Power line carrier communication system for monitoring refrigerated containers

A power line carrier communication system for monitoring refrigerated containers which includes a master monitoring unit and a first power line interface which interchange messages in a first format. The first power line interface translates the first format to a second format suitable for power line environment, and messages in the second format are applied to a power line. Remote monitoring units receive the messages from the power line, and they return messages to the power line containing status data relative to refrigerated containers. The second format includes a message starting preamble having a duration and logic level which is not duplicated by normal operation of the apparatus, enhancing the probability of proper message synchronization and reception over noisy power line environments.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates in general to the monitoring of refrigerated 
containers, and more specifically to monitoring refrigerated containers 
over electrical distribution power lines. 
2. Description of the Prior Art 
Electrical utilities have used their high voltage transmission lines for 
many years for communication with remote switching and substation sites, 
for supervisory control purposes. Transmission lines are ideal for 
communication as they extend from the power generation site to the remote 
sites without intervening obstacles. Use of the distribution power lines, 
however, has been slower to develop, as the distribution power lines are 
susceptible to electrical noise and interference, and they include 
distribution transformers, electrical loads, sectionalizing switches, 
capacitor banks, and the like, which attenuate communication frequencies. 
The increasing desirability of being able to selectively and remotely 
control electrical loads on the distribution system, and the availability 
of low cost encoders for automatic meter reading, have produced a flurry 
of activity in the use of distribution power lines for communication 
purposes. U.S. Pat. Nos. 3,911,415; 3,942,168; 3,942,170; 3,967,264; and 
3,980,954, describe some of the early problems encountered, and solutions 
thereto, when using electrical distribution power lines for communication. 
A specialized use of electrical distribution power lines for communication 
purposes has been disclosed in U.S. Pat. No. 4,234,926. Refrigerated 
containers, called "reefers" are monitored by a central computer, using 
the power lines for polling or interrogating remotely located computerized 
monitoring units associated with the reefers, as well as for receiving 
reefer status data in return. Since large numbers of reefers are stacked 
aboard ships, as well as in ship terminals, the use of the electrical 
power lines connected to the reefers for the additional functions of 
monitoring and control substantially increases the speed and reliability 
of the monitoring process, which is normally manually performed. 
While the use of ship and ship terminal electrical distribution lines for 
communication may seem simple and straight forward compared with the 
attenuating obstacles encountered by the electric utilities in using their 
distribution systems for communication, ship and ship terminal electrical 
systems can have substantial amounts of electrical noise. For example, 
large adjustable speed motor drives which chop the electrical waveform may 
be used, which feed large amounts of electrical noise back into the power 
lines. 
SUMMARY OF THE INVENTION 
Briefly, the present invention increases the reliability of power line 
carrier communications for monitoring refrigerated containers aboard ships 
and in ship terminals by making it easier for the communication system to 
detect the start of a message received over the power line. A master 
monitoring unit (MMU) having a computer directs the monitoring and control 
process, providing messages in a first format for a first power line 
interface also referred to as a network central control unit (NCCU). The 
NCCU translates the messages to a second format more suitable for the 
power line environment, it modulates the message, such as by frequency 
shift keying (FSK), and it places the translated messages on the 
electrical distribution power line. Remote monitoring units (RMUs) gather 
status information from the reefer's refrigeration units. The RMUs include 
second power line interfaces. The second power line interfaces are coupled 
to the power line and they demodulate the high frequency messages. 
The RMUs sample the incoming line at a rate greatly in excess of the 
message bit rate, looking for a valid message starting preamble. The 
message itself has a predetermined logic level, such as a logic zero, in 
every ten message bits. The message starting preamble has at least ten 
consecutive bits at a logic level opposite to the predetermined logic 
level which must periodically occur in every ten message bits. If this 
periodic logic level is a logic zero, then the starting preamble must be 
at the logic one level for at least ten bit times. Since ten logic ones 
cannot occur during normal message composition and transmission, the 
chances of detecting the valid start of a message are greatly enhanced, 
and the chances of false starts once a message begins are minimized.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring now to the drawings, and to FIG. 1 in particular, there is shown 
a functional block diagram of a power line carrier communication system 10 
constructed according to the teachings of the invention. System 10 
includes a plurality of transportable refrigerated containers or reefers, 
such as reefers 12 and 13. Reefer 12 includes a refrigeration unit 14 
which conditions the air in a container 15, and reefer 13 includes a 
refrigeration unit 16 which conditions the air in a container 17. U.S. 
