C2-RS232-RS422-RS485-phan 3
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Transcript of C2-RS232-RS422-RS485-phan 3
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Serial Communication
EIA RS422 and RS485
Recommended StandardThe purpose of this course note is to attempt to describe the main elements in SerialCommunication. This application note attempts to cover enough technical details ofRS422 and RS485.
EIA- Electroonic Industries Association
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Introduction
The purpose of this note is to describe the
main elements of an RS-422 and RS-485
system.
This note attempts to cover enough technical
details so that the system designer will have
considered all the important aspects in the
data system design.
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Balanced Line Drivers
In a balanced differentialsystem the voltage producedby the driver appears across apair of signal lines thattransmit only one signal. (2-6V)
C: Signal Ground. It isnt usedby a balanced line receiver indetermining the logic state ofthe data line.
ENABLE: (Tristate condition ofthe driver)
RS-485 diver must haveENABLE control signal.
RS-422 driver may have thissignal, but it is not alwaysrequired.
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Schmitt Trigger using Op-Amp
vout=A(v2v1)
The comparator will switch when V+=0. Then
The effect of using a Schmitt trigger
(B) instead of a comparator (A).
Typical hysteresis curve (Non-inverting)
Non-inverting Schmitt trigger
Inverting Schmitt trigger
The difference between the trip points
is the hysteresis H and is given as
sss V
R
RVR
RVR
RH
2
1
2
1
2
1 2)(
Let's say you detect a low-to-high transition at 2.5 V. A
100 mV hysteresis would mean that the low-to-high
transition is detected at 2.55 V and the high-to-low
transition is detected at 2.45 V, a 100 mV difference.
Vcc = Voltage Common Collector Bipolar
Vee = Voltage Common Emitter BipolarVss = Voltage Common Source FET
Vdd = Voltage Common Drain FET
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EIA Standard RS-422 Data Transmission Each generator can drive up
to 10 receivers.
The 2 signaling states of the lineare define as follows:
When the A terminal of thedriver is negative with respectto the B terminal, the line is
in a binary 1 (MARK or OFF)state.
When the A terminal of the
driver is positive with respectto the B terminal, the line is
in a binary 0 (SPACE or ON)
state. Like RS232, the data bits are
transmitted in reverse order.
The terminator is placed at the end of a transmission
line or daisy chain bus, designed to match impedance and
hence minimize signal reflections.
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Figure shows the condition of the
voltage of the balanced line for
an RS-232 to RS-422 converter
when the line is in the idle
condition or OFF state.
The A terminal is equivalent to
the - designation.
The B terminal is equivalent to
the + designation.
The same relationship alsoapplies for RS-485 systems.
RS-422 can withstand a common
mode voltage (Vcm) of 7 volts
EIA Standard RS-422 Data Transmission
DVM: Digital voltmeter
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As many as 32 driver/receiver pairs can share a multi drop network
Many characteristics of the drivers and the receiver are the same as RS-
422.
The range of the common mode voltage Vcm that the driver and receiver
can tolerate is expanded to +12 to -7 volts. The driver can be disconnected or tristated from the line.
The signal ground line is also recommended in an RS-485 system
EIA Standard RS-485 Data Transmission
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Tristate control of an RS-485 device
using RTS
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Send data control of an RS-485 device
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Biasing an RS-485 Network
In order to maintain the proper idle voltage state (all drivers are tristated),
bias resistors are must be applied to force the data lines to the idle
condition.
Example: 10 node, RS-485 network with two 120
termination resistors
T i t t ti f
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Transient protection of
RS-422 and RS-485 system
Isolated RS-485 device
RS-485 Device with signal ground
connected to chassis ground
Signal ground connection between two nodes
with 100 resistor
Isolated node with shunt protection to earth ground
Isolated port with ungrounded shunt protection
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Surge Transient protection of
RS-422 and RS-485 system
Fused port protection
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SN65176B, SN75176B
DIFFERENTIAL BUS TRANSCEIVERS
The SN65176B and SN75176B differential bustransceivers are integrated circuits designed for
bidirectional data communication on multipoint
bus transmission lines.
They are designed for balanced transmission
lines and meet ANSI Standards TIA/EIA-422-B
and TIA/EIA-485-A The receiver features a minimum input
impedance of 12 k, an input sensitivity of 200
mV, and a typical input hysteresis of 50 mV.
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Full duplex
SN75179B DIFFERENTIAL DRIVER AND
RECEIVER PAIR
The SN75179B is a differential driver and
receiver pair designed for balancedtransmission-line applications and meets
TIA/EIA-422-B, TIA/EIA-485-A. It is designed to
improve the performance of full-duplex data
communications over long bus lines.
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Half duplex
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CONTROLLING THE DRIVER ENABLE
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CONVERT RS232 TO RS485
Half duplex
A MAX233 converts the RS-232 signals to TTL levels, and
the TTL signals connect to an SN75176Bthat provides the
RS-485 interface.
The MAX220MAX249 family of line drivers/receivers is
intended for all EIA/TIA-232E and V.28/V.24 communications
interfaces, particularly applications where 12V is not
available.
MAX232
MAX233
V.24 is a specification for single-ended communications that includes the definition of
connector pin allocations. It is used together with V.28 to define a specification for
serial asynchronous or synchronous communications.
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CONVERT RS232 TO RS485 Half duplex
Popular chip MAX232 is used to convert RS232 signals from/to TTL level. MAX485 converts TTL level signals to RS485
standard. RTSline is used to change transmission mode of MAX485. RTS signal is first converted to TTL level by MAX232 thenconnected to DEand RE/pins. 2 LEDs are used to indicate current state of the interface.
LM78LXX Series
Output voltages of 5.0V, 6.2V, 8.2V, 9.0V, 12V, 15V
The MAX481, MAX483, MAX485, MAX487MAX491, and MAX1487
are low-power transceivers for RS-485 and RS-422 communication.
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CONVERT RS232 TO RS485 Half duplex
This is a small RS232 to RS485 converter project it use for convert RS232 signal level to RS485 level multidrop. It can use with 32 slave to communicate withPC and embedded systems board for long distance less than 1.2Km( 4000 feet).
Specification
- 9-12Vdc power supply
- Small size only 2.8" X 1.45 "
- Use for RS485 multidrop 2 wire connection.
- Direct connect with PC on DB9 connecter.
- Use RTS signal to control direction .
- 32 Slaves (up to 256 slaves with some transceiver i.e. MAX3088)
J1 and J2 use for jump R terminate(120) at the end of communication line or last slaver. The 75176 use for transceiver.RTS active (logic 0 or -3 to -15V) when we need to send data .
Diode 1N4001 provides
protection against reverse
polarity connection of the
supplyAdding a resistor to a line may limit
damaging current flows that would
otherwise result from short high-voltage
transients, such as those caused by
electrostatic discharge (ESD).
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Data Format and Protocols
Information content passing through peer-to-peerconnection is packed in a very simple structure:
and are bothconfigurable via software (device configurationparameters)
Most common generic Handshakeare
available/selectable with RS232 interface: Hardware (RTS-CTS)
Software XON/XOFF
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