Interfacing 10

7/29/2019 Interfacing 10

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Synchronous vs. Asynchronous

Synchronous- One-way communication

- Receiver and transmitter must have their clocking synchronized, .

- Requiring a constant stream of transmission

- Request for data is made, time is given for the request to be carriedout, and data is read.

Asynchronous- Utilizes start and stop bit to communicate the beginning and end of

data words to the receiver- No need for devices to be synchronized


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Data Word

Start Bit Signals the start of transmission of the data bits

Transition from logic 1 to logic 0

Data Bits

Generally 8 data bits (not including parity bit)

Transmitted and received least significant bit first

Stop Bit

Signals the end of a data word

Logic 1


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Parity Bit

Can be added to the transmitted data to check for and displayan error message when a bit is lost during transmission

Comes in two forms

Odd Parity

Sum of the 1s in the frame will be odd

The logical state (1 or 0) of the parity bit is selected to fulfill this condition

Even Parity

Sum of the 1s in a frame will be even

Number of 1s checked against the parity bit to determine ifany bits were lost during transmission

A two bit loss will go undetected with this method


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Data Word


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Baud and Bit Rates

Baud Rate

A baud is the reciprocal of the shortest pulse duration in

a data word inclusive of start, stop, data, and parity bits

Baud rate is the total number of bits transmitted persecond

Data Bit Rate

The number ofdata bits transmitted in one second.

Does not include the start or stop bits


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In this example:

1 start bit, 1 stop bit, 1 parity bit, 8 data bits, 11 bits per word, 300 baud rate

Bit Time = 1/(Baud rate) = 1/300 = 3.33 msec

Character Time = (total bits in word) x (bit time) = 11 x 3.33 = 36.6 msec

Data Bit Rate (including parity) = (data bits in word)/(character time)= 9/3.33 = 270


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RS232 and RS485Fundamentals


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What is RS232 RS-232 Signals

RS-232 Line Driver

RS-232 Speed

RS-232 SW settings

What is RS-485 RS-485 Line Driver

RS-485 Network

RS-485 Half-duplex

RS-485 Full-duplex RS-485 and DL devices

RS-232 vs RS-485


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RS-232

RS-232 is a popular communications interface for

connecting modems and data acquisition devices (i.e.

GPS receivers, electronic balances, data loggers, ...) to

computers.

RS-232 can be plugged straight into the computers

serial port (know as COM or Comm port).


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RS-232 Signals Architecturally RS-232 is a

bi-directional point to point

link.(serial port - PC side)

Two independent channels

are established for two-way

(full-duplex)

communications.

RS-232 can also carry

additional signals used for

flow control (RTS, CTS)

and modem control (DCD,

DTR, DSR, RI).


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Three major wires for the Serial interface:

Transmit ( Pin 2)

Receive ( Pin 3)

Ground Pin 7 (25 pin connector)

Pin 5 (9 pin connector)

Tx Tx

RxRx

GndGnd

Computer

Device

Transmit connects to Receive


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RS232 is well-established standard, developed by the EIA(Electronics Industry Association) in 1960s

Originally intended as an electrical specification to

connect computer terminals to modems

Defines the interface between a DTE and a DCE

DTE = Data Terminal Equipment (terminal)

DCE = Data Communications Equipment (modem)

A modem is sometimes called a dataset

A terminal is anything at the terminus of theconnection.

VDT (video display terminal), computer, printer, etc


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RS-232C RS-232C

Telephone

network

DTE DCE DCE DTE


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1s and 0s in RS-232C

A 1 is called a mark

A 0 is called a space

The idle state for an RS-232C line is a 1

(mark)

Idle state is called marking the line


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Data Transmission Example

Plot of the asynchronous RS-232C transmission of the ASCII charactera with odd parity:

0 1 0 0 0 0 1 1 0 1

Idle

state

Stop

bitStart

bit

Idle

state

ASCII character a 7 bits

LSB first

Parity

bit

time

TD


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RS-232C Connectors

The original standard specified a 25-pin connector

Today, a 9-pin connector is more common

E.g.,

DB9P

Note:

P = pin

Sometimes called a male

connector The mate for this is a DP25S, or

socket connector the female


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RS-232C Connectors

DB25P

DB9P

DB25S

DB9S

Where is pin 1? Where are pins 2, 3, 4, etc.?

Pin 1

Pin 1Pin 1

Pin 1


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RS-232C Pin Numbers1 2 3 4 5

9 8 7 6

DB9P


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RS-232C Pins, Signals, Directions

DB251

2

3

4

5

67

8

20

22

Signal NameCD Chassis Ground

TD Transmit Data

RD Receive Data

RTS Request To Send

CTS Clear To Send

DSR Data Set ReadySG Signal Ground

DCD Data Carrier Detect

DTR Data Terminal Ready

RI Ring Indicator

Direction-

DTE DCEDTE DCEDTE DCEDTE DCEDTE

DCE-

DTE DCEDTE DCEDTE DCE

DB9

2

3

7

8

65

1

4

9

Pin


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Common 25 pin D-shell connector pinout used for

asynchronous data communications.Pin Signal1 PGND Protective Ground2 TXD Transmit Data3 RXD Receive Data

4 RTS Ready To Send5 CTS Clear To Send6 DSR Data Set Ready7 SG Signal Ground

8 CD Carrier Detect

20 DTR Data Terminal Ready22 RI Ring Indicator

(serial port - PC side)


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RS-232 Line Driver Unbalanced Line Drivers

Each signal appears on the interface connector as a voltage with reference to a signal

ground.

The idle state (MARK) has the signal level negative with respect to common whereas

the active state (SPACE) has the signal level positive with respect to the same

reference.


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RS-232 Speed

How fast can RS-232 be? The maximum speed, according to the standard, is 20kbit/s.