Pat. Nos. 4,402,191; 4,409,797; and 4,424,684, which are assigned to the 
same assignee as the present application, illustrate typical container 
refrigeration units. 
Each reefer includes one of two different types of remote monitoring units, 
depending upon whether the reefer already has a computer based 
refrigeration controller, such as controller 18 associated with reefer 12. 
Controller 18 may be the refrigeration controller disclosed in U.S. Pat. 
No. 4,663,725 entitled "Microprocessor Based Control System And Method 
Providing Better Performance And Better Operation Of A Shipping Container 
Refrigeration System", which is assigned to the same assignee as the 
present application. This patent is hereby incorporated into the 
specification of the present application by reference. 
Reefer 12, having a refrigeration controller 18, requires a remote 
monitoring unit 19, referred to as an integrated remote monitoring unit 
(IRMU). If the reefer does not have a controller 18, such as reefer 13, 
the RMU will include a data acquisition function which obtains status 
information directly from a refrigeration unit, and this type of RMU is 
referred to as a stand alone remote monitoring unit (SRMU) 20. IRMU 19 and 
SRMU 20 will be referred to collectively as "RMUs" when the distinction 
between them is not pertinent to what is being discussed. 
Reefers 12 and 13 are monitored by a master monitoring unit (MMU) 22 which 
includes a central computer for preparing, transmitting and receiving 
messages. A network central control unit (NCCU) or first power line 
interface 24 is disposed between MMU 22 and an electrical distribution 
power line 26 which extends to the locations of the plurality of reefers 
12 and 13. The communications between MMU 22 and NCCU 24 are not over 
power line 26, and thus a first message format is used which may be 
formulated with minimal concern with electrical noise. Thus, a generally 
accepted communications medium, such as an RS-232-C standard may be 
applied. NCCU 24 includes a transmitter/receiver 28 for receiving messages 
in the first format, translating means 30 for translating the first 
message format to a second message format tailored for the severe power 
line environment, a power line carrier transceiver (CCT) 31 for modulating 
the message, and power line coupling means 32 for applying the modulated 
message to power line 26. 
IRMU 19 includes power line coupling means 34, a CCT 36 for demodulating 
messages received from power line 26, and computer means 38 for receiving 
messages from MMU 22 and for preparing and sending messages to MMU 22 when 
required. For example, if the message requested the status of reefer 12, 
IRMU 19 would send status information obtained from refrigeration 
controller 18 back to MMU 22. IRMU 19 would prepare the message in the 
second format in computer 38. CCT 36 modulates the message, and power line 
coupling means 34 applies it to power line 26. NCCU 24 detects the message 
via power line coupling means 32, CCT 31 demodulates the message, 
translating function 30 translates the message from the second to the 
first message format, and transmitter/receiver 28 sends the reformatted 
message to MMU 22. 
SRMU 20 includes a power line coupling 34', a CCT 36', and a computer 38', 
all similar to these functions in IRMU 19. SRMU 20 further includes means 
40 for obtaining data available in digital form from refrigeration unit 
16, and means 41 for obtaining data available in analog form from 
refrigeration unit 16. 
A message format which may be used for the first format is shown in FIG. 2. 
