Development of the Super
Link II Communication Systemfor Building Air Conditioners

Air conditioningmanagement system

KOJI YOKOHAMA*1

SATOSHI HIRATA*2

TAKASHI INABA*2

SEIJI HIRAMATSU*2

1

HIROYUKI OTAKE*2

KAZUYOSHI TAKAHASHI*

There is increasing demand for high-capacity indoor and outdoor units for multi-unit air conditioning systems that havereduced space requirements, improved functionality, and reduced equipment costs. In particular, the requirement will grow for
air conditioning systems that consist of one outdoor unit and 100 or more indoor units. The new Super Link II air conditioner
communication system plays an important role in supporting the further development of air conditioning systems. While maintaining
interchangeability and compatibility with previous communication systems, Super Link II has a new automatic polarity and protocol
adjustment algorithm that permits the mixed operation of conventional and new air conditioners with automatic address setting ofup to 128 indoor units.

encountered when new units replace old ones in a buildingon either an individual or a group by group basis.
For c omp at ibi l it y pu r p os es , S up er L i n k I I us es atransceiver that meets the same RS -485 specifications as
Super Link. The communication system, which is developed
according to MHI’s own method and criteria of evaluation,is capable of connecting up to 172 units, composed of 128
indoor units, 32 outdoor units, and 12 central controllers.
Table 1 compares the outline specifications of the new andprevious systems.
Interconnecting a larger number of units requires anincrease in the volume of communications. The central
controller that manages the air conditioning accounting
system checks the status of each indoor unit every minute.
The communications speed has been quadrupled in the newsystem to maintain this one-minute status checking interval
for an increased number of units.
A faster communication speed means that the effect ofreflected waves on the communications waveform is a more
important issue, and communications are impossible if thedistance is too great. Figure 1 shows the communication
waveforms under such conditions. We found that reflectedwaves were caused by the capacitor in the over-voltage
protection circuit. We solved this problem by redesigning
the over-voltage protection circuit, which was provided
for protection in the event that the main power line wasincorrectly connected to a communication terminal.
We performed a number of tests to confirm our predictionsof communications performance under various branchinga nd con ne c t ion s cena r ios . T he syst em is c apable ofcommunicating over wiring runs of up to 2000 m, witha maximum of 1000 m to the furthest end. Due to theirincreased capacitance, shielded cables were restricted
to a maximum run of 1500 m.

1. Introduction
In 1990, Mitsubishi Heavy Industries, Ltd. (MHI) developedand brought to market the Super Link communication
system for controlling air conditioners used in stores andof f ice buildings. T he system’s installation workabilit y,expandabilit y, and reliabilit y were of such a high levelthat it has been used as the communication system for all
new multi-unit air conditioner models developed since its
introduction.
There is increasing demand for high-capacity indoor
a nd outdoor units w it h reduced space requi rements,
improved functionality, and reduced life-cycle costs. In
particular, the requirement will grow for air conditioning
systems that consist of one outdoor unit and 100 or moreindoor units. Such large air conditioning systems require
a communication system capable of interconnecting 100 ormore air conditioning units.
This paper describes the features and technology of theSuper Link II air conditioner communication system used in
MHI’s multi-unit air conditioners.

2. Communication circuit configuration
The Super Link communication system used in MHI’sconvent ional mult i-unit a ir condit ioner systems has acommunication transceiver with excellent resistance to noisy
environment and reliability that meets the RS-485 standards
for multi-node, long-distance communication. In addition,
previous Super Link uses MHI’s unique communication
circuit configuration and polarity adjustment algorithm that
depolarizes its two communication lines1 to reduce problems
with incorrect connections during installation.
The primary goal of this development was to enable themixed operation of new and previous systems that would be

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 2 (Jun. 2008)*1 Nagoya Research & Development Center, Technical Headquarters*2 Air-Conditioning & Refrigeration Systems Headquarters

Table 1 Specifications of new and conventional communication systems
Specification

ItemRemarks

Super Link II

Super Link

Wiring medium

2-conductor cable

2-conductor cable

Number of devices

172 (maximum)

100 (maximum)

Maximum cable
length2000 m (unshielded cable)
1500 m (shielded cable, 0.75 mm2)1000 m (shielded cable, 1.25 mm2)1000 mBaud rate

38400 bps

9600 bps

4 x as fast as conventional system

Interface

RS-485

RS-485

EIA standard

Access control

CSMA/CD

CSMA/CD

Transmitted waveform

TxD +Received waveform

RxD +Vinyl cabtire cable: 0.75–2.0 mm2
Shielded cable: 0.75 mm2 or 1.25 mm2

NGWaveformon transmission lines

Line-Vd

Effect of reflected waves
24.6 µs (98%)

Fig. 1 Distortion of received waveforms due to reflections on transmission channels
The branching pattern of transmission channels and the number of units connected makes the effect of the reflectiona serious problem, distorting the received waveforms to the point of disrupting normal communication.

