Hands of Data
Communications, Networking
& TCP/IP Troubleshooting
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Topics
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Basics of serial communication
RS-232
RS-422
RS-485
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Serial Communication
 Data is sent and received one bit at a time over
a communication channel.
 Serial ports
 9 pin and 25 pin
 Used to communicate with terminals and
peripheral devices
 Also known as COM port, controlled by
UART
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Parallel Communication
 8 (or 16 or 32) bits are sent and received
simultaneously
 Sent over a bus comprising 8 (or 16 or 32) channels,
one for each bit.
 Usually used within a device.
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Connector – Types
Types of serial port connectors
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DTE vs. DCE
 Devices that use serial cables for their communication
 DTE
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Data Terminal Equipment
E.g. computers, printers
Transmits on Pin 2
 DCE
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Data Communications Equipment
E.g. modems
Transmits on Pin 3
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Asynchronous vs. Synchronous
Asynchronous
 No clock sent
 Level sync
 Character transfer
 Susceptible to noise
 Slow
 Short lengths
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Synchronous
 Clock sent
 Edge sync
 Packet transfer
 Resistant to noise
 Fast
 Long distances
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Serial Communication – Keywords
 Handshaking - a procedure used to check the link
between DTE & DCE before transmitting data
 Baud Rate
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Transmission speed
Number of bits that are actually being sent over the serial
link
 Type of Cables
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Modem Cable
Null Modem Cables
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Modem Cable
Signal Description
9-pin
DTE
25-pin
DCE
Remarks
Carrier Detect (CD)
Receive Data (RD)
1
2
8
3
From Modem
From Modem
Transmit Data (TD)
3
2
From Terminal/Computer
Data Terminal Ready (DTR)
4
20
From Terminal/Computer
Signal Ground (SG)
5
7
From Modem
Data Set Ready (DSR)
6
6
From Modem
Request to Send (RTS)
7
4
From Terminal/Computer
Clear to Send (CTS)
Ring Indicator (RI)
8
9
5
22
From Modem
From Modem
RS-232 Pinouts DB9 to DB25
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Null Modem Cables
Signal description
25-pin
25-pin
Signal description
Receive Data
3
2
Transmit Data
Transmit Data
2
3
Receive Data
Data Terminal Ready
20
6+8
Data Set Ready + Carrier
Detect
System Ground
7
7
System Ground
Data Set Ready + Carrier
Detect
6+8
20
Data Terminal Ready
Request to Send
4
5
Clear to Send
Clear to Send
5
4
Request to Send
Connecting 25 pin to 25 pin for Null Modem
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Null Modem Cables
Signal description
9-pin
9-pin
Signal description
Receive Data
2
3
Transmit Data
Transmit Data
3
2
Receive Data
Data Terminal Ready
4
6+1
Data Set Ready + Carrier
Detect
System Ground
5
5
System Ground
Data Set Ready + Carrier
Detect
6+1
4
Data Terminal Ready
Request to Send
7
8
Clear to Send
Clear to Send
8
7
Request to Send
Connecting 9 pin to 9 pin for Null modem
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RS-232 Standard
 Specifies the method of connection of a DTE and a
DCE.
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A DCE receives data from the DTE and retransmits it to
another DCE via a data communications channel such as a
telephone link.
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RS-232 Connections
Connection between DTE and DCE using DB-25 connectors
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Voltage Levels
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RS-232 Transmitters & Receivers
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Half Duplex Operational Sequence of RS 232
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RS-422 Interface Standard
 Defines a balanced, or differential, data
communications interface
 Uses two separate wires for each signal
 Permits very high data rates
 Minimizes problems with varying ground potentials
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The EIA-422 Balanced Line Driver
Connections
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EIA 422 Voltages vs. Real Life
 EIA voltages: ±6 volts to ±2 volts
 In real systems the voltage is between ±5 volts
and ±.2 volts
 Often the same chips that are used for 485
systems are used for 422
 Typical measured voltage is approximately ±2
volts
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EIA - 422 vs. Real Life
Simplex
Half or full duplex
 10Mbps
 Differential
 10 devices
 No speed defined
 Differential
 Point to point
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RS-485 Interface Standard
 Most versatile EIA standard
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Distances of up to 1200m
Data rates of up to 10Mbps
32 line drivers
32 line receivers
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RS-485
 RS-485 is the most common voltage standard in use today
for multi-drop communication systems.
 This is because …
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It is very resistant to noise
It can send data at high speeds
And can go long distances
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Two-Wire Multidrop
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Four Wire Network
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DO YOU WANT TO KNOW MORE?
If you are interested in further training or
information, please visit:
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