When a channel is configured to use an IEC 60870-5-101Protocol Type(see Specify the IEC 60870-5 Communications Protocol), you need to configure these properties on the IEC 60870-5 tab. The settings you specify for these properties need to match the corresponding settings of the outstation(s) that use this channel.
- Link Address Size—Use the combo box to specify the number of octets that are used to transmit the link address in each Application Service Data Unit (ASDU).
If the Transmission Procedure is Unbalanced, you need to specify the Link Address Size that corresponds to the size of the Link Address that is used by the outstation. Specify 1 Octet or 2 Octets, as appropriate. (You specify each outstation’s Link Address using the Link Address field on each Direct Outstation Form (see Specify the IEC 60870-5-101 Parameters).)
The Link Address is optional if the Transmission Procedure is Balanced (see below) as the address is not needed to identify the outstation. Therefore, with a Balanced transmission you can optionally specify 0 Octets.
- Allow Single Character Acknowledgment—The IEC 60870-5-101 protocol defines two types of message that can be transmitted to acknowledge the receipt of a transmission.
One acknowledgment message is a single-byte response. This is a more efficient type of response, although it may be received spuriously if there is interference on the line. Select the Allow Single Character Acknowledgment check box if interference is not a problem, or if the outstation only sends single character acknowledgments. If the Transmission Procedure is Balanced, and an acknowledgment is requested, ClearSCADA will respond by sending a single-byte message to the outstation, in response to each data packet that it receives successfully.
The other type of acknowledgment message is a response that contains between four and six bytes (depending on the Link Address Size). This response is less efficient, but is appropriate for use on channels that are operating in an environment that is affected by interference. Clear the Allow Single Character Acknowledgment check box if the outstation is to send four- to six-byte acknowledgment messages in response to each request that it receives successfully from ClearSCADA (and vice versa if the Transmission Procedure is Balanced and ClearSCADA receives a request to send an acknowledgment). For an outstation to receive such messages successfully, the outstation has to be able to send long acknowledgment messages.
Regardless of the Transmission Procedure used, any message containing an ASDU that ClearSCADA sends to an outstation will require that outstation to send an acknowledgment of the type specified here. If the Transmission Procedure is Unbalanced (see below), ClearSCADA will not send any acknowledgment messages. If the Transmission Procedure is Balanced, an outstation response containing an ASDU might request that ClearSCADA sends an acknowledgment.
- Transmission Procedure—Use to specify the transmission procedure that applies to the channel. Select the option that applies to your system setup:
- Unbalanced—ClearSCADA controls the data traffic on this channel by scanning the channel’s outstations sequentially. The ClearSCADA server initiates transmissions via the channel. None of the outstations on the channel can initiate transmissions—instead, they can transmit data only in response to a poll from the server.
- Balanced—The channel is used for point-to-point communications, whereby ClearSCADA uses the channel to communicate with a single outstation. With a Balanced transmission procedure, the outstation can initiate communications with ClearSCADA.
- Test Link Interval—Only applies if the Transmission Procedure is Balanced. With a Balanced transmission, there may be long periods of time during which no data is transmitted. ClearSCADA can transmit a test frame at specified intervals, to determine whether the channel is still available. Whenever ClearSCADA sends a test frame, it expects a response from the outstation to indicate that communications are still healthy. If no response is received from the outstation, the communication with the outstation is deemed to have been unsuccessful.
Use the Test Link Interval to specify the maximum amount of time, in seconds, that the Link Layer can remain idle, before ClearSCADA sends a test link frame to the outstation.
Specify 0 if ClearSCADA is not to send any test link frames via this channel.
- Class 2 Poll Interval—Use to specify the minimum time interval between receiving an empty Class 2 poll response and sending the next Class 2 poll response to the same link address. Specify the required Interval in OPC Time Format.