F

200 m

200 m

Terminal blockfor connecting
communication lines

N : 32

200 m

200 m

Communicationtransceiver

BCD

E

N:1

N : 32

N : 32

N : 32

N : 32

DE1
DE2
CTRL

200 m

A

Microcomputer

Overvoltage
protection circuit

TXD

RXD
Input signal
selection circuit

Fig. 2 Example of branching patterns where N is the number ofunits connected
A is a central controller, B–E are indoor units, and F is an outdoor
unit. Communication cables 200 m in length are connected toblocks A–F.

Fig. 3 Communication circuit configuration
Two communication transceivers enable this configuration tocommunicate in positive as well as negative polarity.

Figure 2 shows an example of t y pical connect ionsand Fig. 3 shows the conf iguration of a Super Link II
communicat ion circuit w ith t wo t ransceivers and one
received signal selection circuit. With this arrangement, thecircuit detects the polarity of the communication network
regardless of which way it is connected, and selects theappropriate polarit y for the transmitting and receiving
circuits to enable intercommunication.
Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 2 (Jun. 2008)3. Communication procedure

4. Automatic address-setting function

3.1 Media access control (CSMA/CD)
Super Link uses MHI’s own protocol, which is a typeof carrier sense multiple access with collision detection(CSMA/CD) system.
It is a bus c ont ent ion s y st em where t wo or morecommunication terminals share one transmission line,
and when necessar y, the terminals compete to use thetransmission line. It is also a “1-persistent” access control
in that when one terminal is about to start communication
it checks the transmission line to see if it is already in use,
in which case it continues to wait until the line is free before
using it.
Due to the increased number of expected data collisions
caused by the significant increase in the possible number ofdevices, Super Link II uses a “non-persistent” access control
where, if the transmission line is being used, the transmitter
waits for a random time before retrying the transmission.
Fur ther more, the average leng th of this random wa itinterval has been adjusted to prevent the increased numberof units (up to 128 indoor units for Super Link II instead of
48 for previous Super Link) from causing a deteriorationin the responsibilit y and line availabilit y factor of thecommunications.
3.2 Automatic polarity and protocol adjustment
algorithm

Each controller must have its own address when one
net work covers t wo or more controllers. Multi-unit air
conditioners for buildings have indoor units installed inliving spaces, and outdoor units installed outdoors; these areinterconnected by refrigerant piping. The indoor and outdoor
units communicate with each other to produce the intended
air conditioning performance, and each unit must know theother’s address for this to happen.
T he addressing switches mounted on the controller
are not easily accessible in many cases as the indoor units
in a building are built into the ceiling or the ceiling void.
Therefore, previous Super Link-compliant products have anaddress-assignment function 2 that automatically assigns anaddress to each indoor unit. The function was expanded from48 units to a maximum of 128 in Super Link II.
Pairing the indoor and outdoor units for each refrigerantsystem is important when one network covers two or morerefrigerant systems that managed by a central controller asshown in Fig. 4.
The pairing of the indoor and outdoor units connectedby the refrigerant piping must match the pairing of theindoor and outdoor units connected by the communication
network configured by the crossover wiring. A mismatch
between the pairs linked by the refrigerant piping and by
the communication network can cause accidents, such asmechanical failures or water leaks. MHI has developed andbrought to market an automatic address setting function
for two or more refrigerant systems to avoid such pairingproblems.
The following subsections describe the technologies
used in the automatic address-setting function for multiple
refrigerant systems.

It is important to ensure interoperability of old and newunits when they are being replaced. In a building with ancentrally controlled system in particular, the indoor units
are usually replaced progressively in phases rather than all
at once. When this happens, both the new units and the oldunits must be connected simultaneously to the centralized
control system so that the management of energy saving andair conditioning fee will continue without interruption.
The new auto polarity and protocol adjustment algorithm
allows the previous Super Link system to operate if it detects
at least one older unit when the system is turned on.
T he prev ious Super L i nk auto pola r it y adjust ment
algorithm 1 determines the polarity of the communication
circuit. I f there is already one air conditioner with anestablished polarity on the network, than any newly installed
air conditioners adopt this same polarity. The Super Link II
auto polarity and protocol adjustment algorithm is capableof automatically switching between the previous and new
protocols by adding the previous/new data to the data for
determining polarity.
As the new algorithm gives precedence to the previous
protocol in detection, the most time-consuming configuration
would be with 128 indoor units and 32 outdoor units allof which meet the new specif ications. In this extreme
configuration with all units turned on simultaneously, it
would take about 20 s to switch all the units to the newprotocol.

Outdoor unit

Refrigerant
pipingCentralcontroller

Communicationline
Indoorunit

Indoorunit

Indoorunit

Indoorunit

Indoorunit

Indoorunit

Indoorunit

Indoorunit

Indoorunit

Indoorunit

Indoorunit

Indoorunit
Centralcontroller

Fig. 4 Refrigerant piping and communication lines
The pairing of indoor and outdoor units connected by the refrigerant
piping must match the pairing of the indoor and outdoor units
connected by communication lines.

