Computers in Surveying SVY2301 / E4006 Automated Surveying.
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Transcript of Computers in Surveying SVY2301 / E4006 Automated Surveying.
Computers in Surveying
SVY2301 / E4006Automated Surveying
Communication Ports
• There are two basic types of communication ports available in most computers. These are:
• • serial port; and• • parallel port.
Communication Ports
• serial ports (COM1 or COM2) are generally used for connection of peripherals such as:– Digitisers modems;– Mice plotters.
• parallel ports (LPT1 and LPT2) are generally used for connection to printers or external disk drives.
Interfacing & Communication• Surveyors can be faced with interfacing
problems when trying to establish a connection between a data recorder and a computer.
• Survey equipment manufacturers usually supply standard interfacing cables for the most popular brands of computer but these may not be satisfactory in all cases.
• In addition to these problems interfacing problems can also occur when a surveyor tries to do something different such as connect his total station directly to a computer.
Interfacing & Communication• Surveyors require a good
understanding of at least the basic principles of interfacing and communications
Interfacing & Communication
Number System & Codes• A byte consisting of 8 bits can represent a
total of 256 numbers in the range 0–255.– 00000000 = decimal 0, and – 11111111 = decimal 255.
• a character code system based on an 8-bit byte would allow 256 characters to be coded.
• The ASCII-8 character code system is normally only used internally in the computer.
Interfacing & Communication
Number System & Codes• ASCII 7-bit code is the recommended
data representation code for communications between devices.– allows 128 characters to be represented
by the numbers 0–127. • 1111111 is the binary representation of 127.
• 8th bit of the byte is often used for error detection purposes. – providing a check that the other 7 bits
were not corrupted during transmission.
Interfacing & Communication
Signal Devices• a number of standard interfaces are
adopted by the computing and electronics industries.
• These standards fall into two categories, – serial interfaces, and – parallel interfaces.
• Serial communication interfaces can support longer distances between devices. – telephone is a serial communication device.
Interfacing & Communication
Signal Devices – Serial Interfaces• data signals are transmitted over a
two wire line by sending each bit one after the other as either a high or low voltage
• A standard signal code such as the ASCII 7-bit character code is used to control the data transfer.
Interfacing & Communication
Signal Devices – Serial Interfaces• The most common standard serial
interface in the computer industry is the RS232C standard.
• It was intended to describe the interface between data terminal equipment (DTE) and data communications equipment (DCE).– A ‘dumb’ computer terminal is a DTE – a modem is an example of a DCE.
Interfacing & Communication
Signal Devices – Signal Format (RS232C)• The RS232C is actually a voltage
standard. • Logic state 0 is defined by a voltage
between +5V and +15V whilst logic state 1 is defined by a voltage between –5V and –15V. A line is in logic state 1 or ‘high’ during idle time. The transmission of a data signal can be represented by a block diagram with the 1’s represented as highs and the zeros as lows.
Interfacing & Communication
Signal Devices – Signal Format (RS232C)• The RS232C is actually a voltage
standard. • Logic state 0 is defined by a
voltage between +5V and +15V
Interfacing & Communication
Signal Devices – Signal Format (RS232C)• logic state 1 is defined by a voltage
between –5V and –15V. • A line is in logic state 1 or ‘high’
during idle time.
Interfacing & Communication
Signal Devices – Signal Format (RS232C)• The 7-bit binary code for the
character ‘A’ is 1000001.
Interfacing & Communication
Signal Devices – Signal Format (RS232C)• To advise the receiver that a
character is being sent the transmitter prefaces each character with a start bit, a zero. – the line switches from a negative voltage
to a positive voltage.
Interfacing & Communication
Signal Devices – Signal Format (RS232C)• The character bits are followed by;
– a parity bit, and – one or two stop bits that leave the
signal high until the next character is sent
Interfacing & Communication
Signal Devices – Signal Format (RS232C)• The transmitting device must have at
least as many stop bits as the receiver requires.
• When two way communication occurs both must have the same number of stop bits.
Interfacing & Communication
Signal Devices – Signal Format (RS232C)• The parity bit is often used for a
simple error checking technique.
Interfacing & Communication
Signal Devices – Signal Format (RS232C)• even parity
– The transmitting device counts the number of 1s in each character and if there are an even number then the parity bit is set to 0.
– If the number of 1s added to an odd number then the parity bit would be set to 1 so the total was even.
• When the data bits are received the receiving device counts the number of 1s and if they add up to an even number the character is accepted. If they total to an odd number the transmitter may be asked to re-transmit the character. It should be noted that this simple check only checks an error in one bit as errors in two or more bits may cancel each other and the wrong character is then accepted.
