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 10100

1000

100kbps10kbps1kbps

EIA-422 10100

1000

10Mbps1Mbps100kbps

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

EIA 232D Protocol

Modem Modem

DTR

DSR

RTSCTS

DTRDSR

TxD Data DCD

RxD

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 0 01 1 1

Parity=?

Stop Stop

Discussion of Async

Very simple to implement Inefficient Alternative: synchronous

transmission

Discussion of Async

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

Performance Analysis of Communications Links

Consider the effect of propagation delay and transmission rate

Recall that

a = Propagation time

Transmission time

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

Definitions V = velocity of signal propagation L = frame length (average or

fixed) T = throughput Assuming no overhead,

= frame length

transmission time + propagation delay

T = Number of bits

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+a Start of Reception

t0+1 End of Transmission

t0+1+a End of Reception

Utilization of a Baseband Bus

aRdLV

dV

LR

TL R

adV

LR

=

1

UTR a

= =1

1

Utilization = Throughput/Capacity

T = frame length

Propagation delay + transmission time

U vs. aU

a

1.0

0.8

0.6

0.4

0.2

1 5 10 15 20