Note: Descriptions are shown in the official language in which they were submitted.
CA 02211236 1997-07-23
APPARATUS AND METHOD FOR CONNECTING
COMMUNICATIONS l~l~;l'WORRS
Field of the Invention
This invention relates to a method for interfacing between local-area
nelwofk and a radio communications system. The invention is applicable to,
Electronic Industries Association recomm~n~led st~n~l~rd RS-485 and data
radio communications systems.
Background of the Invention
An Electronic Industries Association (EIA) recommen(led st~n(l~rd RS-
485 is a standard for electrical characteristics of generators and receivel~ foruse in balanced digital multipoint systems. Systems that operate within the
guideline of the RS-485 st?m~l~rd include multiple generators and receiv~ls
that may be attached to a common interconnecting two wire cable. A local-
area network that meets the RS-485 standard is relatively str~ r~ rd to
~le.Q,i~n The data si~n~llin~ rate within the t~n~rd may be up to 10
megabits/second, and the distance between generators or rec~ivels, e.g. data
termin~ , may be up to 340 meters. The termin~l~ are connected in parallel
with one pair of wires there being no control lines.
RS232 is a standard for connecting and controlling via protocols digital
equipment such as termin~l~ and co ~uler to another digital eqllipment via
modems. In the RS232 standard control lines for controlling the data flow are
provided. The control lines are: Request To Send (RTS), Clear To Send (CTS)
Carrier Detected (CD), Data Set Ready (DSR) and Data Terminal Ready
(DTS). RTS and CTS control lines are needed when, for ~mple, a terminal is
requested from a main col~ ler to send data. The terminal is connected to a
modem and sends RTS comm~n-l to a modem. The modem starts to
synchronise with the modem connected to the main co~ er. When the
modems establish communication the modem that is connected to terminal
sends a CTS command to the terminal and then the terminal starts to
transmit data to the main computer.
Digital equipment that needs to control data flow cannot be connected
to RS485 networks because control lines are not provided. The,~rofe,
connecting a RS-485 network to another network is not possible.
, CA 02211236 1997-07-23
This invention seeks to provide a method and apparatus for in~rf~çinE
between RS485 local-area network to other networks which mitigates the
above mentioned disadvantages.
5 Sllmm~qrY of the Invention
According to the to a first aspect of the invention there is provided an
apparatus for connecting via a communication system, having at least one
communication device, an RS485 local-area network to at least one other
10 network. The apparatus includes: a modem for transmitting and receiving
data messages and an interface for interfacing the modem to the
communication system. The interface includes: a first input and a first
output for, in use coupling the interface to the modem, a second input and
second output for coupling the interface to the communication device, a
15 means to generate a request to send mess~Ee to the modem and a means to
simulate a clear to send message for the modem.
By use of the present invention an RS485 network can be connected to
an other network via a communication system such as radio communication
system, telephony communication system, infra red communication system
20 or other communication system because the clear to send comm~n-l that is
required for modem con~ ication is ~imlll~ted.
Preferably, the interface indudes an input driver having an input and
an output, and an output driver having an input and an output, wherein the
input of the input driver and the output of the output driver are coupled to an
25 input of the RS485 local-area network, a switch operably coupled to the
output of the output driver and to the input of the input driver having an
output operably coupled to a control unit and an input and a control line
operably coupled to the modem and the control unit for generating a request to
send message and to simulate a clear to send meSs~Ee which control unit
30 having a first output for controlling the communication device and a second
output operably coupled to the modem for sending data messages.
In this manner, the means to generate a request to send me~s~qEe and
the means to simulate a clear to send message includes an input buffer having
an input for receiving data messages from the RS485 local-area network, a
35 stack for storing the received messages, a delay unit for delaying the
transferring of the data messages for a certain period of time and an output
buffer for transferring the data messages to the modem.
CA 02211236 1997-07-23
In the preferred embodiment of the invention the coll~ ic~tion
system is a radio communication system and the commlmic~tion device is a
radio transmitter.
In the preferred embodiment of the invention the commlmic~tion
5 system is a telephony communication system having telephone lines and the
communication device is a telephone.
In the preferred embodiment of the invention the co~....ic~tion
system is a telephony communication system having telephone lines and the
communication device is a dial-up modem.
