UDR register configuration - Arc protection - Auto synchronization - Bay control and measurement - Merging unit - Transformer protection - 2 winding - Feeder protection - Voltage regulation - Capacitor bank protection - Petersen Coil control - Grid automation - Busbar differential protection (high impedance) - Back-up protection - Motor protection - Busbar protection (voltage and frequency) - Transformer protection - 3 winding - Interconnection protection - Generator protection - Power management/Load shedding - Modbus Communication Manual - REX640 Protection and control - PCL4 - IEC - ANSI

UDR registers are read-only registers mirrored both on input register (3x) and holding register (4x) areas. After adding a Modbus register value into the user-definable memory area, it is possible to leave it in exactly the same format as the source Modbus value. Alternatively, it is possible to change the user-definable value outlook to better suit the Modbus client.

The UDR value configuration can be divided in three parts.

Initial properties. Redefine the user-definable register type and how the Modbus value source is retrieved.
Rescaling. Applies a new scaling to the UDR value.
Presentation. Modifies the presentation of the UDR value. These modifications are only needed for special retrofit cases.

Columns in the tool do not necessarily appear in the same order as in the table.

Table 1. Configuration columns in Modbus Communication Management tool in PCM600
Setting	Alternatives	Description
Initial properties
Scale value format	Regular Modbus register value ¹	UDR value initially inherits scaling from the source Modbus register. This includes the source value `Primary scale in use` setting. If checked, the value is automatically presented as a primary value, according to the system CT and VT ratio settings.
Scale value format	DA value from system level	UDR value is initially the same as the IEC 61850 value received from system level. For current and voltage measurements this means pu value (1.0 corresponds to 1.0 × nominal value).
UDR register size (v2)	Same as the source register ¹	If the Modbus source register is a 16-bit register, then the UDR register is automatically also
		16-bit. This principle applies to 32-bit registers, too.
	16-bit	UDR register is forced to 16-bit, regardless of the source register size.
	32-bit	UDR register is forced to 32-bit, regardless of the source register size.
UDR register type	Same as the source register ¹	If the Modbus register is an unsigned register, also the UDR register is unsigned. This principle applies to signed source registers, too.
	Unsigned	The value is interpreted as a positive value only.
	Signed	The value is interpreted as a signed two's complement value.
Rescaling
Scaling	No scaling ¹	No rescaling of the Modbus UDR value. Scaling arguments are ignored.
	Ratio scaling	Linear scaling of the source value in the source range `Min In...Max In` into UDR value range `Min Out...Max Out`. The scaled UDR value saturates at `Min Out` and `Max Out` values.
	Multiplicative scaling	The source value is multiplied by scaling argument 1 (similar to argument in ratio `Min In`). Other scaling arguments are ignored.
	Divisor scaling	The source value is divided by the scaling argument 1 (similar to argument in ratio `Min In`). Other scaling arguments are ignored.
Min In	Any real value	Scaling argument 1. Used also as the scaling factor for multiplicative and divisor scaling.
Max In	Any real value	Scaling argument 2
Min Out	Any real value	Scaling argument 3
Max Out	Any real value	Scaling argument 4
Presentation: 32-bit register presentation
Word swap	Unchecked ¹	32-bit register value expressed according to “de facto” standard. Word order High-Low.
Word swap	Checked	32-bit register's value word order changed to Low-High
Presentation: Special retrofit value manipulation
Byte swap	Unchecked ¹	16-bit register value expressed according to Modbus standard. Byte order High-Low.
Byte swap	Checked	16-bit register value byte order changed to Low-High.
Bit size	0 ¹	Register bit size reduction not in use. All bits in register used by the value.
Bit size	2...31	Register bit size reduced to this value. Saturation limits of the value according to the configured bit size, including possible sign bit.
Justification	Right ¹	After a value bit size reduction, the remaining value bits are aligned to the right (LSB) side of the register.
Justification	Left	After a value bit size reduction, the remaining value bits are aligned to the left (MSB) side of the register.

Multiplicative and divisor scaling are similar operations, because any real value can be used as scaling argument. For example, divisor scaling by 4 is the same as multiplicative scaling by 0.25.

¹ Default value