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 2 (Jun. 2008)4.1 Network connector
Automatically and electrically establishing the correct
pairing of indoor and outdoor units for each refrigerantsystem requires isolating the communication network foreach refrigerant system and having the units within eachsingle network segment communicate on the network sothey recognize the presence of the others. A communicationline connector called a network connector is used to isolate
the communication lines conveniently. Figure 5 shows aconceptual connection diagram for connecting two or morerefrigerant systems for communications purposes. Figure 6
is a schematic diagram of the network connector.
The communication lines and the refrigerant piping ina multi-unit air conditioning system usually run parallel
through the building, and are covered by insulation as they
connect with rooftop outdoor units. Network connectors
are used to isolate the communication network for eachrefrigerant system. Figure 7 shows the procedure for
automatically setting the addresses in a system with two or
more refrigerant systems.
4.2 Procedure for setting tentative addresses
As the address information determined through theautomat ic address set t ing procedure is stored in t henonvolatile memory of the indoor control circuit board, it
remains intact even if the power supply is cut off as would
happen in a power blackout. This address information is
not present when the unit is shipped from the factory. Thesystem follows a special procedure for setting tentative
indoor unit addresses when the power is turned on for thefirst time. This particular tentative address setting function
has been expanded to handle a maximum of 128 indoor units
in Super Link II, and expansion from the previous limit of 48.

Communication line
(crossing over multiple refrigerant systems)

Indoorunit

Indoorunit

Indoorunit

Indoorunit

Indoorunit

Indoorunit

Indoorunit

Indoorunit

Indoorunit

Indoorunit

Indoorunit

Centralcontroller

Control circuit board
Network connector

Other systems In/out

Centralcontroller

Control box schematicFig. 5 Configuration of system with automatic address setting for
multiple refrigerant systems
The communication line runs parallel to its associated refrigerant system.
The outdoor units are wired in crossover mode to connect two or morerefrigerant systems. The network connectors isolate the communicationlines for each refrigerant system.

Setting procedure
Set outdoor unit
address manually

Disconnect network
connectors

Fig. 6 Schematic diagram of network connector layout
Terminal blocks are provided for separate connection of units
associated with a particular refrigerant system and for crossover
electrical wiring with other refrigerant systems.

Network connector isolates Network
according to refrigerant system

Input the starting address
and number of units
connected into outdoor unit

Permanent addresses
for indoor units will be set
Pairing of indoor and outdoor units
will be established

Turn on power to indoor
and outdoor units
Connect network connectors
Tentative addresses
for indoor units
will be determined00

01

Network will be established

31

11/01

121/31

01/0012/01

122/31

13/01

123/31

20/01

127/31

02/00
Carry out “finalization”
operation from any outdoor unit

Connect network
connector after pairing

Communication
line
00/00

10/00

Refrigerant piping

Refrigerantpiping

Network
connector

Communicationline(in/out)
Indoorunit

Addresses for indoor units
will be set automatically

Outdoor unit’s address
will be obtained automaticallyFig. 7 Procedure for automatic address setting of multiple refrigerant systems

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 2 (Jun. 2008)4.3 Procedure for setting permanent addresses
The tentative address numbers are assigned in ascendingorder starting from 000 to 127 for each refrigerant system.
MHI has developed an algorithm that works as follows. The
outdoor unit is given the number of indoor units connected
and the starting address number. Based on the tentativeaddresses, the permanent addresses for the indoor units areautomatically determined from the starting address number
and the number of units connected.
4.4 Pairing indoor and outdoor units
W hen the indoor units obt ain the st ar ting address
number from the outdoor unit, they simultaneously obtainthe outdoor unit address number. The outdoor unit obtains
the address numbers of two or more indoor units using theprocedure for requesting registration of the permanent
address numbers from the indoor units. This series ofprocedures establishes the pairing of the indoor and outdoor
units sharing the same refrigerant system.
4.5 Matching polarity
Finally network connectors connect all the refrigerantsystems to one communicat ion net work. However, as
the net work connectors isolate the dif ferent segments
dur ing st a r t up, the communicat ion pola r it y that wasestablished independently for any refrigerant system does
not necessarily agree with the polarity established for theothers. The system is designed such that a “finalization”
process takes place on the outdoor unit of one selectedrefrigerant system so that the devices associated with otherrefrigerant systems adopt the communication polarity of theselected system.
4.6 Automatic address setting time
Testing showed that automatic address setting took 1 min
to obtain tentative addresses for the maximum of 128 indoor
units, and less than 5 min from startup to complete thepermanent address number assignment of 128 indoor unitsand one outdoor unit simultaneously.

5. Conclusion
MHI is progressively expanding its range of multi-unit air
conditioner models for buildings and centralized controllers
equipped with Super Link II. MHI expects to see an increasein business for new and replacement installations given theincreased expandability provided by the improved Super
Link II communication system described here.
M H I is com m it ted to developi ng i nnovat ive a i r
conditioning products to satisf y the diversified market
needs of its customers. These products not only provide thesophisticated functions that customers demand, but they arealso easy and quick to install, and reduce installation errors.
References
1. Japan Patent No. 2566323, US Patent No. 5257160, European
Patent No. EP0451276B1
2. Japan Patent No. 3169959

Koji Yokohama

Satoshi Hirata

Takashi Inaba

Seiji Hiramatsu

Kazuyoshi Takahashi

Hiroyuki Otake

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 2 (Jun. 2008)10