Interfacing & Communication
Signal Devices – Signal Format (RS232C)• even parity
– The transmitting device counts the number of 1s in each character and• if there are an even number then the parity bit is set to 0. • If the number of 1s added to an odd number then the parity bit would be set to 1 so the
total was even.
– the receiving device also counts the number of 1s and • if they add up to an even number the character is accepted.• If they total to an odd number the transmitter may be asked to re-transmit the character.
– only checks an error in one bit as errors in two or more bits may cancel each other and the wrong character is then accepted.
Interfacing & Communication
Signal Devices – Signal Format (RS232C)• Odd parity
– the parity bit is set such that the number of 1s in the data bits (including the parity bit) add up to an odd number.
• No Parity– If ‘none’ is selected then no parity check is carried out by
the receiver. – This option is only available when 8-bit words are used as a
parity bit is not sent.
Interfacing & Communication
Signal Devices – Signal Format (RS232C)• Communication Parameters
• Basic Characteristics. – Most equipment that surveyors use
would communicate using asynchronous, half-duplex signals at RS232C voltage levels.
• Baudrate. – Both devices must be set to the same
baud rate – i.e. bits per second.
Interfacing & Communication
Signal Devices – Signal Format (RS232C)• Communication Parameters
• Parity. – Both devices must be set to the same
parity.
• Start and stop bits. – All devices attach one start bit to the
data bits and either one or two stop bits. – Some devices allow the user to select
either one or two stop bits. – Both devices must be set to the same
number of stop bits.
Interfacing & Communication
Signal Devices – Signal Format (RS232C)• Communication Parameters
• Word length. – The standard word length is 8 bits (7
data bits + 1 parity bit). – Many devices however allow the user
to select a 9 bit word (8 data bits for the character + 1 parity bit). In this case if a parity bit is selected then only one stop bit may be used.
Interfacing & Communication
Signal Devices – Signal Format (RS232C)• Communication Parameters
• ACK/NAK. – Normally the transmitting device sends all of the
data continuously, one block after another. – When protocol is selected as a parameter
however, the transmitter waits at the end of each block of data until it receives acknowledgement from the receiver that the data has been received. The character ACK is sent from the receiver to the transmitter in this case.
– If the data block was not received or was corrupted then the character NAK is sent to the transmitter. In this case the transmitter re-transmits the data block.
Interfacing & Communication
Signal Devices – Signal Format (RS232C)• Communication Parameters
• XON/XOFF. – Some devices use these control
characters to control the flow of characters.
– When a device receives an XOFF character (DECIMAL 19) during output, it suspends transmission until an XON character (Decimal 17) is received from the other device, after which it continues with the transmission.
Interfacing & Communication
Signal Devices – Signal Format (RS232C)• Communication Parameters
• Last characters. – The user is often able to select the
last characters that will be transmitted after a block of data.
– Normally the characters CR and LF (carriage return and line feed) are sent to end the data block.
– With some devices the user may select CR only as the last character.
Interfacing & Communication
Signal Devices – Signal Format (RS232C)• Communication Parameters
• Most devices have a set-up or configuration program that enables the user to select the required parameters from a menu that can be scrolled through all of the alternative settings.
• Alternatively the user must carefully set a series of microswitches on the device. These switches are often called dip switches.
Interfacing & Communication
Signal Devices – Pin & Line Assignment (RS232C)
• The Canon D range of 25 way connectors – only between 3 and 10 wires are used
for computer communications.
Interfacing & Communication
Signal Devices – Pin & Line Assignment (RS232C)Pin Signal Function
1 PG Protective Ground2 TXD DTE TRANSMITS DATA RXD DCE RECEIVES DATA3 RXD DTE RECEIVES DATA TXD DCE TRANSMITS DATA4 RTS REQUEST TO SEND5 CTS CLEAR TO SEND6 DSR DATA SET READY7 SG SIGNAL GROUND8 DCD DATA CARRIER
DETECT20 DTR DATA TERMINAL
READY
Interfacing & Communication
Signal Devices – Pin & Line Assignment (RS232C)
• The connector may be male or female i.e. having pins or holes respectively. Generally – a DTE device will have a male
connector, and– a DCE will have a female connector
Interfacing & Communication
Signal Devices – Pin & Line Assignment (RS232C)
• An RS232 interface may be successfully achieved with just three wires if no hardware ‘handshaking’ is required.
• The lines used in this case are lines 2, 3 and 7.
DTE DEVICE DCE DEVICE
Comment Signal Pin Data Flow Pin Signal Comment
Transmits Data TXD 2 2 RX Receives Data
Receives Data RXD 3 3 TX Transmits Data
Signal Ground SG 7 7 SG Signal Ground
Interfacing & Communication
Signal Devices – Pin & Line Assignment (RS232C)• the transmit pin on one device is connected to the
receive pin of the other device. – Obviously this must also be the case if communication is
to occur.
