Bill Farmer

Random thoughts on random subjects

Smart Meters

by Bill Farmer. Categories: Electronics .


Smart meters are not a new idea. I was a member of a project in the early 1980s which designed and rolled out a prototype smart meter which was far ahead of it’s time. The unit was called a CALMU (Credit and Load Management Unit). It was capable of recording electricity consumption with several tariffs, gas consumption, with a connection to a suitable gas meter and water use, with a connection to a suitable water meter. It had a remote display panel with a vacuum fluorescent display which could be mounted in a suitable position.


The electricity meter used a variant of an Intel 8051 processor which included an eight bit A-to-D. To increase the accuracy the current and voltage measurements were made against a slowly ramping base level and aggregated over a time period. The samples were taken synchronised with the supply frequency at various points on the wave. I argued at the time that it would be more accurate to sample unsynchronised at a small difference from the supply frequency so the sample points drifted across the waveform over time. Who was right I have no idea.

Radio teleswitch

The meter was connected to the customers’ telephone line via a multi-purpose modem/radio teleswitch receiver. The receiver used two nested phase lock loops, the inner loop synchronised to the carrier phase, the outer loop synchronised to the message (PDF) bits. The messages were 50 bits long with a 13 bit CRC and a start bit. This was implemented by a ferrite rod aerial connected to a simple transistor amplifier connected to an input pin on the processor, an Intel 8051 variant with an EPROM. Synchronisation to radio teleswitch messages was achieved by a successful 13 bit CRC and start bit check.


Clock crystal used for the radio teleswitch processor.


The input from the telephone line was connected to an interrupt pin on the processor. When an interrupt occurred due to an incoming message from the telephone exchange the processor checked for a signal long enough to check for a valid message and either processed the message or went back to radio teleswitching. The modem used the CUTS cassete tape standard for communication, which AFAIK was never intended for use over telephone lines. However it worked.

Data Concentrator

At the telephone exchange was a data concentrator which interrogated the CALMUs on demand from the computer centre. It was an off the shelf Intel 8086 development board in an Intel enclosure. The code was written by the CEGB in some obscure real time language which was probably popular at the time. However a telephone exchange was a high electrical noise environment at that time as it was full of busy electro-mechanical exchange equipment. So the concentrator crashed regularly and had to be reset.

Display Panel

How the display panel communicated with the CALMU I don’t remember except that it used 4 core telephone wire and probably some sort of serial protocol. I also don’t remember how the modem processor communicated with the metering processor.

Super Snooper

I did develop several useful gadgets for testing purposes, one was the CALMU Super Snooper which had a two line LCD display and a hexadecimal keypad. This allowed interrogating a CALMU by sending a query message and displaying the reply. It used the EPROM variant 8051 processor. It had a piezo buzzer mounted on the pcb which provided audio feedback for the keypad. If you held a button down while turning it on it played the tune from Super Trouper by Abba.

LCD Display

Similar LDC display

BBC Micro receiver

Another was a radio teleswitch receiver using an 8051 variant which was connected to a BBC Micro digital I/O port. This, in conjunction with some software on the BBC Micro, enabled the radio teleswitch messages to be displayed.

Acorn Atom terminal

The other was an EPROM that turned an Acorn Atom into a serial terminal. The standard keyboard scanner in an Atom was horrible to type on, it only registered a key press when you released the key. So I wrote my own, included in the EPROM. I can’t remember why we wanted a serial terminal, but we must have done or I wouldn’t have had the idea.

Other projects

There were other metering projects going on at the time, one used broad band communication down the customers’ electricity supply cable from the nearest substation. This worked during times of low demand on the power network, but became unreliable during times of high demand due to the high noise level.


The various utilities were all nationalised industries at the time, so there was not too much difficulty working with other utilities on an interesting project such as this. Shortly after the end of the project BT rolled out a new tariff I think they called Bitstream to offer communication with devices on a customer’s premises via the telephone line. The prices they wanted for the service were completely out of scope for meter reading. So that was the end of that. Since then all the utilities have been privatised, so any joint metering projects became exceedingly unlikely. They have since all done their own thing.