In a second aspect of the present invention a method for connecting via
a communication system, having at least one communication device, an
RS485 local-area network to at least one other n~wolh is provided. The
RS485 network includes an apparatus having an interface unit and a modem.
The method of connecting includes the steps of: monitoring the RS485 local-
15 area network for mess~ges destined for the at least one other ne~wo.l~,
simlll~ting a clear to send message to the modem and senlling via the
communication system a data mess~q~es from one of the RS485 local-area
network to the at least one other netwolh.
In this manner, the step of simul~ting a clear to send mess~e includes
20 the steps of: generating a request to send (RTS) commAn-l for the modem and
delaying the data messages for a constant period of time.
Preferably, the interface lmit includes means for moni~rinF and
controlling an operating mode of the communication device and the step of
sen~ling via communication system data messages further includes the steps
25 of: sending to the communication device a connect comm~ntl for l~h:lnging theoperating mode of the collllll~lication device to a connect mode, starting a
time delay and sending data mess~ges at an end of the time delay to the
network.
In the preferred embodiment of the invention the co.~ . . .ic~tinn
30 system is a radio communication system and the communication device is a
radio transmitter.
In the preferred embodiment ofthe invention the co~....ic~tiQn
system is a telephony communication system having telephone lines and the
communication device is a telephone
A preferred embo-lim~nt of the ~llvelllion will now be described by way
of example only, with reference to the drawing.
Brief Description of the Drawings
CA 02211236 1997-07-23
FIG. 1 is a block diagram of a radio co.. ~ ication system connected
to local-area network according to a preferred embodiment of the invention;
FIG. 2 is a block diagram of a modem having an electronic circuit for
interpreting binary ~ign~l~ to a radio transceiver controls accoldillg to a
preferred embodiment of the invention;
FIG. 3 is a flow chart showing a method for interfacing between a local-
area network and a radio co~'-cations system accordillg to a preferred
embodiment of the invention; and
FIG. 4 is a timing diagram of data m~~.s~Fes tr~n~mitted between a
transmitting side of a first local area network to the receiving side of a second
local area network in a radio co~ ~ication system accor~ g to a preferred
embodiment of the invention.
Detailed Description of the Drawin~s
Referring firstly to FIG. 1, a co~ l~ic~tion~ system 1 comprises a
plurality of sub-nelwolks linked together. The system includes local-area
nelwolks 11 and 21 and a point to multipoint data radio co~ ;c~tion~
system comprising radio stations 12, 19 and 18.
Each local-area network 11, 21 includes colll~uler termin~l~ 10, 23 and
printers 22, the interaction between which is governed by a LAN protocol
such as AppleTalk and Ethernet. The local-area nelwolh~ 11, 21 are also
connected to a respective one of the radio stations 12, 19 and 18.
The radio stations are nominally i-lçntic~l and therefore, only radio
station 12 will be described in detail. It can be seen from FIG. 1 that radio
station 12 includes an apparatus 13 which includes a modem 15 by means of
which the radio station 12 co~unicates with the local-area netw~l~ 11 and
an interface 14 the function of which will be later described. The radio station12 also includes a radio transmitter 16 connected to the modem 15 and an
~ntenn~ The radio station 19 also includes a radio transmitter 20 and an
apparatus 26 which includes a modem 24 and an interface 25.
Having given a general system overview, the apparatus 13 will now be
described in greater detail with reference to FIG. 2.
Apparatus 13 has a two wire input 33 which connects to a port in the
local-area network 11. The two wire input is connected to an input driver 34.
The input driver 34 is connected to a switch 36. An output from the switch 36
is connected to an input of an input buffer 46 of a control unit 32.
CA 02211236 1997-07-23
The input buffer 46 has three outputs. The first output is a buffer full
flag 53 connected to a comm~n-l generator 41 and to a delay unit 42. The
second output is a buffer empty flag 52 connected to the comm~n~ generator
41 and the third output is connected to a stack 44. An output 47 from the
command generator 41 is input to the radio transmitter 16.
An output from the delay unit 42 is connected to a switch 43. The
switch 43 makes and breaks a connection between two units 45 and 40.
Buffer 46 is connected to the stack 44 which in turn is connected to an
output buffer 45. The output buffer 45 is in turn connected to a modem
10 transmitting unit 40 the connection being made and broken by the switch 43
described earlier.
