# 20.4 Configuring data endpoints To use WCC Lite as a programmable logic controller, it needs to be configured in a particular way. The PLC functionality of the WCC Lite only allows for the use of data that is has been configured in the Excel configuration spreadsheet. This has been done for security purposes and to preserve transmission medium only for data that is available. Only topics defined in the configuration can post or get data. If a certain data entry exists but it has not been linked to a PLC program, all calls from PLC runtime application to redis database will be ignored. Therefore it is highly advised to prepare and upload the Excel configuration before using this signal in the PLC application. Some parameters are mandatory for PLC usage. These parameters are shown in two tables below (one for Devices, one for Signals tab). Please note that other parameters can be used as well, but are not covered because they aren’t specific to PLC functionality. Table. Mandatory parameters for Devices tab
Parameter | Type | Description |
name | string | User-friendly device name |
device\_alias | string | Device alias to used in configuration |
enable | boolean | Enabling/disabling of a device |
protocol | string | Selection of protocol (IEC 61499) |
Parameter | Type | Description |
signal\_name | string | User-friendly signal name |
device\_alias | string | Device alias from a Devices tab |
signal\_alias | string | Unique signal name to be used |
source\_device\_alias | string | device\_alias of a source device |
source\_signal\_alias | string | signal\_alias of a source signal |
Outputs of variable type ANY cannot be directly wired to inputs of the same type and therefore need to be explicitly typed using transitional function blocks.
No more than 20 tags should be published over a period of 5 seconds, as this may overfill the queue. A ”publish only on change” policy is advised.
Currently only PUBLISH\_1 and SUBSCRIBE\_1 function blocks are supported.
The functionality of the subscribe function is dependent on the presence of the "F\_STRING\_TO\_" function block, for example (F\_STRING\_TO\_REAL). Likewise, the publish function's proper operation relies on the availability of the "F\_ \_TO\_STRING" FB, for example (F\_REAL\_TO\_STRING).
If every step until now has been successful, the user can now proceed to debug the PLC application.