The first format, referenced 42, will first be described as it is prepared 
by MMU 22. The first message format 42 may start with an optional 
synchronization field 43 of any desired number of bytes, followed by a one 
byte start-of-text (STX) character 44 which delimits the start of a valid 
message. NCCU 24 strips the synchronization field 43, if used, from the 
start of a message. When a valid start byte 44 is detected, the next two 
bytes 45 are used to define an unsigned integer which indicates the packet 
length in bytes of the following data. The following data includes a one 
byte task number 46 assigned by MMU 22, which is an identification number 
used only by MMU 22. The identification number is followed by a one byte 
character 47 which informs NCCU 24 as to the message type, ie., the type 
of processing required. For example, the message may be directed only to 
NCCU 24, such as setting up internal configurations; or it may be a 
message which is to be applied to power line 26 directed to all of the the 
RMUs, or to a specifically addressed RMU. A variable length data field 48 
follows the processing type identifier 47, with its contents depending 
upon the type of message to be processed. For example, if the message is 
to be applied to power line 26, the data field would contain an address 
tag, a universal or a specific RMU address, plus data relative to the type 
of command being sent. 
When the communication is between NCCU 24 and a local power line, the 
address tag must be set to a predetermined value before the RMUs will 
respond to any message. For purposes of example, it will be assumed that 
the predetermined value is an ASCII "A" (41H). The address tag allows 
expansion of the monitoring system to include remote power lines, by using 
the address tag to address one or more remote power lines which are 
communicated with by radio. When an interface associated with a remote 
power line is addressed by the address tag, the remote interface will swap 
an ASCII "A" for the value in the address tag before applying the message 
to the associated power line. Thus, all reefers, regardless of which power 
line they are connected to will automatically be set up to respond to 
messages from MMU 22. 
A two byte error check field 49 follows, such as a field generated by using 
the cyclic redundancy check (CRC), ie., a polynomial calculation performed 
on the message data bits. 
The first message format 42 when prepared by NCCU 24 for transmission to 
MMU 22 includes the optional sync field 43, the starting character 44, and 
the two byte packet length 45. The task number used in field 46 is the 
same task number previously assigned to the MMU request for which this 
message is a response. NCCU 24 uses the one byte field 47 used by MMU 22 
to indicate type of message, to indicate success or failure in 
implementing the command in the message received from MMU 22. The data 
field 48 includes the data resulting from the MMU command, e.g., RMU 
status data, followed by the CRC error checking field 49. 
FIG. 3 indicates the second message format 51, which is used for power line 
communications. A message starting preamble 53 includes at least three 
transition changes, required to synchronize communications between carrier 
receivers "listening" to the power line 26. The transitions are followed 
by a predetermined logic level which persists for at least the length of 
time required for 10 message bits. The message following the starting 
preamble 53 consists of a power line prefix 55, 57 and 59 in ASCII, and a 
data field 61 in non-ASCII characters, controlled in ANSI X3.28 format. 
Since each valid character consists of framing bits surrounding a byte of 
information, valid characters cannot be formed in either ASCII or 
non-ASCII which do not have a logic zero in every ten bits of a continuous 
message. The ten continuous bits of a predetermined logic level in the 
starting preamble have a logic level opposite to the periodic logic zero, 
and they are thus at the logic one level. 
The ten logic one bits in the starting preamble 53 must be detected before 
any message will be received. As hereinbefore stated, the formats of the 
ASCII and non-ASCII characters do not allow ten logic ones to be valid 
characters and they cannot occur within a message if the message is 
transmitted continuously. FIG. 4 sets forth the format 65 of a character 
byte. Format 65 has a low start bit, eight data bits BO through B7, and a 
high stop bit. Thus, it is important that in the second message format 
that there be no delay between bytes of a packet which exceeds one-half 
bit, as the line will be held high while awaiting more data. For example, 
if the eight data bit locations are high, the stop bit will make nine high 
bits in a row. A delay of one bit at this point will delay the normal low 
start bit and emulate a valid starting preamble 53. Since the starting 
preamble 53 cannot be transmitted by normal UART operation, the starting 
preamble will not interfere with higher level protocols, as would be the 
case if the starting preamble would require synchronizing on the reception 
of a particular valid asynchronous character. 