• both devices may be configured as DTEs. • In this case both transmit on pin 2 and receive on
pin 3. The normal solution is to cross the lines to achieve a successful interface,
DTE DEVICE DCE DEVICE
Comment Signal Pin Data Flow Pin Signal Comment
Transmits Data TXD 2 2 RX Receives Data
Receives Data RXD 3 3 TX Transmits Data
Signal Ground SG 7 7 SG Signal Ground
Interfacing & Communication
Signal Devices – Pin & Line Assignment (RS232C)
• both devices may be configured as DTEs. – both transmit on pin 2 and receive on pin 3.
• The normal solution is to cross the lines to achieve a successful interface,
DTE DEVICE DCE DEVICE
Comment Signal Pin Data Flow Pin Signal Comment
Transmits Data TXD 2 2 RX Receives Data
Receives Data RXD 3 3 TX Transmits Data
Signal Ground SG 7 7 SG Signal Ground
Interfacing & Communication
Signal Devices – Pin & Line Assignment (RS232C)
• both devices may be configured as DTEs. – both transmit on pin 2 and receive on pin 3.
• The normal solution is to cross the lines to achieve a successful interface,
DTE DEVICE
Data Flow
DTE DEVICE
Signal Pin Pin Signal
TXD 2 2 TXD
RXD 3 3 RXD
SG 7 7 SG
Interfacing & Communication
Signal Devices – Pin & Line Assignment (RS232C)• the flow of characters can only be in one
direction at a time – Communications would normally have to be software
controlled
• Normally other lines on the interface are used to control the flow. When these control lines are used the term ‘handshaking’ is used to describe their function DTE DEVICE
Data Flow
DTE DEVICE
Signal Pin Pin Signal
TXD 2 2 TXD
RXD 3 3 RXD
SG 7 7 SG
Interfacing & Communication
Signal Devices – Pin & Line Assignment (RS232C)
• Normally other lines on the interface are used to control the flow. When these control lines are used the term ‘handshaking’ is used to describe their function
Interfacing & Communication
Signal Devices – Pin & Line Assignment (RS232C)
Protective GroundDTE transmits dataDCE receives dataRequest to sendClear to sendData carrier detectData set readyData terminal
readySignal ground
DATA RECORDER COMPUTER
PG 1 1 PG
TXD 2 2 TXD
RXD 3 3 RXD
RTS 4 4 RTS
CTS 5 5 CTS
DCD 8 8 DCD
DSR 6 6 DSR
DTR 20 20 DTR
SG 7 7 SG
Interfacing & Communication
Signal Devices – Pin & Line Assignment (RS232C)Pin 20 is set high
by each device when it is switched on. Thus each device informs the other that it is ready
DATA RECORDER COMPUTER
PG 1 1 PG
TXD 2 2 TXD
RXD 3 3 RXD
RTS 4 4 RTS
CTS 5 5 CTS
DCD 8 8 DCD
DSR 6 6 DSR
DTR 20 20 DTR
SG 7 7 SG
Data terminal ready
Signal ground
Interfacing & Communication
Signal Devices – Pin & Line Assignment (RS232C)When a device is
ready to transmit data pin 4 is set high
DATA RECORDER COMPUTER
PG 1 1 PG
TXD 2 2 TXD
RXD 3 3 RXD
RTS 4 4 RTS
CTS 5 5 CTS
DCD 8 8 DCD
DSR 6 6 DSR
DTR 20 20 DTR
SG 7 7 SG
DCE receives data
Request to sendClear to sendData carrier
detectdetected on pin 8 of the other device
Interfacing & Communication
Signal Devices – Pin & Line Assignment (RS232C)If it is ready to
receive a message it will respond with a clear to send (CTS) on pin 5
DATA RECORDER COMPUTER
PG 1 1 PG
TXD 2 2 TXD
RXD 3 3 RXD
RTS 4 4 RTS
CTS 5 5 CTS
DCD 8 8 DCD
DSR 6 6 DSR
DTR 20 20 DTR
SG 7 7 SG
DCE receives data
Request to sendClear to sendData carrier
detectdetected on pin 8 of the first device
Interfacing & Communication
Signal Devices – Parallel Interfaces• each bit of a data byte travels down its
own wire. – for an 8-bit byte the interface would consist of
at least 8 wires.
• Normally used for one way communication between a computer and an output device such as a printer or plotter.
• If two-way parallel communication is required then additional cables are required for hardware handshaking.
• The Centronics pin connectors have become the standard parallel interface for printers. This connector has 36 pins.