This article introduced the RS-485 main line
theory, the RS-485 main line system the question which exists in the
data communication process, as well as in RS-485 actual project
application.
1st, outline
Along with the digital technology development and the
computer widespread application, now a system often is composed day by
day by many computers, needs to solve the multi- stations, the remote
communication question. When the request signal distance is
several dozens meters to on kilometer, widely uses the RS-485
transceiver. The RS-485 transceiver uses the balance
transmission and the difference receive, therefore has the ability
which the suppression altogether mold disturbs, in addition the
receiver has the high sensitivity, can examine lowly reaches 200mV the
voltage, therefore the transmission signal can obtain the restoration
outside the kilometer. Uses the RS-485 main line, a pair of
twisted pair line can realize the multi- stations networking,
constituted the distributional system, the equipment simple, the price
is inexpensive, can carry on the long distance correspondence the
merit enable it to obtain the widespread application
2nd, RS-485 main line theory
In the automated domain, along with the distributional
control system development, the urgent need one kind of main line can
suit the long-distance range the digital communication. In in
the RS-422 standard foundation, EIA studied one kind to support the
multi- pitch points, the long-distance range and receives the high
sensitivity the RS-485 main line standard.
The RS-485 standard picks the useful balance -like transmission,
the bad fraction receive data transceiver actuates the main line,
concrete specification requirement:
Receiver input resistance RIN >= 12k Omega
The driver can output ±7V altogether mold voltage
Input end electric capacity <= 50pF
In the festival points is 32, has disposed in 120 Omega
terminals resistances situation, the driver at least also can output
voltage 1.5V (terminal resistance size with use twisted pair line
related parameter)
The receiver input sensitivity is 200mV (namely (V+) -
(V-) >= 0.2V, indicates signal "0"; (V+) - (V-) <= -0.2V,
indicates signal "1")
Because of the RS-485 long-distance range, the multi-
pitch points (32) as well as the transmission line cost low
characteristic, enables EIA RS-485 to become in the industrial
application the data transmission first choice standard. Based
on this, RS-485 automated domain application extremely widespread, but
the RS-485 main line utilization still has very many problems in the
actual project, has affected the project quality, brought for the
project construction have been very many is not convenient.
1st, impedance not continuously
Signal in transmission process if meets the impedance to
change suddenly, the signal can cause the reflection in this place,
this kind of signalling principle, enters another kind of medium with
the light from one kind of medium to have to cause the reflection is
similar. Eliminates this kind of reflection the method, is
maintains the transmission line impedance to be continual as far as
possible, in the actual project bridges in the electric cable line
terminal is for reduce the signalling with the electric cable
characteristic impedance similar size terminal resistance principle.
Theoretically analyzes, is transmitting the electric cable
terminal so long as has bridged the terminal resistance which matches
with the electric cable characteristic impedance, can the effective
reduction signalling. But, in realization application, because
transmits application environment and so on electric cable
characteristic impedance and communication baud rate related, the
characteristic impedance is impossible and the terminal resistance
closely coincide, therefore the more or less signalling also can
exist. The signalling logarithm according to the transmission
influence, is in the final analysis because the reflection signal has
triggered the receiver input end comparator, caused the receiver to
receive the wrong signal, caused CRC to verify the mistake or the
entire data frame is wrong. This kind of situation is unable to
change, only has avoids it as far as possible.
2nd, RS-485 earth question
Merely uses a pair of twisted pair line each connection
A, the B end connection to get up, but to the RS-485 correspondence
link signal ground, also may not work in certain situations, but
buried the hidden danger to the system. The RS-485 connection
uses the difference way transmission signal certainly not to need to
examine the signalling system regarding some reference point, only had
to examine two between the potential difference to be allowed.
But should pay attention is the transceiver only has under
altogether the mold voltage does not surpass the certain scope (-7V to
+12V) the condition can the normal work. When altogether the
mold voltage surpasses this scope, can affect the correspondence to be
reliable until the damage connection. Like chart 1 shows, when
transmitter A to receiver B transmission data, the transmitter A
output altogether mold voltage is VOS, because two systems have each
one independent earth system to have terrestrial electricity potential
difference VGPD, then the receiver input end altogether the mold
voltage can achieve VCM=VOS+VGPD. RS-485 standing operating
procedure VOS <= 3V, but VGPD possibly can very large scale have
(several volts even dozens of volts), and the possible partner to have
强干?the signal to cause the receiver altogether mold to input
VCM to surpass normally encircles, lightly produces the interference
current on the holding wire to affect the normal correspondence, heavy
piece of damage equipment.
3rd, RS-485 main line structure and transmitting range
RS-485 supports the half-duplex or entire duplex
pattern. The network analysis situs generally uses the terminal
match the main line structure not to support ring-like or the star
network, best uses a main line to serially connect each pitch point.
From the main line to each pitch point lead-out wire length
should as far as possible short, in order to causes in the lead-out
wire the reflection signal to be lowest to the main line signal
influence. When uses the RS485 connection, passes through
regarding the specific transmission line, from the generator the
biggest electric cable length which permits to the load its data
signalling is the data signal speed function, this length data mainly
is influence and so on signal distortion and noise limits. When
the data signal speed reduces to 90Kbit/S below, when the hypothesis
biggest permission signal loss is 6dBV, then the electric cable length
is limited in 1200M. In fact, when is practical is definitely
may obtain compared to its big electric cable length. When uses
the different line diameter the electric cable. Then obtains the
biggest electric cable length is not same.