However, modern equipment can operate much faster than this. (i.e. Lynxcan reach 115200 baud.)

The length of the cable also plays a part in maximum speed.The longer the cable and the slower the speed at which you can obtainaccurate results.

A large wire capacitance and inductance limits the maximum length of thecable and/or the maximum speed;

Moreover higher is the capacitance of the cable higher is the interferencebetween two adjacent signal wire.

50 feet (15m) @ max baudrate is commonly quoted as the maximumdistance.

RS-232 switch settings


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RS-232 switch settings

One byte of async data has:

Start Bit = 1 (always)

Data Bits = 8 (or 7)

Stop bits = 1 (or 2)

Parity = NONE (or EVEN or ODD)


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Computer and device must have the same settings for

Baud Rate

Parity

Stop bits

Baud Rate - data transmitted in bits/second

Parity - Check against faulty data transfer. If used, 8 (typically) data bits sent plus

parity bit. Responsibility of device to check parityStop Bits - denotes end a data string. Use 2 stop bits with SR510.

(Recommended)

S232 i l f


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RS232Typical Data format

Data Format:

Bit 0: Start bit

Bit 1-8: ASCII code of Data

Bit 9 : Parity Bit

Bit 10-11 : Stop bits

Each ASCII character requires about 12 bits to be transmitted or about 833s forcharacter.

Start

BitLSB

ASCII

MSBParity &

Stop Bits

Data in above example is sent LSB first (eg. ASCII A=01000001)

Logic of Data may be inverted. Control lines use positive logic

Time

Voltage

RS232 i h C l Li


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RS232 with Control Lines

(control lines not necessarily required in RS232) - some devices require them.

Handshake Control Lines.

CTS - Clear to Send - (eg. pin 5 on SR510 lock-in)

Set high (Asserted) by instrument to tell computer that instrument is ready to

receive data

DTR - Data Terminal Ready - (pin 20)

Set high by computer to tell instrument that Computer is ready to receive data.


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Testing out the RS232

Use Hyper Terminal (Windows XP)

Programs>>Accessories>>HyperTerminal

Set up Communications : Com 1, 19200 Baud, 8 Bits, no parity, 2 stop bits.

With Fiber-optic link connected to RS232, monitor the TX BNCconnection on the Oscilloscope.

Type commands in Hyperterminal to read/write data to the fiber-optic link.

(Connect TX to RX directly to make sure sense of logic is correct, etc.)

NOTE: Do not need flow control with our simple fiber link


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What is RS-485 RS-485 is a EIA standard interface which is very common

in the data acquisition world

RS-485 provides balanced transmission line which also

can be shared in Multidrop mode.

It allows high data rates communications over longdistances in real world environments.

RS-485 was designed for greater distance and higher

baudrates than RS-232.

According to the standard, 100kbit/s is the maximum

speed and distance up to 4000 feet (1200 meters) can be

achieved.

RS 485 Li D i


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RS-485 Line Driver Balanced Line Drivers

Voltage produced by the driver appears across a pair of signal wires thatransmit only one signal. Both wires are driven opposite.

RS-485 driver has always the Enable direction control signal.

Differential system provides noise immunity, because much of thecommon mode signal can be rejected by the receiver. So ground shiftsand induced noise signals can be nullified.

RS 485 Network


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RS-485 Network RS-485 provides Half-Duplex, Multidrop communications over a single twisted pair

cable.

The standard specifies up

to 32 drivers and 32

receivers can share a

multidrop network

Terminator resistors avoid

reflected signal

RS 485 Half duplex


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RS-485 Half-duplex Data logic uses Half-Duplex configurations for Data Collecting and

Master/Slave layouts.

Usually we talk about Multidropnetwork (i.e. MX4000,DPS9000)

Slave 1

TX

ENABLE

RX

Slave 2

TX

ENABLE

RX

Slave N

TX

ENABLE

RX

RTX485+

RTX485-

485GND

Master

TX

ENABLE

RX

RS 485 Full duplex


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RS-485 Full-duplex Potentially RS-485 interface can also use 4-wires to comunicate in multidrop mode but...

Scanner

TX

ENABLE

RX

TX485+

TX485-

RX485+

RX485-

485GND

TX

ENABLE

RX

Other device


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RS-232 vs RS-485

The architectural difference between RS-232 and RS-485 isthat 232 is a bi-directional point to point link, whereas 485 is

a single channel bus.

Electrically, each 232 signal uses a single wire with

symmetric voltages about a common ground wire. 485 uses

two wires to carry the single signal differentially.

The main difference is that only one device on a 485 bus can

transmit at a time, however there is no similar limitation inthe transmission for RS232 since it has a peer-to-peer link .


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RS-232 vs RS-485RS-232 RS-485

Mode of Operation SINGLE-ENDED DIFFERENTIAL

Total Number of Drivers 1 DRIVER 32 DRIVERand Receivers on One Line 1 RECEIVER 32 RECEIVER

Maximum Cable Length 50 FEET 4000 FEET Maximum Data Rate @Max length 20kb/s 100kb/s

Driver Output Signal Level(Loaded Min.) Loaded +/-5V to +/-15V +/-1.5V

Driver Output Signal Level(Unloaded Max) Unloaded +/-25V +/-6V

Driver Load Impedance 3k to 7k 54

Max. Driver Current inHigh Z State Power On N/A N/A

Max. Driver Current inHigh Z State Power Off +/-6mA @ +/-2v +/-100uA

Slew Rate (Max.) 30V/S N/A

Receiver Input Voltage Range +/-15V -7V to +12V

Receiver Input Sensitivity +/-3V +/-200mV

Receiver Input Resistance 3k to 7k 12k

Interfacing 10

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