Communication and System

carrier current applications since they do not have the intelli-

gence needed to distinguish between real messages and

noise induced phantoms

The difficulty in designing special protocols arises out of the

special nature of the AC line an environment laden with the

worst imaginable noise conditions The relatively low data

rates possible over the AC line (typically less than 9600

baud) make it even more imperative that systems utilize the

most sophisticated means available to ensure network effi-

ciency

With these facts in mind the designer is referred to a publi-

cation intended to aid in the development of carrier current

systems This is literature

570075 The Bi-Line Carrier Cur-

rent Networking System a 200 pp book that functions as

the ‘‘bible’’ of Bi-Line system design It has sections on

LM1893 circuit optimization protocol design evaluation kit

usage critical component selection and the Datachecker

DTS case study

viously mentioned publications for advanced techniques)

At the beginning of a received transmission the first 0 to 2

bits may be lost while the chip’s receiver settles to the DC

bias point required for the given transmitterreceiver pair

carrier frequency offset

With proper data encoding

dropped start bits can be tolerated and correct communica-

tion can take place One simple data encoding scheme is

now discussed

Generally a CCT system consists of many transceivers that

normally listen to the line at all times (or during predeter-

mined time windows) waiting for a transmission that directs

one or more of the receivers to operate If any receiver finds

its address in the transmitted data packet further action

such as handshaking with the transmitter is initiated The

receiver might tell the transmitter via retransmission that it

received this data waiting for acknowledgement before act-

ing on the received command Error detecting and correct-

ing codes may be employed throughout The transmitter

must have the capability to retransmit after a time if no re-

sponse from the receiver is heard - under the assumption

that the receiver didn’t detect its address because of noise

or that the response was missed because of noise or a line

collision (A line collision happens when more than 1 trans-

mitter operates at one time - causing one or more of the

communications to fail) After many re-transmissions the

transmitter might choose to give up Collision recovery is

achieved by waiting some variable amount of time before re-

transmission using a random number of bits delay or a de-

lay based on each transmitter’s address since each trans-

ceiver has a unique address

An example of a simple transmission data packet is shown

in

Figure 32 The 8 bit 50% duty-cycle preamble is long

enough to allow receiver biasing with enough bits left over

to allow the receiver controller to detect the square-wave

that signals the start of a transmission If there had been no

transmission for some time the receiver would simply need

to note that a data transition had occurred and begin its

watch for a square-wave If the receive controller detected

the alternating-polarity data square-wave it would then use

the sync bit to signal that the address and data were imme-

diately following The address data would then be loaded

assuming the fixed format and tested against its own If the

address was correct the receiver would then load and store

the data If the address was not correct either the transmis-

sion was not meant for this receiver or noise has fooled the

receiver In the former case when the transmission was not

meant for the receiver the controller should immediately

return to watching the incoming data for its address If the

later case were true then the receive controller would con-

tinue to detect edges tieing itself up by loading false data

and being forced to handshake The square-wave detection

and address load and check routines should be fast to mini-

mize the time spent in loops after being false-triggered by

noise If the controller detects an error (a received data bit

that does not conform to the pre-defined encoding format) it

should immediately resume watching the LM1893’s Data

Out for transmissions the next bit would be shifted in and

the process repeated

A line-synchronous CCT system passing 3 bits per half-cy-

cle may replace the long 8 bit preamble and sync pulse with

a 2 bit start-of-transmission bias preamble The receive con-

troller might then assume that preamble always starts after

bit 1 (the first bit after zero-crossing) so that any data tran-

sition at a zero crossing must be the start of the address bits

and is tested as such The line synchronous receiver oper-

ates with a simpler controller than an asynchronous system

Discussion has assumed that the controller has always

known when the Data Out is high or low The controller

must sample at the proper time to check the Data Out state

Since noise shows itself as pulse width jitter symmetrically

placed about the no-noise switch-points optimum Data Out

sampling is done in the center of the received data pulse

The receive data path has a time delay that at low data

rates is dominated by the impulse noise filter integrator and

is nominally

bit At a 2 kHz data rate an additional delay

of approximately

bit is added because of the cumulative

delay of the remainder of the receiver

Figure 33 shows that

Data Out sampling occurs conveniently at the transmitted

TLH6750 – 35

FIGURE 32 A simple encoded data packet generated by the transmit controller is shown

19