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Practical Troubleshooting & Problem Solving of Ethernet Networks:
RS-232 Fundamentals
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DTE vs DCE
• DTE– Data Terminal Equipment– Transmits on Pin 2 for 25-pin connector
• DCE– Data Communications Equipment– Transmits on Pin 3 for 25-pin connector
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RS-232 Connections (DB25)
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EIA-232
• Electrical Signal levels– Logic 0 =+3 to+25V = Control ON– Logic 1 = -3 to -25V = Control OFF
• Mechanical Interface– DB-25, DB-9 connectors
• Circuit Functionality
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Voltage Levels for EIA-232
Data Line Driver
Control Line Driver
0 (Space)
1 (On)
1 (Mark)
0 (Off)
Data Lines
Driver Receiver
Control Lines
Data Receiver
Control Receiver
+25 V
+25 V
+25 V
+25 V
-25 V
-25 V
-25 V
-25 V
+5 V
+5 V
+3 V
+3 V
-5 V
-5 V
-3 V
-3 V
0 V
0 V
0 V
0 V
ON
OFF
0
1
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Pin AssignmentsPin Number
(DTE)DB9 Connector IBM Assignment
DB25 ConnectorEIA-232 Pin Assignment
1 Received Line Signal Shield
2 Received Data Transmitted Data
3 Transmitted Data Received Data
4 DTE Ready Request to Send
5 Signal Common/Ground Clear to Send
6 DCE Ready DCE Ready
7 Request to Send Signal Common/Ground
8 Clear to Send Received Line Signal
9 Ring Indicator + Voltage (Testing)
20 - DTE Ready
22 - Ring Indicator
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Limitations• Point-to-Point
• Distance Limitation = 2500pf (50ft @ 50pf/ft)
• 20kbps
• -3 to -25V and +3 to +25 V
• Note Table 2.1 Page 30 !
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Typical Approach
• Check basic Parameters
• Identify which is DTE and DCE
• Clarify handshaking compatibility– either Hardware– or Software
• Check actual protocol– use protocol analyzer
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Test Equipment
• Multimeter
• LED
• Breakout Box
• PC Based Protocol Analyzer
• Dedicated Protocol Analyzer
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Breakout Box
LEDs usually Bi-Color
Red commonly Positive Green commonly Negative
DIPSwitches
Common
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RS-232 Breakout Box
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Protocol Analyzer Connections
Protocol Analyzer
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Typical RS-232 Problems
• Mechanical Problems– Keep wires short– Stranded wire instead of solid wire– Only one wire soldered per pin– Bare wire should not be visible– Back shell must secure wire
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Typical RS-232 Problems
• Setup Problems– Software setup compatible
• 8N1or 7E1 or 7O1
– Baud rate correct– Code correct - Binary/Hex or ASCII– Software set up for handshaking
• Hardware or Software control
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Typical RS-232 Problems
• Noise Problems
– Ground Induced Noise
– Induced Noise on TX or RX lines
– Induced noise on Control Lines
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