1. ANALOG AND DIGITAL PHYSICAL INTERFACES
Habib Youssef, Ph.D
Department of Computer Engineering
King Fahd University of Petroleum and Minerals
Dhahran, Saudi Arabia
COMPUTER NETWORK

2. The RS-232/CCITT V.24 and V.28 Interface
Computer
Computer
Data
Data
Modem
Modem
DTE
DCE
DCE
DTE
Out of Band Control
Out of Band Control
DTE : Data Terminal Equipment
DCE : Data Circuit Termination Equipment

3. The RS-232/CCITT V.24 and V.28 Interface (Cont.)
Data processing (DTE) to modem (DCE) interface
The CCITT V.24 Recommendation defines the interchange circuits
V.28 defines the electrical characteristics

4. The RS-232/CCITT V.24 and V.28 Interface (Cont.)
In EIA, known as RS-232-C (the third [-C] version of RS-232)
More recent version of RS-232-D (now EIA-232-D)
Sometimes TIA-232-D (Telecommunications Industry Association)

5. The RS-232/CCITT V.24 and V.28 Interface (Cont.)
A 25-pin connector/interface
ISO 2110 is used
Is not part of the RS-232-C standard
Bit serial data (full duplex)
Out of band control lines

6. The RS-232/CCITT V.24 and V.28 With Null Modems
DTE
DCE
Null Modem
Data
Data 2 2
Data
Data 3 3
Req to Send
Req to Send 4 4
Clear to Send
Clear to Send 5 5
Data Set Ready
Data Set Ready 6 6
Signal Detect
Signal Detect 8 8
Data Terminal Ready
Data Terminal Ready 20 20
Signal Ground 7 7
Note : There are many variations to Null Modem Cross Connection

7. Pin Assignments for V.24/EIA-232
Reserved for testing
Secn. CTS
Unassigned
Shield
Clear to Send
GND
Rx Data
Secn. Recv. Line Signal Detector
DCE Ready
Carrier Detect
Tx Data
Reg to Send
Reserved for testing 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Transmitter signal element timing
Secondary RTS
Test Mode
Transmitter signal element timing
Remote Loopback
Data Signal Rate Select
Transmit signal element timing
Local Loopback
DTE Ready
Secondary received data
Ring Indicator
Secondary Tx Data

8. RS-232/CCITT V.24 & V.28 Related Products
It is often convenient to switch RS-232/V.24 signals from a computer to one of several devices
For example, to different types of printers
Simple “multiple” switches are available for this purpose

9. RS-232/CCITT V.24 & V.28 Related Products (Cont.)
Specialized companies have been developed to handle the interface market with products such as
Multiple switches
RS-232/V.24 cables
Null modems
RS-232/V.24 “gender changers”
Breakout boxes to monitor control signals

10. Limitations of RS-232/V.28 An upper data rate of about 20 kbit/s
An upper cable length of about 50 to 100 feet (about 20 to 40 m)
Some products are available to extend these, but a new approach is needed

11. The Evolution of RS-232-C RS-232-C RS-449 signals
RS-422/423 electrical (1977)
RS-442 balanced circuits
RS-443 unbalanced circuits
RS-232-C
V.35
Balanced Circuits
RS-530 (1987)
Balanced Circuits
EIA-232-D (1987)
Unbalanced circuits

12. Synchronous Transmission
Has a known timing relationship between bits and characters
Characters are sent one after the other
The receiver recovers this timing from transitions in the arriving data 1
Start
End
Characters

13. V.24/EIA-232 dial-up operation

14. RS-423/CCITT V.10 Single Ended Interchange Circuit
Noise
Error
Recvr
Trans
Signal return
Note: V.10 is the same as X.26 or RS-423-A (unbalanced)

15. RS-422/CCITT V.11 Differential Interchange Circuit
Noise
Sensitive to differential signal
Recvr
Trans
Noise was rejected
Termination resistor
Note : V.11 is the same as X.27 or RS-422-A (balanced)

16. CCITT X.21 Interface Physical-level interface between DTE and DCE
For synchronous operations on public data networks
X.21 uses control transitions and ASCII characters rather than using separate signal lines

17. CCITT X.21 Interface (Cont.)
The X.21 electrical characteristics are
CCITT X.27 (balanced; same as V.11 and RS-422)
CCITT X.26 (unbalanced; V.10 and RS-423)
(Note: For operation above 9600 bit/s, X.27 is required)
X.21 mechanical characteristics are
15-pin connector per ISO Standard 4903

18. CCITT X.21 Interface (Cont.)4
Switched 64 kbit/s
X.21
DSU
Bridge

19. CCITT X.21 Interface (Cont.)
Circuit Name Direction
To DCE / To DTE
G Ground,Common Return
Ga DTE Common Return X
Gb DCE Common Return X
T Transmit X
R Receive X
C Control X
I Indication X
S Signal Timing X
B Byte Timing (Optional) X

20. CCITT X.21 bis
As an interim (perhaps longer term) provision, we have X.21 bis
X.21 bis utilizes RS-232 for use with X.25
Particularly used in countries where X.21 has not yet become available

21. CCITT X.21 bis (Cont.)
RS-232 signals are used to represent X.21 events
To initiate the call
Some X.21 features are not supported
Call progress signals

22. ISDN Interface Terminal Equipment (TE) Network Equipment (NE)
Power Source 3 b b c c d d
Transmit
Transmit
Receive e
Receive e f f g g
Power Source 2
Power Sink2 h h