The ten logic one bits also have the advantage of flushing the receiver's 
character buffer by allowing it to finish any character reception in 
progress and guarantee no new character will be received until the first 
character of the next valid message 
As hereinbefore stated, the valid starting preamble 53 is followed by 
fields 55, 57 and 59 collectively referred to as the power line prefix. 
Field 55 is used for the one byte message type, as prescribed by the 
message-type byte 47 described relative to the first message format 42 
used between MMU 22 and NCCU 24. A reply message from an RMU will also use 
this same byte character in field 55. 
Field 57 contains an address tag byte which is the same as that provided in 
the first byte of the data field 48 of the first message format 42. As 
hereinbefore stated the address tag is set to an ASCII "A" in 
communications between NCCU 24 and a local power line; and to a unique 
value recognized by power line carrier systems associated with remote 
power lines, when the monitoring system includes a radio link for 
communicating with one or more remote power lines. 
Field 59 consists of ten bytes which define an address field. The address 
placed in field 59, which is the same address contained in the data field 
48 of the first format 42, may be an address unique to a specific RMU, or 
it may be a universal address recognized by all RMUs. A universal address 
is used during system reset, and in the process of initially giving a RMU 
an unique address. When MMU 22 is polling for reefer status data, it will 
insert a specific RMU address. 
The one byte message type 55, the one byte address tag 57, and the ten byte 
RMU address 59, collectively called the power line prefix, are all in 
ASCII characters. 
The next field 61, variable in length, is the data field, and it contains 
data in non-ASCII characters, including control characters in ANSI X3.28 
format. It will contain the specific command or task to be performed by an 
RMU. As hereinbefore stated, FIG. 4 indicates the format 65 of all 
characters, including prefix and data, which format includes a low start 
bit, eight data bits, and a high stop bit. Thus, there will always be at 
least one logic zero, i.e., the start bit, in every ten bit data 
character. The message ends with a two byte CRC error check field 63. 
When a RMU responds to a command from MMU 22, it will prepare a message in 
the second format 51 just described, inserting the data requested by the 
specific MMU command in the data field 61. 
FIG. 5 is a more detailed block diagram of a power line carrier 
communication system 70 which utilizes the same reference numerals as FIG. 
1 where appropriate. NCCU 24 receives messages in the first format 42 and 
translates them to the second format 51. When the message is for RMUs 
connected to the local power line 26, MMU22 will have inserted an ASCII 
"A" into the address tag field 47 of the first format 42, which tells NCCU 
24 that the message is for the local power line 26. NCCU 24 will maintain 
the ASCII "A" in the second format, appearing in field 57, and it will 
apply the message to power line 26. 
When one or more remote power lines, such as remote power line 72, include 
reefers to be monitored, messages in the second format 51 are obtained 
from NCCU 24 and sent by radio link 74 to a remote power line interface. 
Radio link 74 includes a local central control unit (LCCU) 76 which 
includes a radio transmitter/receiver. Each remote power line, such as 
remote power line 72, includes a remote central control unit (RCCU) 78 
which also has a transmitter/receiver. NCCU 24 translates messages 
received from MMU 22 from the first message format 42 to the second 
message format 51. The address tag located in field 48 of the first 
message format 42 an in field 57 of the second message format 51 provides 
a predetermined power line identification, which, as hereinbefore stated, 
will be assumed to be an ASCII "A" for the local power line. Thus, an 
ASCII "B", for example, may be used for messages destined for a first 
remote power line 72, etc. All RMUs, regardless of which power line they 
are connected to will only respond to messages which have an ASCII "A" in 
the address tag 57. If the message received by NCCU 24 from MMU 22 has an 
ASCII "A" in the address tag field 48, NCCU 24 simply applies the message 
to the local power line 26. If it has an identification other than an 
ASCII "A", LCCU 76 transmits the second formatted message by radio to all 
remote power lines. Each RCCU 78 receives the broadcast message, and if 
the address tag bears its unique identifier, the addressed RCCU 78 swaps 
its identifier for an ASCII "A" in the address tag field 57 and applies 
the message to its associated remote power line 72. Responses by the RMUs 
connected to the remote power line 72 are received by RCCU 78 which then 
swaps its identifier for the ASCII "A" in the address tag field 57 and 
broadcasts the message back to the LCCU 76, which directs the message to 
MMU 22 via NCCU 24. Thus, all RMUs, regardless of which power line they 
are connected to will see an ASCII "A" in the address tag field 57, and 
reefers and RMUs may be connected to any power line without modification. 
