> For the complete documentation index, see [llms.txt](https://docs.videc.de/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.videc.de/acron-9.3/en/anlagenkonfiguration/prozessanbindung/provider/providerid/treiberauswahl/opc_historical-data-access-client.md).
# OPC Historical Data Access Client
#### Background
The OPC HDA interface driver works with all OPC HDA servers as from OPC HDA version 1.0. Historical measurement data and service counters can be imported. The ACRON Provider cyclically polls the servers for new values and save them to the ACRON database.
At present the driver supports synchronous reading of the RAW data. This method should be provided by all HDA servers.
The driver notes the last status (time stamp) for each data point and resumes from that point on restarting. This means no data can be lost, even if the Provider is not running. This information is administered in the `OPCHDA.DEF` file in the local plant cache path `C:\ProgramData\Acron\PlantCache\MyPlantName`.
If data from the past is to be re-applied, this file can be deleted before restarting the Provider.
If data of individual variables is to be re-applied, this file can be adapted before restarting the Provider. To do so, simply delete the line with the relevant variable name from this ASCII file.
#### Syntax of external variable
Full name of the variable as defined in the control system
#### Import external variables
Import predefined variables:
#### Import external variables
1. Set all parameters for the Provider
2. Start the control system
3. Run "Import from control system" in the "External variables" area
Select the variables you want to import into ACRON.
## Driver
### Driver file
Display the name of the selected driver and select a driver from the list of installed drivers
### Version
Version of the driver file
### Interface type
Display the driver interface type
### Data transfer type
Description of the data transfer type
## Driver parameters
### Server name or Server name L
Name of OPC server
Note
If a blank followed by a 'L' is entered after the server name, ACRON interprets the server time stamps in local time.
### Type
L- the OPC server is running on the local computer.
R- the OPC server is running on the remote computer.
#### Advanced settings of the Type parameter
Note
Use these parameters only if problems occur in communication with the OPC server you are using.
#### Explanatory notes to the parameters
**Connection type**
| L | ACRON initially tries to establish an Inproc connection and, if it fails, tries a local connection |
| - | -------------------------------------------------------------------------------------------------- |
| R | ACRON establishes a remote connection and, if it fails, it aborts |
| i | ACRON establishes an Inproc connection and, if it fails, it aborts |
| l | ACRON establishes a local connection and, if it fails, it aborts |
These four prefix letters can be suffixed by a second letter (such as LX):
| | |
| - | --------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| X | The ACRON OPC Client restarts communication completely if permanent errors occur. So this option can be useful in the case of OPC servers which sporadically hang up. |
`lX`
Meaning: Local connection, restart in the event of permanent errors.
### Node name
Name of the computer on which the OPC server is running.
### User password and domain
(optional)
User name, password and domain name, if accessing the server on a different computer. Separate entries by a blank space.
## Settings
### Measure interval
This setting indicates the time interval in which values are polled by the control system/PLC. The minimum value depends on the efficiency of the corresponding interface and the number of external variables being logged.
Recommended entry 60 sec.
### Polling delay after start
Start of data logging in seconds after the ACRON Provider has successfully established process communication. This is useful, for example, if the opposite party takes some time after a restart to initialize all the variables.
Recommended entry: Depending on the startup duration of the OPC Server, about 10 seconds
### Always display provider window on top
Provider windows are always displayed on top
### Activate test mode
Display a window with diagnostic messages
On startup the ACRON Provider opens a window with diagnostic messages. The log file is stored under the relevant provider name in the ACRON installation path, in the `Log` directory.
Note
As diagnosis mode heavily influences the performance of the application, and the recorded messages can normally only be evaluated by Technical Support, you should only activate it when prompted to do so by Support.
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