A user interface unit 
User Interface Unit UIU(UIU) is a remote access terminal for STS prepayment 
The Standard Transfer Specification STSmeters. The device is placed inside the user's premises, while the energy meter is mounted outside and mostly not direct accessible. Some meter manufacturers are using the term customer interface unit (CIU). The functions are the same.
The main purpose is to charge the meter with credit. By various short-codes, it's possible to do many kinds of queries for meter status and consumption. The short codes are not unified, so refer to your user manual for operation.
UIUs are paired with the energy meter when leaving our factory. Depending on the utility order, CLOU interface units are communicating with the meter by radio frequency (RF), power line (PLC) or by wired RS-485.
If the UIU is not communicating with the energy meter, you can pair it again by typing 1593570 and the 11 or 13 digit meter number, then press the enter key (red or blue). The meter number is printed on the meter and on your customer card, which you use to buy credit.
After successful pairing, the display will show OK.
Note: This is only working for CLOU energy meters.
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There can also be other reasons for a communication fail. If the UIU is connected by power line and you are disconnected, batteries need to be inserted in the UIU to recharge the meter or query the status. If this meter is in tamper mode, a recharge is impossible before clearing the tamper.
How to get a Clear Tamper Token?
Editor's note: This article was originally published in August 2020 and has been updated for comprehensiveness.
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Hello,
I pack into a new apartment and notice the clou meter has not been used for over four years. I notified the energy provider in my area and it was fixed, but the monitor is not working . Several attempts to pair with other clou monitor failed . Now they got me a new monitor of 2013 while the meter is 2015, I am still trying to pair it's show Err 06. Please what how can this be fix.
Hello,
thank you for your message.
Based on your description, the issue is very likely related to the STS TID rollover that occurred on 24 November 2024.
If the meter has not been updated with the required key change, it may reject communication and tokens, which can also result in pairing errors such as "Err 06".
In this case, the meter needs to be updated by your energy provider. This includes issuing and entering a key change token pair to switch the meter to the new base date.
Please contact your utility and ask them to perform the TID key update for your meter. Once this is completed, the pairing with the monitor should work normally.
More info is here.