The modem tran~milLillg unit 40 is connected by connection 48 to the
radio transmitter 16.
The radio transmitter 16 has an output 49 which is a ~h~nnel monit~r
15 (CM) comm~ntl line. This is connected to a modem feceivel- 39. The modem
receiver 39 has two outputs 37 and 38. Output 38 is connected to the switch
36 and provides a control signal path allowing the modem ~eceivel 39 to
control the switch 36 in a m~nner to be later described. Output 37 is
connected to an output driver 35 via the switch 36. The switch 36 ~revellts
20 the modem 15 receiving and tran~ illg meS~s~ges at the same time.
The way in which messages are transmitted from local-area nelw~lL
11 to the other networks will now be described with reference to FIG. 4.
A tran~ illillg side 200 timing diagram includes several graphs
showing activities taking place from the moment that a stream of data
25 mess~ges 202 is received on the two line input 33. The tran~ ling side
timing diagram includes the followings graphs: a starting of tr~nRmi~ion
graph 201, an input buffer graph 203, a push to talk (PTT) graph 204, a delay
unit graph 205, a stack status graph 207 and a transmitted audio from the
modem to the radio transceiver graph 211.
A receiving side timing diagram 220 shows the activities taking place
from the moment that a stream of data messages 202 is received on the
.
recelvmg slde.
The receiving side timing diagram includes the followings graphs: a
channel monitor (CM) graph 221, a receiving audio graph 224, a ca~Tier detect
35 (CD) status graph 226, a detected data levels graph 228 and a detected data
in RS485 level graph 230.
Having given a general overview on the timing diagram, the system
operation will now described in details with references to FIGs. 1,2 and 4.
CA 02211236 1997-07-23
Firstly, the tran~millillg side 200 will be described. A stre~m of
messAges 202, as it is shown in graph 201, destined for n~lwolk 21 for
example, will be transmitted by the radio station 12. The meSsA~e stream
202 is received on the two line input 33 and is passed by the input driver 34
and switch 36 to input buffer 46. The input buffer 46 has a capacity to store
one message only. In essence the operation that is described above is that the
CO.l~p~ ~er terminal 10 sends a transmit data (TD) commAntl to the modem 15
in the sense that the modem 15 receives a stream of data 202 that is to be
transm~tted.
When the input buffer 46 is full, the buffer full flag 53 is turned to "on"
state, as it is shown in graph 203. The delay unit 42 is responsive to the
buffer full flag 53, after one mesqAge, to open the switch 43, as shown in graph205. The commAn(l.q generator 41 is also responsive to the buffer full flag 53
to generate a push-to-talk (PTT) comm~n(l to the radio-transceiver 16, as
15 shown in graph 204, and the radio transceiver then starts to trans_it to
establish communication with the radio station. The point marked ~' on
graph 205 is a generation of a request to send c-mmAn-l (RTS). In modem
terms RTS commAn-l was received by the modem 15.
The modem 15 starts to send a ll ail~illg sequence 209 to synchronise
20 the modem in the network 21, as shown in graph 211. The delay unit 42 then
counts for ~prox ,mately 10 seconds 206, as is shown in graph 205, and the
mes~Ages 208 are loaded from the buffer 46 to the stack 44 as shown in graph
207. From the stack 44 the messages are passed to the output buffer 45.
When the delay unit 42 stops counting the switch 43 is then closed and the
25 messAges 210 are passed to the modem transmitter 40, as is shown in graph
211. The point marked "Z" on graph 205 is the ~imlll~tion of a clear to send
command (CTS). In modem terms a CTS message was initiated, showing
that the modem 15 is ready to transmit data messages. The modem 15
starts to transmit the messages 210 as shown in graph 211. The modem
30 operation described in detail above involves a "mimicl~ng" of normal modem
RTS - CTS operation in which a sending modem sends an RTS messA~e and
then awaits a CTS message from the remote modem.
The modem transmitter 40 then transmits the messAges to the
transmitter 16 from whence they are transmitted to the radio station 19.
When the last message has been transmitted, the buffer empty flag 52
turns to "on" state and the buffer full flag will turn to "off" state at point
dotted X on graph 203. The delay unit 42 will open the switch 43 after a
certain delay, for example the time it would take 1.5 characters to enter the
CA 02211236 1997-07-23
buffer, and then break the connection between the output buffer 45 and the
modem transmitter 40. The commAnd generator 41 then stops sçn(ling a PTT
command to the transceiver 16.
