Question
Is a dedicated OPC Client driver required for each OPC Server?
Answer
No. The OPC Client driver is a generic driver to support the OPC specification. Only one OPC Client driver is required for all fully compliant OPC Servers.

Question
Does the OPC Client driver support Citect's redundancy?
Answer
Yes, the OPC Client driver supports Citect's redundant architecture by a) checking the status of the OPC Server itself, and b) by checking the 'ServerState' returned from the OPC Server. If either of these tests reveal a problem, the OPC Client driver will go offline, and Citect will swap to the redundant OPC Client / Server pair. An OPC Server does not need to explicitly cater for redundancy for Citect's redundancy to operate.

Question
Can the native addressing format of the PLC be used when configuring Citect?
Answer
This is highly dependant on the OPC Server implementation. Citect simply requests 'OPC Items' from the OPC Server, and the format of those items is specified by the OPC Server design. The address field in the Citect Tags form is used to specify the 'OPC Item'. It may be that an OPC Server allows items to be specified in a format similar to the native addressing format of the device, though this is not necessarily the case.

Question
Can the OPC Client driver read Arrays from OPC Servers?
Answer
Yes.  See KB Article Q3392

Question
Do the 'Response times' shown in the Citect kernel (eg. "Page Driver") accurately reflect actual response time from the PLC, and can they be compared with other Citect drivers. ?
Answer
No, the 'Response times' do no accurately reflect response times from the PLC. The OPC Client / Server act like a buffer, and Citect can only determine how quickly the OPC Client driver accesses that 'buffer'. This measured time is usually insignificant compared with the actual response time from the PLC. This means that the response times shown in Citect for the standard drivers (eg. TIDIRECT, SSEVEN, ABTCP etc.) cannot be compared against the response times of the OPC Client driver. Some other method of comparing the drivers should be used.

Question
The OPC specification uses tag names rather than physical addressing to request the value of a tag. How does this affect the performance in Citect?
Answer
When Citect reads tags from a PLC, it is sometimes quicker to request a block of tags which includes several of those needed by Citect. This is because each request has a specific overhead in time independent of how many tags are being requested, so by making one larger request instead of many smaller requests reduces the average overhead per tag. By using tag names to access the value of a tag, Citect cannot logically 'block' several requests into one request and so the overhead cannot be eliminated. While the exception reporting nature between the Citect OPC Client and the Server reduces or negates this concern at this point, performance may still be degraded at two places:
a) between the OPC Server and device. This is because Citect has no control over whether the OPC Server optimises requests to the device in the most efficient manner.
b) between a Citect Client and a Citect IO Server. As Citect cannot block individual requests when using tag names to access the tags values, each Citect Client will request each tag individually across the network. This will signifcantly load the network and have a corresponding affect on the performance of the Citect system as a whole.

Question
I have a customer who tries to connect to an OPC Server and they always get the 40007 error. This seems to indicate that the tag address has an incorrect item id, but the correct value has been entered. This error may also indicate that the OPC Server requires that an "Access Path" be provided for each item. The "Access path" can be specified when using the FactorySoft OPC Client when you are adding an item. Is there any possibility to use the Access path in Citect?
Answer
The Access Path is a server specific option that is intended to allow the client to provide a 'recommendation' to the server regarding how to get to the data. Citect does not allow the ability to specify access paths for items. The OPC specification isn't exactly clear as to whether it is legal for the OPC Server calls to fail if an access path is not specified, but it does go as far as saying that by "convention" a server should select an access path if one is not provided. With this in mind, Ci Technologies suggest that an OPC Server should handle the case where an access path is not specified.