I - INTRODUCTION
I.1 - GeneralPurpose
In the ApplicationNote AN535, we described how
to implement the basis of a power line network
usingthededicatedmodemchipST7537CFN.This
network has beenvalidatedwith ST7537MASTER
& SLAVE systems, and shows the feasibility of
power line control.But Home automation systems
allow more than switching a light, or changingthe
value of a digit. The possibilities of the network
increaseeach time we connecta new device on it.
For instance, adding a phone remote system will
allow the remote (from anywhere on the planet)
control of devices connected on the networkonly
by callingyour house.The phoneremote systemis
now demonstratingthisfact : atelephoneinterface
that allows the householderto give instructions to
appliancesfromoutside.
I.2 - Description
The phone remote system is a device connected
to bothtelephoneline and power line network.It is
able to detect the phone ring, to hang up the line,
and ”converse”withthehouseholder.Thisdialogue
is done by using the DTMF (digital tone multi-
frequency) generated by the remote phone. To
answerback,thephoneremote systemisbeeping.
Of course,thisis a low cost choice andthe system
can be improved by using a voice synthesiser with
on line help andso on...
PHONE
REMOTE
SYSTEM REMOTE
OBJECT 2
Mains
Telephone
Line
REMOTE
OBJECT 1
AN488-01.EPS
Figure 1 : Phone RemoteSystem Connected
to the Power Line Network
In a classical electric installation, this device would
have used one control cable for each controlled
object.Withpowerlinecontrol,youjusthavetoplug
it and the link isdone with all devices controlledby
mains network.
I.3 - Improvements
The part II describes several improvements done
to the communication program explained in
AN535.Thesemodifications allow new features :
- softwarecarrierdetect,
- C.S.M.A.timings(Carrier SenseMultiple Access),
and the use of communication modules in phone
remote program.
II - POWER LINE COMMUNICATION USING
ST7537CFN
TheST7537 CFNpower line modem complieswith
the regulation described in the CENELEC
EN50.065-.1 document, (so frequency, bit coding
and otheruseful features are included in the chip).
Nevertheless,the designerofan applicationhas to
take in account the power line communication
specification when writing his protocol. For in-
stance, access protocols are required for coexis-
tence on the medium.
II.1 - C.S.M.A. Timings
The following paragraphs give a concrete case of
communicationon powerline network.In first, only
2 devices are connected to the network. Then,
othersdevices are connected,and we will be con-
frontedto a real networkspecification.
Communication between 2 units
This is the simplest case of communication.Only
twodevicesareabletosendandreceivepacketon
thenetwork.Oneunitsendsanorder,theotherone
isreplying withan acknowledge(see Figure2).
TheunitTchecksthatthenetworkisfreeforaccess
thanks to the Carrier Detector, then it sends its
frame and waits an acknowledge during a delay
Twack. The unit R receive a frame and sends an
acknowledgeif R is the destinationof the frame.R
mustsend the ack. framebefore the endof Twack.
If it is not the case, or if T has not received the
acknowledge, T tries again to send the order. Of
course the frame format must allows error detec-
tion by the receiver (order checksum) an by the
transmitter(checksuminack.frame).Inmostofthe
case,communication occurs without error,and the
both units must keep silent for a delay Tsilent in
order to leave time for application control. To allow
thetwo unitsto transmit,the unitThasto waitmore
than R, so R willbe the first to take the channel if
it needs to transmit.
If the two units send a frame exactly at the same
time, they will not receive ack. frame, so they will
retryafteradelay(Twack). Arandomvalueisadded
ST7537
PLM
COM.
Unit T Unit R
Order Frame
Ack. Frame
Mains
ST7537
PLM
COM.
AN488-02.EPS
Figure 2 : Two Units on the Mains
PHONE REMOTE SYSTEM APPLICATION NOTE
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