Communications
The Etherlog 3000 features built-in licence-exempt 433MHz radio communications and an RS-232 port with GSM and landline modem compatibility, enabling you to communicate with it easily. This makes uploading accumulated data from the logger an easy task in many difficult situations such as when it is measuring things at the top of a mast, in the middle of a muddy field or underneath a bridge span.Furthermore, maintenance operations such as downloading revised data logging programs are greatly simplified by obviating the need to physically access the logger.
The logger can be programmed to make outgoing calls as well as receive incoming modem calls. This is potentially very useful when exceptional conditions have been detected and action needs to be taken.
Where a GSM modem is used, the flexibility of the Etherlog is greatly enhanced by its ability to send and receive SMS text messages. These can be used to interrogate the status of the logger, start logging programs, and control attached equipment. The logger can also be programmed to send alarm messages via SMS when situations requiring immediate attention have been detected, such as a sustained over-temperature reading or a detached sensor.
Radio communications
Modems
GSM and SMS
Radio communication: operation and options
The Etherlog 3000 communicates with a host PC, which is equipped with a low cost radio transceiver connected to its RS232 port. All the software required for the host PC is supplied with the data logger. The operation of the radio is largely determined by considerations of power consumption - it is simply not feasible for the logger to listen continuously for a request to connect. Instead the logger sends out 'advertisements' at regular (programmable) intervals, inviting users to connect.The maximum range achievable is about 200m in the open field. This may be subject to degradation where a direct line of sight is obscured.
Radio communications in the Etherlog are built on the ISO LAP protocol family, and feature sequenced packet delivery and 16 bit CRC error checking. Each logger has an "address" associated with it, permitting two users to communicate with their respective loggers within range of each other at the same time.
Security features include password controlled access to the logger (but the password is never transmitted over the radio). Session key encryption protects data transferred during a communications session.
Radio alarms
The radio advertisements that the Etherlog sends out each have
an alarm code attached to them. The alarm code is set from within a lucid
program, and may therefore be used to signal any condition which is of concern
to the user. For example, an alarm condition could be triggered when:
- a digital input changes state
- an analogue input goes outside a pre-defined range
- battery voltage is low
- data storage space is running out
The host PC software can be configured to respond to alarm conditions and,
depending on the cause of the alarm, optionally execute a pre-defined script to
clear the condition automatically. For example, if an alarm is generated as
storage space runs out, the PC can automatically connect to the logger, offload
the data and delete the file, clearing space to allow data acquisition to
continue.
Modems
The Etherlog supports both landline and GSM modems with the capability to set
up modem initialisation strings and dialup number. The lucid language
provides commands to initiate a modem connection and to interrogate the modem
connection state, permitting logging programs to call the host PC when, for
example, some error condition exists. The host PC can dial into the logger as
required, provided that the logger has been programmed to answer.
GSM and SMS
SMS text capabilities add a whole dimension to the degree of control that a
user has over the use of the Etherlog. Text messages are processed by the logger's
command interpreter, and can be used to start a task whose sole purpose is to read
a sensor value, send it back in another text message, and stop itself. This
enables the user to
pseudo-interactively control the logger from a mobile phone.
In addition, the logger can send its own text messages under the control of a lucid program, and can report numerical or textual information to the user.
Coffee machine demonstration
As an example, consider our office coffee machine, which (although an admittedly
contrived application) illustrates the use of SMS and Etherlog well.
One of the Etherlog's digital outputs is used to control a solid-state relay which powers the coffee maker, and a temperature sensor hangs inside the jug so that, when the liquid level rises high enough (and the sensor's reading increases a lot), we can conclude that the coffee is made. The coffee machine is started (say from the car) by sending it a message "log start coffee", which starts a lucid program which turns on the relay and waits for the temperature sensor to indicate the coffee is made. It then sends a message back to the sender of the text message saying the coffee is ready, and how many Watt-hours were used making it.
However, let's say the temperature sensor fails. If it starts to read below zero, the coffee machine program knows that the sensor has failed to open-circuit.
In this situation, it turns the relay off immediately, and sends a different text message indicating the failure. In this case, the recipient will not be the caller who started the coffee machine, but the engineer who is responsible for its maintenance.
As we said, the example is contrived, but it really exists and demonstrates the usefulness and flexibility of the system.