Although the more modernized substitution
technology increases day by day, but the RS-485 technology still was
maintaining the mainstay status in the innumerable correspondences
network. Below is the inspection common breakdown and the
establishment quite troublesome RS-485 network 8 steps of methods.
1. RS-485 uses a for non- balanced difference signal, this meant
in the network each equipment all must connect through a signal
channel, by smallest data line in noise. The data transmission
medium is composed by a pair of twisted pair line, should add on the
shield level in a noise bigger environment.
2. in the overwhelming majority RS-485 network, the question
which the terminal pitch point causes can solve it must be more than.
Which pitch point in order to inspect to stop the work, needed
to shut off each pitch point the power source and separates it from
the network. Uses the ohm meter to survey receiving end A and B
or + and - the between resistance value. The breakdown pitch
point reading usually is smaller than 200 ohms, but the non- breakdown
pitch point reading will be able to be much bigger than 4,,000 ohms.
Is 3. which line A, which line is B continuously all is not very
clear. The different manufacturer uses the different label to
stipulate, even if the B line should forever be under the idle
condition voltage higher that. Therefore, the A line is equal to
-, the B line is equal to +. May examine under the network idle
condition with the voltmeter. If the B line has not been higher
than the A line voltage, then can have the connection problem.
4. when does not have the equipment to carry on the
transmission, all equipment all are at the monitor condition time, in
the RS-485 network can have 三 the condition. This will
cause all drivers to enter Gao Zutai, will cause the hanging condition
to pass on all RS-485 receiving end. The pitch point designer in
order to overcome this non-steady state model the method is:
Installs under receiving end A and the B line pulls pulls the
resistance with on to simulate the idle condition. In order to
inspect this bias, should surveys the B line under the network power
supply and the idle condition to the A line voltage. In order to
guarantee is far away does not decide the condition which like in the
chart shows, the request at least has 300mV the voltage. If has
not installed the terminal resistance, the bias request is extremely
loose.
5.1 twisted pair lines add the place the RS-485 network to be
allowed on the line to transmit the data with the under good place.
Because does not have two transmitting ends to be able in the
identical time successfully communication, therefore last the
transmission finished after in the data time internally, the network
performance for is idle condition, but the pitch point has not caused
its driver to enter 三 the condition in fact. If another
equipment attempts in this in the compartment to carry on the
communication for a while, will be able to have the conflict which the
result might not be forecast. In order to examine this kind of
conflict, the use digital oscilloscope catches several bytes 1 and 0.
Determined a pitch point when end of transmission enters time
which 三 the condition needs. Guarantees RS-485 software not
to attempt to respond (slightly is bigger than 1ms compared to a byte
time shorter request under the 76.8kb/s speed).
6. each kind of reliable center long distance networking
technology all has some kind of form in to set at the isolation,
besides RS-485. It must depend on the system designer to
guarantee the network not including any earth return. Will
isolate each pitch point by the magnitude degree increase network
reliability.
7. although the isolation is resists the power source surge the
together defense line, but increases the multistage surge blanker to
be allowed to slacken the bigger surge disturbance, guaranteed they
are in the scope which the network isolation may tolerate.
Should better be has the high performance 接地?position in
the network to install the surge blanker. Connects in the
identical spot it the earth, likes other network equipments or the
factory electrical system is same.
Under 8. once the RS-485 network establishment and the movement,
should record its disposition each detail. Including terminal
information, bias, linear and spare parts information. If may
bear results in, should purchase some spare parts and deposit in the
cabinet. Because PC machine tacitly approves only has the RS232
connection, some two methods may obtain on PC the position machine
RS485 electric circuit:
(1) the string mouth RS232 signal transforms through the
RS232/RS485 switching circuit PC machine the RS485 signal, should
better be selects regarding the situation quite complex industry
environment guards against the surge belt isolation ?product.
(2) through the PCI multi- strings mouth card, may directly
select the output signal is the RS485 type expansion card. RS485
connection
RS485 uses the difference signal negative logic, + 2V ~
+ 6V expressed "0", - 6V ~ - 2V expresses "1". RS485 has two
systems and 四线?two kind of wiring, 四线?only can realize
the point-to-point mailing address, presently very little uses, now
uses is two systems wiring way, this wiring way develops the plain
structure for the main line type to be most on the identical main
line may hang meets 32 points. Generally uses in the RS485
correspondence network is advocates from the mailing address, namely
main engine belt many from machine. In very many situations,
connects when the RS-485 correspondence link only is simply with a
pair of twisted pair line each connection "A", "B" the end connection
gets up. But has neglected the signal place connection, this
connection method in many situations is can the normal work, but
actually buried the very big hidden danger, this had two reasons:
(1) altogether mold disturbance question: The RS-485
connection uses the difference way transmission single mode, certainly
did not need a relative Yu reference point to examine the signal, the
system only must examine two between the potential difference to be
allowed. But the people often neglected the transceiver to have
the certain altogether mold voltage scope, the RS-485 transceiver
altogether mold voltage scope are -7 ~ 12V, only had satisfies the
above condition, the entire network could the normal work. When
the network line Chinese Communist Party mold voltage surpasses this
scope can stably affect the correspondence to be reliable, even
damages the connection.
(2) EMI question: In the transmission driver output signal
altogether mold partial need returns to the circuit, if does not have
a low resistivity to return to the channel (signal), can radiate the
form returns to the source end, the entire main line can like a giant
antenna to the external radiation electromagnetic wave.
In the RS485 network process another need idea question is the
terminator resistance question, will not add the terminator resistance
entire network in the equipment few distances short situation to be
able the very good work but to reduce along with the distance increase
performance. Theoretically, carries on the sampling when each
receive data signal center point, so long as reflection signal when
starts the sampling weakens to enough is low may not consider the
match. But this in with difficulty grasps in fact, American
MAXIM Corporation has an article to mention an experience principle
may use for judges when any type data rate and the electric cable
length to need to carry on the match: When the signal switching
time (rise or fall time) surpass the electrical signal to need the
time along the main line unidirectional transmission above 3 times may
not match.
The general terminal match uses the terminal resistance method,
RS-485 should all connects in parallel fashion the terminal resistance
in the main line electric cable start and the terminal. 终接
the resistance takes 120 in the RS-485 network Omega. Is equal
to the electric cable characteristic impedance resistance, because of
majority twisted pair line electric cable characteristic impedance
approximately in 100 ~ 120 Omega. This match method simple is
effective, but has a shortcoming, the match resistance must consume
compares the high efficiency, not too suits regarding the power loss
limit quite strict system. Moreover one comparison province
electricity match way is the RC match. May save the majority of
powers using an electric capacity C room partition direct component.
But the electric capacity C value is a difficulty, needs carries
on compromised in the power loss and the match quality. Also
some one kind uses the diode the match method, this kind of plan
although realization true "match", but it can rapidly weaken the
reflection signal using the diode clamping action, achieved the
improvement signal quality the goal, the energy conservation effect is
remarkable.
Recently two years some companies have completed based on the
partial enterprises information implementation, in the factory already
laid down has extended to workshop each office, the control room local
area network present situation, promoted the string mouth server to
substitute for the multi- strings mouth card, this mainly was using
the enterprise already the local area network resources reduction line
investment which had, saved the cost, was equal to puts through tcp/ip
the multi- strings mouth card on the scene.
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