Kenco KT Series Manuel utilisateur

DDA Interface Manual

“KT” Series Magnetostrictive

Liquid Level Transmitters

Kenco Engineering Company

10001 East 54th Street

Tulsa, Oklahoma 74146

Phone: 918-663-4406

Fax: 918-663-4480

Website: www.kenco-eng.com

Email: [email protected]

79862

(Rev. #)

[5-2-19]

DDA Interface Manual

Table of contents

1. Terms and Deﬁnitions.................................................................3

2. Introduction.................................................................................5

3. Safety Instructions......................................................................5

4. Quick Start-Up Guide..................................................................5

4.1 Before You Begin .................................................................. 5

4.2 Quick Start-Up Procedure ..................................................... 5

5. Display Menu..............................................................................5

5.1 Operation Modes................................................................... 5

5.1.1 Run Mode ...................................................................... 5

5.1.2 Program Mode............................................................... 5

5.2 Display Diagram.................................................................... 6

5.3 Menu Structure..................................................................... 6

6. Alarms.........................................................................................6

7. Error Codes (Faults)....................................................................7

8. DDA Interface..............................................................................7

9. Hardware and Software Environments.......................................8

10. DDA Command Decoder Examples ..........................................8

11. DDA/Host Computer Communication Protocol.........................9

12. DDA Command Deﬁnitions .....................................................13

12.1 Special Control Commands........................................... 13

12.2 Level Commands .......................................................... 13

12.3 Temperature Commands............................................... 14

12.4 Multiple Output Commands .......................................... 14

12.5 High-Level Memory Read Commands .......................... 15

12.6 High-Level Memory Write Commands .......................... 16

12.7 Diagnostic/Special Command Set ................................. 18

12.8 DDA Error Codes........................................................... 18

13. Dashboard, Setup Software ..................................... 19

13.1 Installing Dashboard..................................................... 19

13.2 Home Screen ................................................................ 19

13.3  ................................................................ 20

13.4 Signal Settings............................................................. 20

13.5 Level Settings .............................................................. 20

13.6 Temperature Settings................................................... 21

13.7 Flash Settings .............................................................. 21

13.8 Save Settings ............................................................... 22

13.9 Programming the Display ............................................ 22

13.9.1 Data from Device Tab........................................ 22

13.9.2 Calibration......................................................... 22

13.9.3 Factory.............................................................. 22

DDA Interface Manual

1. Terms and deﬁnitions

6A Heavy Oils

‘Generalized Crude Oils’, Correction of Volume to 60 °F against API

Gravity.

6B Light Oils

‘Generalized Products’, Correction of Volume to

60 °F against API Gravity.

6C Chemical

‘Volume Correction Factors (VCF)’ for individual and special applica-

tions, volume correction to 60 °F against thermal expansion coefﬁ-

cients.

6C Mod

An adjustable temperature reference for deﬁning VCF.

API Gravity

The measure of how heavy or light a petroleum liquid is compared to

water. Allowable values are 0 to 100 degrees API for (6A) and 0 to 85

degrees API for (6B).

DDA

‘Direct Digital Access’ – The digital protocol for use in intrinsically safe

areas.

Density

Mass divided by the volume of an object at a speciﬁc temperature. The

density value should be entered as lb. / cu. ft..

Explosion proof

Type of protection based on enclosure in which the parts which can

ignite an explosive gas atmosphere are placed within, and which can

withstand the pressure developed during an internal explosion of an ex-

plosive mixture, and which prevents the transmission of the explosion

to the explosive gas atmosphere surrounding the enclosure.

Flameproof

Type of protection based on enclosure in which the parts which can

ignite an explosive gas atmosphere are placed within and which can

withstand the pressure developed during an internal explosion of an ex-

plosive mixture, and which prevents the transmission of the explosion

to the explosive gas atmosphere surrounding the enclosure.

FOUNDATION™ ﬁeldbus

An all digital, serial, two-way communications system that serves as

the base-level network in a plant or factory automation environment.

Developed and administered by the ﬁeldbus FOUNDATION™.

GOVI

‘Gross Observed Volume of the Interface’ – The total volume of the tank

occupied by the interface liquid. The GOVI is only given when mea-

suring two liquids and is calculated by subtracting the volume of the

product from the total volume of liquid in the tank (GOVT – GOVP).

