Measurand registers are mirrored on both 3x and 4x Modbus register areas. Registers can be accessed in the Modbus Communication Management tool in PCM600, under tabs: 3x – input registers and 4x – holding registers.
Register values are received from the IEC 61850 system level in two formats. Measurands are usually received as floating point values and counters as integer values. The Modbus register values are always presented as integer values. To make the source floating point value decimals visible in the Modbus register, the received IEC 61850 value can be multiplied with, for example, values 10, 100 or 1000. The Modbus register rounds the integer part and truncates all decimals that are left in the source value.
| Setting column | Alternatives | Description |
|---|---|---|
| Register address | Not adjustable | The 3x and 4x Modbus memory map addresses where the register resides as default. If the field is empty, the register is not visible in the Modbus memory map as default. The register can always be further mapped into the user-definable area. |
| Data category | 0 1 | Register does not belong to any data category group. |
| 1...16 | Register belongs to data category group number N. If any object belonging to this group changes its value, bit (N-1) in SSR4 gets the value 1. The SSR4 bit value is automatically reset when client reads it. | |
| Primary scale factor in use | Unchecked 1 | In case of current and voltage values, the value is now a PU (Per Unit) value, where 1.0 corresponds to 1.0 × Nominal value. |
| Checked | In case of current and voltage values, the value is now a primary value, based on the configured CT and VT ratio values. Current is in [A] and voltage is in [kV]. | |
| Scale | Any real value | Modbus value is scaled as Value × Scale + Offset. The result is a correctly rounded integer value. If the Modbus value exceeds the register size limit after the scaling, then it saturates at the register max value. |
| Offset | Any real value |
The Primary scale factor in use setting has a meaning only for values that are related to current or voltage measurements. Usually the check box should be visible only for these kinds of values. In case the setting occurs for another type of value, it does not affect the source value.
The primary scale factor setting cannot be applied afterwards in the user-definable register. The setting must be done at the source register value, and the UDR setting Scale value format must be configured as “Regular Modbus register value”.