The way in which the receiving side 220 will h~nt~le the transmitted
5 messages will now be described.
When the transceiver 16 starts to receive the training sequence 209
from the modem of the radio station 12, as shown in graph 224, on the output
49, the modem channel monitor (CM) comm:~ntl line is turned to "on" state as
shown in graph 221. The modem 24 starts to synchronise with the modem 15
by recovery of the clock 226 reference of modem 15 from the ~lail~llg
sequence 209 and by setting is own clock to match modem 15 clock. The
modem 24 then turns the CD commAntl to "on" state as shown in graph 226,
which indicates that the modem 24 is ready to receive data. The received
m~s~Age 223 is than passed through output 50 to the modem receiver 39.
15 The switch 36 is responsive to the voltage level on output 38 to allow the
modem 15 to transfer the received messages to the ne~wolh and to prevent
any new messages from the network being transmitted by the modem. The
modem receiver 39 detects the received meSs2ges as shown in graph 228.
Detected mess~qEes 227, as shown in graph 228, are passed by the modem
20 receiver 39 and via the switch 36 to the output driver 35. From the output
driver 35 they are placed onto the network via the wires 33 as is sho~,vn in
graph 230.
The local-area network according to the preferred embo-limçnt of the
invention is typically a network that is based on the Open System
25 Interconnection (OSI) model. The OSI model has seven layers. The first layer
is the physical layer wherein an electrical and a mechAnisAl connection of the
data device is defined. RS485 is one of the standards for those layers. The
others layers, that is to say a link layer, a network layer, a transport layer, a
session layer, a presentation layer and an application layer will not be
30 discussed here.
Referring now to FIG.3, a flow chart showing a method for int~ in~
between a local-area network and a radio communications system according
to a preferred embodiment of the invention is shown.
The method starts with initialisation of the apparatus 13, step 100, and
35 waiting for receiving data messAges from the local-area nelwolk in step 101.
When data messages from the RS485 local-area nelwo,h are received
by the apparatus 13 the input driver 34 converts the data messAges to digital
CA 02211236 1997-07-23
sign~ls, step 102 and control comm~ntls are created for the apparatus 13 and
the radio transmitter 16.
The process of creation includes the steps of waiting f~r the input
buffer 46 of the control unit 32 to be full, as in step 103; checking the radio
5 transmitter operational mode as in step 104; and generating RTS comm~nd
for the modem 15, as in step 105.
In response to a RTS comm~nd the delay unit 42 starts a time delay,
step 106, and the radio transmitter 16 push to talk (PTT) comm~nll is issued,
sçn-ling to the radio transmitter 16 a PTT comm~nfl for ~-h~nging the
operating mode of the radio trans_itter 16 to transmit mode, step 106.
In a next step, 107, the modem 15 sends a llail~illg symbols to the
modem 24 and simulates the CTS comm~n-l, step 109. The delay unit 42
stops the time delay after a constant period of time, for example 10 milli-
seconds, step 110.
If the input buffer 46 is not empty, as in step 111, the data mçs~q~ges
are moved to the stack 44 and from the stack 44 to the output buffer 45, as in
step 112. A next step, step 113 involves sending over the air of the data
messages at the end of the constant period of time delay.
If the input buffer 46 is empty, for e~mrle for more than the time that
20 it would take 1.5 character to be received by the modem 15, as in step 111,
the modem 15 stops the radio transceiver 16 tr~n~ g by turning the PIT
off, as in step 114. If the input buffer is not empty then the steps 111 to 113
are repeated.
The method of interfacing enables the connection via two wires or four
25 wires digital equipment to a radio modem co-n-n----ic~1;on system and to
control the receiving and the transmitting of a data meSs~es without the use
of an extra control lines.
In alternative embodiments of the invention the communication
system can be a telephony system, a micro-wave system, a fiber-optic
30 system or satellite system. The way that the modem establishes
communication with the other modem is different from system to system but
the way of "mimicking" the RTS and the C~TS is the same in all of the
systems. For example in telephony system after RTS comm~nd is initiated
by the modem, the modem than dials to make connection to the other modem
35 using a telephone or a dial-up modem.