GOVP

‘Gross Observed Volume of the Product’ – The total volume of the tank

occupied by the product liquid. When measuring only one liquid, it is

also the total volume of liquid in the tank (GOVT). When measuring two

liquids it is the total volume of liquid in the tank minus the volume of

the interface liquid (GOVT – GOVI).

GOVT

‘Total Gross Observed Volume’ – The total volume of liquid in the tank.

When measuring only one liquid it is equal to the volume of the product

(GOVP). When measuring two liquids it is equal to the volume of the

product and interface liquids (GOVP + GOVI).

GOVU

‘Gross Observed Volume Ullage’ – the difference in volume between the

working capacity of a tank and the total volume in the tank (Working

Capacity – GOVT).

HART®

A Bidirectional communication protocol that provides data access

between intelligent ﬁeld instruments and host systems.

Interface

Noun; The measurement of the level of one liquid when that liquid is

below another liquid.

Interface

Adj.; The Software Graphical User Interface (GUI) that allows the user

to access software protocols (HART, DDA, MODBUS).

Intrinsic safety

‘Intrinsically safe’ - Type of protection based on the restriction of

electrical energy within apparatus of interconnecting wiring exposed to

potentially explosive atmosphere to a level below that which can cause

ignition by either sparking or heating effects.

DDA Interface Manual

Mass

The property of a body that causes it to have weight in a gravitational

ﬁeld, calculated by density at the reference temperature multiplied by

the volume correction factor (Density * VCF).

MODBUS

A serial communications protocol published by Modicon in 1979 for use

with its programmable logic controllers (PLCs). It has become a de fac-

to standard communications protocol in industry, and is now the most

commonly available means of connecting industrial electronic devices.

NEMA Type 4X

A product Enclosure intended for indoor or outdoor use primarily to

provide a degree of protection against corrosion, windblown dust and

rain, splashing water, and hose-directed water; and to be undamaged by

the formation of ice on the enclosure. They are not intended to provide

protection against conditions such as internal condensation or internal

icing.

NPT

U.S. standard deﬁning tapered pipe threads used to join pipes and

ﬁttings.

NSVP

‘Net Standard Volume of the Product’ – The temperature corrected

volume for the product liquid in the tank, requires the transmitter to

be ordered with temperature measurement capabilities. The NSVP is

calculated by multiplying the volume of the product liquid by a volume

correction factor based on temperature (GOVP * VCF).

Reference Temperature

The temperature at which the density measurement is given, the allow-

able values are 32 °F to 150 °F (0 °C to 66 °C).

Speciﬁc Gravity

The density ratio of a liquid to the density of water at the same condi-

tions.

Sphere Radius

The internal radius of the sphere that contains the liquid, the value is

used to calculate the volume along with the Sphere Offset.

Sphere Offset

An offset value that accounts for additional volume in a sphere from

non-uniform sphere geometry, the value is used to calculate the volume

along with the Sphere Radius.

Strap Table

A table of measurement correlating the height of a vessel to the volume

that is contained at that height. The transmitter can contain up to 100

points.

TEC

‘Thermal Expansion Coefﬁcient’ - a value correlating the change in tem-

perature for an object with the change in its volume. Allowable values

are 270.0 to 930.0. TEC units are in 10 E-6/Deg F.

Temperature Correction Method

One of ﬁve product correction methods used to correct the product

volume in the tank due to changes in temperature from 60 °F including

(6A, 6B, 6C, 6C Mod, and Custom Table.

Volume Calculation Mode

One of two methods use to calculate volume measurements from level

measurements, including Sphere and Strap Table.

VCF

‘Volume Correction Factor’ – A table of measurements correlating

temperature points with correction factors for the liquids expansion/

contraction. The transmitter can contain up to 50 points.

Working Capacity

The maximum volume of liquid that the user desires for their vessel to

hold, typically 80% of the vessels maximum volume before overﬁll.

DDA Interface Manual

2. Introduction

2.1 Purpose and use of this manual

The content of this technical documentation and of its various

annexes is intended to provide information on mounting, installation

and commissioning by qualiﬁed service personnel according to

IEC 60079-14 and/or factory trained service technicians and local

regulations.

