response Delay parameters are set to 0 ms, the device does not perform collision avoidance on the multidrop channel.

For more information about the above properties, see Configure Serial Multidrop Settings.

Message transmission

The SCADAPack x70 device takes the following actions when it transmits a DNP3 message on an RS232 (RTS) Keyed interface:

1. Waits the Response Delay time.
2. Activates RTS on the RS232 port.
3. Waits the Tx Warmup Delay time.
4. Transmits the DNP3 data.
5. Waits for the RS232 transmission to complete, then waits the Postamble Delay time.
6. Deactivates RTS on the RS232 port.