FIG. 6 is a detailed block diagram illustrating an exemplary embodiment of 
power line communication system 10 shown in FIG. 1. NCCU 24 includes an 
RS-232 transmit/ receive chip 90, such as Maxim's MAX231, an asynchronous 
communications interface adapter (ACIA) chip 92, such as RCA's CDP65C51, a 
microcomputer 94 having an internal UART 95, such as Intel's 80C31, a chip 
select chip 96, an address latch 98, a random access memory (RAM) 100, an 
erasable programmable read-only memory (EPROM) 102, and the hereinbefore 
mentioned carrier current transceiver (CCT) 31, such as National 
Semiconductor's LM1893. The power line coupling means 32 for applying 
messages to, and receiving messages from, power line 26 includes a 
coupling transformer 106 having a winding connected to CCT 31 and a 
winding connected to the three phases and neutral of distribution power 
line 26 via coupling capacitors 108, 110 and 112. 
A message prepared by MMU 22 is sent to RS-232 chip 90 and then to ACIA 92, 
with ACIA 92 being a program controlled interface between microcomputer 94 
and the serial data provided by chip 90. Microcomputer 94 reformats the 
message from the first to the second formats, and the internal UART 95 
sends the message to CCT 31 which FSK modulates the message and applies it 
to power line 26 via the coupling transformer and coupling capacitors 
broadly referenced 32, as they perform function 32 shown in block form in 
FIG. 1. 
IRMU 19 includes coupling capacitors and a coupling transformer, broadly 
referenced 34, as they perform function 34 shown in block form in FIG. 1. 
A CCT chip 36 similar to chip 31, is connected to a microcontroller chip 
114 which includes an internal UART 116. An ACIA chip 118, along with 
optoisolators 120, interface IRMU 19 with refrigeration controller 18. A 
chip select chip 122, an address latch 124, an EPROM 126, and a RAM 128, 
are also included, all of which may be similar to the chips in NCCU 24. In 
addition, IRMU 19 includes a RAM 130 backed by a battery 132. RAM 130 
contains an image of the latest status information provided by controller 
18 for rapid dumping to the power line 26 when requested by MMU 22, and it 
also includes a maintenance record of the reefer 12 it is associated with, 
downloaded by MMU 22. IRMU 19 also includes a nonvolatile RAM 134, which 
contains the unique address of the IRMU 19. 
SRMU 20 does not require a computerized refrigeration monitor and 
controller 18 for data gathering purposes. Functions in SRMU 20 which may 
be the same as just described for IRMU 19 are given like reference 
numerals with a prime mark, and will not be described again. The data 
gathering function 40 shown in FIG. 1 for obtaining data from 
refrigeration unit 16 which is available in digital form is provided by a 
digital I/0 chip 136 which includes input status and an output latch under 
the control of computer 114'. The data gathering function 41 shown in FIG. 
1 for obtaining data from refrigeration unit 16 available in analog form, 
such as data from temperature sensors, includes a multiplexer 138, such as 
RCA's 4051, which is addressed by an address latch 140 which is under the 
control of computer 114'. The output of multiplexer 138 is connected to 
computer 114'via an A/D converter 142. In the SRMU 20 the ACIA function 
118'is connected to a plug-in receptacle 144 via an RS'232 chip 146 for 
optional communication with a hand held data retriever (not shown). 
In addition, SRMU 20 also includes a battery and charger (not shown) so 
that data may be logged from reefer 13 by SRMU 20 without being connected 
to power line 26, and a timer 148 is provided to periodically provide 
signals which initiate the retrieval of status information from 
refrigeration unit 16.