2.2 Used symbols and warnings

Warnings are intended for your personal safety and for avoidance

of damage to the described product or connected devices. In this

documentation, safety information and warnings to avoid dangers that

might affect the life and health of personnel or cause material damage

are highlighted by the preceding pictogram, which is deﬁned below.

3. Safety instructions

3.1 Intended use

The purpose of this document is to provide detailed information on

the protocol interface. All safety related information is in the product

speciﬁc operation manual. Consult the operation manual before

connecting to the level transmitter.

4. Quick start-up guide

4.1 Before you begin

Note:

You must use a RS-485 converter with “Send Data Control” and the

Set-up Software to ensure proper operation.

Example:

B & B Electronics 485BAT3 (815-433-5100 www.bb-elec.com).

FTDI USB-RS485-WE-1800-BT (www.ftdichip.com)

4.2 Quick start-up procedure

1. Connect +24 Vdc to terminals.

2. Connect data lines to terminals.

3. Connect the PC (or other device) to data lines.

(If you are using a PC, use a RS-232 to RS-485 converter. See

Note above for more information.)

4. Turn on power to the transmitter.

5. Start the Dashboard. Select Com Port and Address. Factory

default address is ‘192 ’ for DDA.

Before starting the operation of the equipment read this documentation

thoroughly and follow the safety information.

Symbol Meaning

NOTICE This symbol is used to point to situations

that may lead to material damage and/or personal

injury.

6. Change the address to one that is suitable for the installation

network.

7. Verify proper operation of product and or interface ﬂoats and

temperature.

8. Turn off power to the transmitter.

9. Remove data lines.

10. Install the transmitter into the vessel (see Operation Manual).

11. Reconnect power and data lines.

12. Calibrate current tank level using setup software (optional).

13. Verify communications with the host system.

Default communication parameters

DDA: 4800 BAUD 8, E, 1

5. Display Menu

All liquid level transmitters are shipped with a Stylus (Kenco Part #

79857) to be used for manipulating the display. For single and dual

cavity housings, the Stylus is designed to allow for programming of the

unit without removing the housing. When using the Stylus make sure

to align the Stylus with the shape outline around the buttons in the same

orientation. Failure to correctly align the Stylus can cause the display to

not function properly.

Note:

Do not use any device other than the Stylus to operate the display.

Note:

Improper use of the Stylus can cause the display to not function

properly.

5.1 Operation Modes

The level transmitter runs in one of the following modes of operation.

You can use these modes to calibrate and set up various operating

parameters.

5.1.1 Run Mode

Run mode is the primary mode of operation. This mode will perform

measurements, display data, and respond to DDA commands.

5.1.2 Program Mode

Program mode is the primary mode for commissioning and trouble-

shooting the level transmitter. The full menu and available functions are

shown in section 5.3 Menu Structure. To enter program mode use the

Stylus and press the Enter Key as shown in section 5.2 Display Diagram.

Program Mode is protected by a password to keep unwarranted chan-

ges from occurring. The factory default password is 27513. When in

program mode, remote communications are not functional. An automa-

tic timeout feature is provided so that the transmitter does not remain

inadvertently in program mode. The timeout is set for 1 minute before

prompted for additional time. Total timeout is 2 minutes.

DDA Interface Manual

Note:

Whenever program mode is exited from the display the unit will reset

itself to insure all changes have been accepted. The reset will take

approximately 5 seconds before the level transmitter is able to respond

to commands.

Note:

In program mode, the transmitter will not respond to incoming DDA

commands. A busy error will be sent to the controller to notify the

unit is in program mode. This function will prevent a user at a remote

terminal from programming the unit while a user is accessing program

mode from the display.

5.2 Display Diagram

UP Arrow – Used to move cursor on screen up and to increment num-

ber.

DOWN Arrow – Used to move cursor on screen down and to decrease

number.

SCROLL Arrow – Used to move cursor on screen to the right, cursor will

cycle back.

ENTER Key – Used to Enter Program Mode, select Highlighted Item, and



EXIT Key – Hidden key in the middle of the display that is used to exit

menu at any time.

MEASURED VARIABLE – The process variable that is selected to display.

The display will automatically scroll between selected variables.

MEASUREMENT – The numerical value for the MEASURED VARIABLE

shown on the display.

UNITs – Unit of measurement for the MEASURED VARIABLE shown on

the display.

TEMPERATURE – Average temperature for the product in the tank. Only

shown if the level transmitter was purchased with temperature.

