OK, attached is an app note from SAIA Burgess Controls, whoever they are, showing a few possible configurations for an EN62053-31 type SO output, along with a couple of configurations for the interface.
The output is the transistor side of an optocoupler, maybe with protection circuitry. The energy for the pulses comes from an external power supply, and there’s an external limiting resistor in series with the output. The circuits shown would give an ON voltage ranging from the saturation voltage of the output phototransistor to maybe a volt, and an OFF voltage ranging from a high of nearly the input supply voltage to something dependent on the value of the internal zener diode, the internal limiting resistor, and the external limiting resistor. All of that is consistent with the abbreviated schematic in the OP’s datasheet.
The purpose of the output isn’t entirely clear to me. SAIA offers a pulse counter that can be driven directly from the pulse output, presumably to provide a remote reading of the meter for, maybe, a customer, while keeping the meter itself in a secure enclosure to prevent tampering. The pulse output might serve as the input to a pulse recorder, so that the utility can analyse the pulse rate and thus the average power over an interval, to calculate a demand charge for customers that have to pay them. My experience with electric metering says that a new meter - say, electronic as opposed to electromechanical - have to be backward-compatible with a couple of generations of the previous technology - say, electromechanical meters with pulse initiators - if they want utilities to even consider buying them.
In the attached document, some of the possible configurations show an internal series resistor and a parallel zener diode. If the diode voltage is lower than the external DC supply, some non-zero current will flow through the output. It’s not clear to me what levels are permissible, but it might be more than some minuscule value. So, it’s possible that it’s hard to get very close to the voltage of the external supply on the output.
If the actual circuit is anything like the examples, it might be possible to use it with 5V and a fairly big resistance, say 10K, without any other interface circuitry, and connect directly to an Arduino digital input. Or, that may not work, if the voltage drop across the reverse-protection diode is too big, the internal limiting resistor is too big, or the ON voltage of the transistor is too high. Whatever the output is for, it doesn;t look like it’s intended to be super-friendly to 5V microcontrollers.
Applicationnote_S0puls_output_EN_V1.1_01.pdf (1.06 MB)