## Analogue output function

Channels 1 and 2 of the PD 640 module can be configured to work as either an analogue input or an analogue output.

When configured as an analogue output, the analogue input is still active and can be used as a read back feature to monitor the output current.
All analogue input channel features, such as notification, alarms etc., continue to work in the same way as if the channel were configured for analogue input.

An analogue output can be configured as 0-20mA or 4-20mA.

The control signal for the analogue output can be written to the channel in two ways:
– Percentage value
– Scaled SI unit value

Writing in percentage (0-100) will set the output current to the required percentage of the selected output range: 0-20mA or 4-20mA.

Writing in Scaled SI unit, will calculate the output signal based on ZeroPoint and Span, using the same calculation principle as described for Analogue Input channels.

Writing to one of the two output control registers will always result in the value being automatically re-calculated and written to the other register. Both registers will thus always show the actual value.

## Example 1:

An analogue output channel controls a 90° turning valve.
ZeroPoint = 0 and Span = 90.
Output configured to 4-20mA.
The valve can now be controlled by writing the requested opening percentage (0 to 100) into the Percentage register or by writing the requested opening angle (0 to 90) into the Scaled output register.
The value 50 is written to the Scaled output register. This corresponds to (50-0)/90*100 of the scale = 55.5%. This value is written to the percentage register.
The output current is set to 55.5% of the selected output range. For a 4-20mA channel this is: ((20-4)*55.5/100)+4 = 12.88mA.
Likewise, by writing 55.5 into the Percentage register would result in the current being set to 55.5% = 12.88mA, and the Scaled output register would be re-calculated to 55.5% of the range which equals 50°

## Example 2:

An analogue output channel controls a frequency converter. The frequency converter is configured to control the motor from a minimum of 500 RPM to a maximum of 4000 RPM.
Output configured to 4-20mA.
The motor can now be controlled by writing the requested speed percentage (0 to 100 %) into the Percentage register or by writing the requested real speed (500 to 4000 RPM) into the Scaled output register.
If the value 3200 is written into the Scaled output register, this corresponds to (3200-500)/3500*100 of the scale = 77.1%. This value is written into the percentage register.
The output current is set to 77.1% of the selected output range. For a 4-20mA channel this is: ((20-4)*77.1/100)+4 = 16.3mA.
Similarly, writing 77.1 into the Percentage register would result in the current being set to 77.1% = 16.3mA, and the Scaled output register would be re-calculated to 77.1% of the range, which equals 3200 RPM

## Related Topics

Transmitter installation