Intro to Serial Transceivers
April 2010
What is a Transceiver?
• Transmitter converts CMOS/TTL levels to
Electrical Interface Standards
• Receiver converts Electrical Interface Standard
levels to CMOS/TTL outputs
CMOS/TTL
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Transceiver
Electrical
Interface
Standards
What are Electrical Interface Standards?
• Definition of electrical characteristics in order
for two or more systems or equipment to
communicate
• Electrical Interface Standards
• TIA/EIA
• RS-232, RS-485 RS-422, RS-423, RS-449
• PROFIBUS
• ITU-T
• V.10, V.11, V.28, V.35, V.36, X.21
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RS-232 (EIA-232) Standard
• Single-ended
• Point-to-point
• Output driver voltage: ± 7V typical with 5KΩ
load
• Receiver input voltage: ± 3V minimum
• Distance of up to 15 meters
• 115.2 kbps maximum data rate
• Currently RS-232 Transceiver max data rate is 1
Mbps
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RS-485 (EIA-485) Standard
• Differential
• Point-to-multipoint
• Up to 32 drivers and 32 receivers
• 10 Mbps maximum data rate up to 50 meters
• 200 kbps data rate up to 1200 meters
• -7V to +12V common mode input voltage range
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RS-422 (EIA-422) Standard
• Differential
• Point-to-multidrop
• 1 driver and up to 32 receivers
• 10 Mbps maximum data rate up to 50 meters
• 200 kbps data rate up to 1200 meters
• -7V to +7V common mode input voltage range
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Differential Signaling
•
Signal on B is inverse of A
• Difference between VA and VB is 2X the amplitude
• Improved noise rejection, speed, distance and reliability
A
A
Tx
Rx
B
B
A-B
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Differential Cancels Noise
• Noise and external
interference has
equal impact on
each wire in the pair
Input
Signal
A
Noise is coupled
onto both signals
equally
• A-B differential
cancels out noise
B
• Common-mode
offset (ground shift)
is also cancelled
out
A-B
Diff.
Noise is
cancelled out
by differential
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Comparison of PROFIBUS and RS-485
PROFIBUS is SUPERSET of RS-485
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Specification
RS-232
RS-422
RS-485
PROFIBUS
Transmission Type
Single Ended
Differential
Differential
Differential
Maximum Cable
Length
15 m
1200 m
1200 m
1200 m
Minimum Driver
Output Voltage
±5 V
±2 V
±1.5 V
±2.1 V
Driver Load
Impedance
300 Ω
100 Ω
54 Ω
54 Ω
Receiver Input
Resistance (min.)
3kΩ to 7kΩ
4kΩ
12kΩ
20kΩ
Receiver Input
Sensitivity
±3 V
±200 mV
±200 mV
±200 mV
Receiver Input
Voltage Range
−15 V to +15 V
−7 V to +7 V
−7 V to +12 V
−7 V to +12 V
Serial Communication Standards
ITU-T
• V.10 – Single Ended electrical circuits for data communication at up
to 100 kbps
• V.11 - Differential electrical circuits for data communication at up to
10 Mbps
• V.28 - electrical characteristics for single ended double-current
interchange circuits
• V.35 - specifications from V.11 for the differential signals, while the
control signals remain single ended.
• V.36 – Modems spec. for synchronous data transmission using 60108 kHz group band circuits
• X.21 - All signals are balanced.. Electrically the X.21 signals are the
same as RS-422
• EIA-530 defines pinout on a 25 pin connector
• RS-449 defines the pinout on a 37 pins connector
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