IEC 62056-21
IEC 61107 or currently IEC 62056-21, was an international standard for a computer protocol to read utility meters. It is designed to operate over any media, including the Internet. A meter sends ASCII (in modes A..D) or HDLC (mode E) data to a nearby hand-held unit (HHU) using a serial port. The physical media are usually either modulated light, sent with an LED and received with a photodiode, or a pair of wires, usually modulated by a 20mA current loop. The protocol is usually half-duplex.
The following exchange usually takes a second or two, and occurs when a person from the utility company presses a meter-reading gun against a transparent faceplate on the meter, or plugs into the metering bus at the mailbox of an apartment building.
The general protocol consists of a "sign on" sequence, in which a handheld unit identifies itself to the metering unit. During sign-on, the handheld unit addresses a particular meter by number. The meter and hand-held unit negotiate various parameters such as the maximum frame length during transmission and reception, whether multiple frames can be sent without acknowledging individual frames (Windowing), the fastest communication rate that they can both manage (only in case of mode E switching to HDLC) etc. Next the meter informs the hand held unit about the various parameters that are available with it in various security settings viz. the 'no security logical group', ' the low security logical groups' and ' the high security logical groups'.
If the parameter required is in no security group, just a get.request will provide the HHU with the desired response. If the parameter required is in low security group, a password authentication of the HHU is required before information can be read.
In case of high security parameters,the meter challenges the hand held unit with a cryptographic password. The hand held unit must return an encrypted password. If the password exchange is ok, the meter accepts the hand held unit- it is "signed on."
After signing on, the hand held unit generally reads a meter description. This describes some registers that describe the current count of metered units (i.e. kilowatt hours, megajoules, liters of gas or water) and the metering unit's reliability (is it still operating ok?). Occasionally a manufacturer will invent a new quantity to measure, and in this case, a new or different data type will appear in the meter definition. Most metering units have special modes for calibration and resetting meter registers. These modes are usually protected by anti-tampering features such as switches that sense if the meter enclosure has been opened.
The HHU may also be given limited rights to set or reset certain parameters in the meter.
The hand held unit then sends a sign-off message, and the meter automatically signs off after a previously negotiated time interval after the last message, if no sign-off message is sent.
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