NOTIFICATIONS – Four squares with letters. Top left square will always

be present indicating a D for DDA mode or a M for Modbus mode. Top

right square, A, will only show when there is an alarm. Toggle the UP

Arrow key to view alarms. Bottom right square, F, will only show when

there is a fault. Toggle the DOWN Arrow key to view error codes. Bot-

tom left square, P, will only show when the units is being programmed

remotely.

5.3 Menu Structure

• Data From Device

Display

Units

`Length Units

`Temp Units

Address

Signal Strength

`Prod Trig Lvl

`Int Trig Lvl

• Calibrate

Product Level

`Current Level

`Offset

Interface Level

`Current Level

`Offset

• Factory

Settings

`Gradient

`Serial Number

`HW Revision

`SW Revision

`SARA Blanking

`Magnet Blanking

`Gain

`Min Trig Level

Temp Setup



Auto Threshold

Reset to Factory

6. Alarms

The DDA output is equipped with several alarms that will be shown on

the display. Use the stylus to tap the Up arrow to view alarms. The

DDA output is set to fail high, greater than the order length, if there is an

issue and the level value output cannot be trusted.

Fig. 1: DDA Display

DDA Interface Manual

101 Missing

Magnet





• Verify Float(s) are not in inactive zone.

• Verify Auto Threshold is enabled.

• Cycle power to sensor. If proper operation

is not restored, Contact Factory.

102 Internal

Fault 1

Cycle power to sensor. If proper operation

is not restored, Contact Factory.

103 Internal

Fault 2

Cycle power to sensor. If proper operation

is not restored, Contact Factory.

104 Internal

Fault 3

Cycle power to sensor. If proper operation

is not restored, Contact Factory.

105 Lobe Fault 1 • Verify Auto Threshold is enabled.

• Cycle power to sensor.

• If proper operation is not restored, Contact

Factory.

106 Lobe Fault 2 • Verify Auto Threshold is enabled.

• Cycle power to sensor.

• If proper operation is not restored,

Contact Factory.

107 Delta Fault Contact Factory to discuss application.

108 Internal

Fault 4

Cycle power to sensor. If proper operation

is not restored, Contact Factory.

109 Peak Fault • Verify Auto Threshold is enabled.

• Cycle power to sensor.

• If proper operation is not restored,

Contact Factory.

110 Hardware

Fault 1

Cycle power to sensor. If proper operation

is not restored, Contact Factory.

111 Power Fault • Cycle power to sensor.

• Verify Power Supply rating.

• Verify wiring.

• If proper operation is not restored,

Contact Factory.

112 Hardware

Fault 2

Cycle power to sensor. If proper operation

is not restored, Contact Factory.

113 Hardware

Fault 3

Cycle power to sensor. If proper operation

is not restored, Contact Factory.

114 Hardware

Fault 4

Cycle power to sensor. If proper operation

is not restored, Contact Factory.

115 Timing

Fault 1

Cycle power to sensor. If proper operation

is not restored, Contact Factory.

116 Timing

Fault 2

Cycle power to sensor. If proper operation

is not restored, Contact Factory.

117 Timing

Fault 3

Cycle power to sensor. If proper operation

is not restored, Contact Factory.

118 DAC Fault 1 Cycle power to sensor. If proper operation

is not restored, Contact Factory.

119 DAC Fault 2 Cycle power to sensor. If proper operation

is not restored, Contact Factory.

120 DAC Fault 3 Cycle power to sensor. If proper operation

is not restored, Contact Factory.

Fault

Code

Description Corrective Action

Fault

Code

Description Corrective Action

121 DAC Fault 4 Cycle power to sensor. If proper operation

is not restored, Contact Factory.

122 SPI Fault 1 Cycle power to sensor. If proper operation

is not restored, Contact Factory.

123 SPI Fault 2 Cycle power to sensor. If proper operation

is not restored, Contact Factory.

124 Setpoint

Fault

The analog setpoints are too close. Mini-

mum distance is 150 mm (6 in.) for analog

and 290 mm (11.5 in.) for SIL. Adjust

programmed setpoints as needed. (Analog

only) If proper operation is not restored,

Contact Factory.

125 Loop 1 Out

of Range

Verify that magnets are positioned within

expected measuring range. Adjust program-

med setpoints as needed. (Analog only) If

proper operation is not restored, Contact

Factory.

