Use the fields within the Pre/Post Amble section of the Scan Parameters tab to define any additional bytes that are transmitted before and after each DNP3 frame that is sent over a communications channel.
The bytes defined within this section are the default bytes that are applied if no Pre-Amble and Post-Amble bytes are specified on an outstation’s configuration Form (see Specify Different Pre- and Post-Amble Message Strings for an Individual Outstation).
In many cases, you will not need to specify any pre-amble or post-amble bytes. However, you might need to specify additional bytes to activate a particular communications channel, or to provide extra time to synchronize the channel.
- Pre-Amble—Use to specify the bytes that are transmitted before each DNP3 frame.
- Post-Amble—Use to specify the bytes that are transmitted at the end of each DNP3 frame.
Specify both the Pre-Amble and Post-Amble in hexadecimal format.
Example: 1
Geo SCADA Expert is to communicate with a number of DNP3 outstations over a radio network that supports data streams. A data stream is specified using a specific pair of bytes (a Stream ID) at the start of each transmission. Stream 13 (decimal) is to be used for the DNP3 communications between Geo SCADA Expert and the outstations. The Pre-Amble of the channel is therefore set to 0D (as 0D is the hexadecimal equivalent of the decimal number 13).
Example: 2
Geo SCADA Expert is to communicate with a DNP3 outstation using a dial-up modem. After some initial testing, it has been found that the phone line and modem combination sometimes corrupts the first few bytes of a transmission. The Pre-Amble is used to ‘pad’ each transmission with a specified number of bytes that are discarded by the receiving device.
Example: 3
Geo SCADA Expert is to communicate with a DNP3 outstation using analog radio. The radio needs time to power up at the start of each transmission. The Pre-Amble FFFF is used to allow the radio time to power up.
ATTENTION: We advise that you do not use a Pre- or Post-Amble of 0564, as the DNP3 protocol uses 0564 to indicate the start of a DNP3 frame.
For more information, see Defining any Default Pre- and Post-Amble Message Settings for a Channel.