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Understand the EIA-232D Interface Standard, signaling standards, signals, and synchronization techniques. Learn about bit and character synchronization, asynchronous communication, and performance analysis of communication links.
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Interfaces and Synchronization Martin Weiss
EIA 232D Interface Standard • Synonymous with ITU V.24 • Asynchronous interface • Up to 19.2kbps • 50 foot maximum distance between DCE and DTE
Other Interface Standards • EIA 449 - Uses EIA-422 (balanced) or EIA-423 (unbalanced) signalling standards • EIA 530 - Same as EIA 449 except with a 25 pin connector
Signalling Standards Circuit Type Max. Separation (m) Max. Bit Rate EIA-423 10 100 1000 100kbps 10kbps 1kbps EIA-422 10 100 1000 10Mbps 1Mbps 100kbps
Main EIA 232D Signals • Receive data (RxD) - Pin 2 • Transmit data (TxD) - Pin 3 • Request to Send (RTS) - Pin 4 • Clear to Send (CTS) - Pin 5
Main EIA 232D Signals • Data Set Ready (DSR) - Pin 6 • Data Terminal Ready (DTR) - Pin 20 • Ring Indicator (RI) - Pin 22 • Carrier Detect (CD) - Pin 8
Modem Modem DTR DTR DSR RTS CTS DSR Data TxD DCD RxD EIA 232D Protocol
Questions • For data transfer, when would we want to transfer bits one at a time, and when as a group? • Why do we have to worry about synchronization? • How can we begin to quantify performance issues?
Bit Synchronization • The receiver must know when a bit starts and when it stops • Normally, synchronization sequences are necessary
Character Synchronization • When does a character start?
Asynchronous Communications • Characters are transmitted when they are generated • The receiver must be configured so that the gross transmission characteristics are identical to the transmitter
Implementation of Async • Line is normally in a logical “High” state • Preceding a character is a “Start Bit” (Low for one bit time) • Subsequent bits may be low or high • Character may be ended by stop bit(s)
Implementation of Async Start 0 0 1 1 0 1 0 Stop Stop Parity =?
Discussion of Async • Very simple to implement • Inefficient • Alternative: synchronous transmission
Synchronous Transmission • Transmitter and receiver are synchronized at the bit and character level prior to transmission • Messages may still arrive asynchronously • Synchronous systems are normally more complex
Propagation time a = Transmission time Performance Analysis of Communications Links • Consider the effect of propagation delay and transmission rate Recall that
Definitions • Let U = (throughput)/(capacity) = T/R • Throughput is the number of bits actually transmitted per unit time • Capacity is the number of bits that could be transmitted per unit time • R = data rate of the channel • d = maximum distance between any two stations
frame length = transmission time + propagation delay Definitions • V = velocity of signal propagation • L = frame length (average or fixed) • T = throughput • Assuming no overhead, Number of bits T = Time from source to destination
Illustration (a < 1) t0 Start of Transmission t0+a Start of Reception t0+1 End of Transmission t0+1+a End of Reception
Illustration (a > 1) t0 Start of Transmission t0+1 End of Transmission t0+a Start of Reception t0+1+a End of Reception
frame length T = Propagation delay + transmission time Utilization of a Baseband Bus Utilization = Throughput/Capacity
U vs. a U 1.0 0.8 0.6 0.4 0.2 a 1 5 10 15 20