Encoder receiver transmitter
Encoder receiver transmitter is a packet radio protocol developed by Itron for automatic meter reading. The technology is used to transmit data from utility meters over a short range so a utility vehicle can collect meter data without a worker physically inspecting each meter.
The ERT protocol was first described in. More technical detail is explained in later.
Technical details
ERT is an OOK modulated radio signal which is transmitted in the unlicensed 900-920 MHz band. The message is transmitted in the clear and uses Manchester encoding. The protocol uses frequency-hopping, a multiple access method to avoid interference with other nearby meters. SCM and IDM packet formats are described inSCM packet
SCM messages are 12 bytes. Each message contains single, cumulative meter reading value along with the meter serial number, commodity type and checksum and tamper flags.| Field | Length | Default Value | Notes |
| Sync bit | 1 | 1 | |
| Preamble | 20 | 0xF2A60 | |
| ERT ID MS bits | 2 | highest two bits of meter serial number | |
| Reserved | 1 | ||
| Physical Tamper | 2 | ||
| ERT Type | 4 | commodity type: water, gas or electric | |
| Encoder Tamper | 2 | ||
| Consumption Data | 24 | meter reading value | |
| ERT ID LS bits | 24 | lower bits of meter serial number | |
| Checksum | 16 | A BCH code with generator polynomial: |
IDM packet
IDM messages are 92 bytes and contain time of use consumption data.| Field | Length | Value | Notes |
| Training Sync | 2 | 0x5555 | |
| Frame Sync | 2 | 0x16A3 | |
| Packet Type | 1 | 0x1C | |
| Packet Length | 2 | 0x5CC6 | Number of remaining bytes and Hamming code of first byte |
| Version | 1 | 0x01 | |
| ERT Type | 1 | 0x17 | LSB nibble is equivalent to SCM's ERT Type. |
| ERT Serial Number | 4 | ||
| Consumption Interval Count | 1 | ||
| Module Programming State | 1 | ||
| Tamper Count | 6 | ||
| Async Count | 2 | ||
| Power Outage Flags | 6 | ||
| Last Consumption Count | 4 | ||
| Differential Consumption Intervals | 53 | 47 intervals of 9-bit integers | |
| Transmit Time Offset | 2 | ||
| Serial no. CRC | 2 | CRC-16-CCITT of ERT Serial Number | |
| Packet CRC | 2 | CRC-16-CCITT of packet starting at Packet Type |
Later patents describe further variations of packets with variable length.
Implementations
Several vendors have implemented ERT receivers Notably, Digi sells an ERT gateway, and Grid Insight sells a PC-based product called the .It should be possible to decode ERT signals using general purpose UHF packet radios such as the or . A software-defined radio receiver has been implemented using inexpensive hardware: , and the software will decode SCM messages. Kismet has an .