3 plans designs
Below explains RS-485 by an example the application:
A university cafeteria has three, some 12 POS machine, each
building has 4 POS machine, they are the RS-485 connection, controls
the POS machine computer to meet the RS-232 connection, the
traditional solution is:
Uses this kind of old style wiring structure to have
following several kind of flaws.
1>, the RS-485 main line the difference signalling
characteristic limit main line length about 1,200 meters, is generally
used this kind of old style wiring structure, but the wiring cover
area extremely is limited (generally is several hundred square
meters). Sometimes in order to open up the transmitting range to
be able not but to use the repeater. Simultaneously also
enlarged in the wiring process to have the problem the possibility.
2>, the general large-scale checking attendance 门禁 system
all is loaded with for the number multitudinous 门禁 controller,
uses this kind of old style wiring structure, all 门禁 controllers
sharing identical root RS-485 main line. When any controller
RS-485 port short-circuits, can affect the entire 门禁 system the
normal work. Searches the breakdown in the multitudinous 门禁
controller to be at, not only has enlarged the work load, also
increased the maintenance cost.
3>, because the RS-485 main line wiring quite is complex, the
major part project expense and the time all wastes in the checking
attendance 门禁 system in the wiring link. But the RS-485 main
line wiring complex degree and the main line length and the equipment
hang meets the number to have the very big relations. Specially
appears in the large-scale system by for is prominent. Used this
kind of old style wiring structure to increase the wiring difficulty
without doubt.
4>, as a result of the geographical environment reason, in is
distanced between the certain distance equipment always to have the
electric potential not balanced problem. Sometimes even if is
away from the very near question still to exist. This kind of
environment creates factor, in old style wiring structure very
difficult comprehensive solution. Even creates the overall
system to be unable to start. Although may temporarily solve the
partial problems through the processing electric potential, but
时隔不久 the similar question can appear once more.
Some flaws appears which in view of above, the trillion
Vietnamese company develops a section new product, it uses unique and
so on the rank difference isolation technology and the highly
effective main line division centralism technology can in the
effective addressing project wiring the common terrestrial electricity
position difference, the impedance matching and is struck by lightning
the question. The user may easily improve the RS-485/RS-422 main
line structure, the division webpage, enhances the correspondence
reliability. When is struck by lightning or the equipment
failure produces, has the problem the webpage to isolate, guarantees
other webpages the normal work. Again solution as follows:
May the effective solution use the first kind of plan
existence using this plan the flaw, it has the following several
advantage:
1>, uses the star structure to connect the RS-485 main line, the
wiring cover area greatly enhances in the effective use connection
situation (generally is several square kilometers).
2>, has eight lower positions machine ports, also each port all
has short-circuits the protection function, and can work is shutting
off the pattern. To the large-scale checking attendance 门禁
system, through shares 门禁 the controller not only to be allowed to
reduce individually the RS-485 main line load to eight ports,
simultaneously effective enhancement overall system reliability.
When any controller RS-485 port short-circuits, only can affect
its in the RS-485 main line system, cannot affect other connection
connections the RS-485 system normal work.
3>, may cause the RS-485 system wiring process to change simple
and quick is clean, thus effective reduced project expense and time.
Between 4>, various ports has the 3000V isolation.
Regarding wiring question which brings by the environment
question, only must carry on the question remarkable region with the
independent port connection centralism processing, will be able the
effective solution electric potential to bring wiring question.
4th, matters needing attention
4.1 very many people often all mistake are the RS-422
serial interface are the RS-485 serial interface entire duplex
editions, in fact, they have many differences in the electric
appliance characteristic, altogether the mold voltage scope and the
receiver input resistance differently cause this two standards to be
suitable for the different application domain. The RS-485 serial
interface driver may use in the RS-422 serial interface application,
because the RS-485 serial interface satisfies all RS-422 serial
interface performance parameter, otherwise cannot establish.
Regarding the RS-485 serial interface driver, altogether the
mold voltage output scope is between -7V and +12V; Regarding the
RS-422 serial interface driver, this performance index only has ±7V.
The RS-422 serial interface receiver smallest input resistance
is 4K Omega; But the RS-485 serial interface receiver smallest
input resistance is 12K Omega.
4.2 RS-485 main line always appears some wiring in the actual
project the question, the attention, when meets the transmission line
certainly must use the same twisted pair line or the same electric
cable, a some people of section of use twisted pair line, because the
twisted pair line length is insufficient or in on center indirect
section of telephone lines or is other lines, such impedance not
continuously, produces the very big reflection signal, the
correspondence is cannot normally carry on.
5th, concluding remark
The RS-485 main line, has high characteristic and so on
noise abatement, width altogether mold scope, long transmitting range,
conflict protection, but also needs to consider the reasonable
application and the network layout, the continual signal channel, the
complete protective measures and so on, should have the overall plan
at the beginning of the design. Along with the optical fiber
communications ofc and the ethernet technology development, the RS-485
main line data transmission distance can be farther.
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