126 Loop 2 Out

of Range

Verify that magnets are positioned within

expected measuring range. Adjust program-

med setpoints as needed. (Analog only) If

proper operation is not restored, Contact

Factory.

127 EEPROM

Fault 1

Cycle power to sensor. If proper operation

is not restored, Contact Factory.

128 EEPROM

Fault 2

Cycle power to sensor. If proper operation

is not restored, Contact Factory.

129 Flash Failure Cycle power to sensor. If proper operation

is not restored, Contact Factory.

130 Internal

Error

Cycle power to sensor. If proper operation

is not restored, Contact Factory.

7. Error Codes (Faults)

8. DDA Interface

8.1 Data line termination and biasing

Termination and biasing of RS-485 data lines are as follows:

Biasing

Each transmitter uses a Fail-safe, Slew Rate-Limited RS-458/RS-422

Transceiver. No additional biasing resistors should be present on the

connecting devices (PLC, DCS, PC, Converter).

Termination

Each transmitter uses a Fail-safe, Slew Rate-Limited RS-

458/RS-422 Transceiver. No additional termination resistors are

necessary in the connecting devices (PLC, DCS, PC, Converter).

8.2 Communication parameters

The 2-wire differential communication interface and all data transmissi-

ons must be at half duplex. Only one device (either the master or a

single transmitter) can transmit data at any given time. BAUD rate

limitations are listed below.

DDA: 4800 BAUD 8, N, 1

(Reference) Monitor: DDA RTU Variable BAUD Rate 8, E, 1

DDA Interface Manual

9. Hardware and software environments

The DDA digital transmitter operates in a networked, intrinsically safe

RS-485 DDA software environment. This environment supports up to

8 multi-dropped transmitters on one communication line. The network

requires a 4-wire bus to provide both power and communications to

each of the transmitters located in the hazardous area. The transmitters

are connected in multi-point conﬁguration (see Figure 2).

The RS-485 network operates in a master/slave mode where the master

(host computer or similar type network controller) interrogates each

slave (DDA transmitter) for a speciﬁc type of data. Each slave has a

. . .

+24 Vdc

power

supply

RS-485

Examples:

PC, PLC, DCS

I.S.B.*

Approved intrinsic

safety barrier

(3 required)

General application

4-wire bus

Intrinsically

safe area

Maximum stub

length = 200 feet

Up to 8

transmitters

(DDA interface)

unique switch programmable hardware address that is issued by the

host computer to activate a particular transmitter. In addition, the DDA

hardware supports a command decoder that supports up to 128 different

commands. The host computer interrogates a transmitter for data by

sending an address byte, followed immediately by a command byte. The

addressed transmitter will ‘wake up’, identify itself by transmitting an

echo of its own local address followed by the received command, and

then perform the requested action. After the requested action has been

completed, the data (if any) will be transmitted back to the host computer

on the RS-485 network. Refer to Section 10 for ‘DDA Command decoder

examples’ and for more information.

10. DDA command decoder examples

10.1 Serial data transmission format

Example 1:

0X X X X X X X X P1

D1 bit Stop bit

Start bit Parity bit

D8 bit

After the DDA address decoder circuitry receives the 11-bit word, an

even parity check is performed across the 8-bit data ﬁeld. If a parity error

is found, the word is ignored and the decoder circuitry resets for the next

transmission. If the parity check is good, the decoder circuitry checks for

a valid address byte. The address decoder circuitry uses the ‘D8’ bit to

distinguish the difference between address bytes and command bytes.

Address bytes are deﬁned as having the most signiﬁcant bit ‘D8’ set equal

to one. Valid address byte values include ‘C0’ hex to ‘FD’ hex (192 to

253 decimal). Address byte values from 80 hex to ‘BF’ hex are reserved

for future use, address byte values ‘FE’ and ‘FF’ hex are reserved for test

functions. (see Example 2).

Fig. 2: Typical Electrical Connections - Intrinsically Safe System

DDA Interface Manual

10.2 Address byte

Example 2:

X X X X X X X 1

D8 bit = 1

(8-bit word - shown as D1 bit)

If the received address byte matches the local DDA address, the DDA

power supply circuitry is activated. If a valid address byte has been

found, the decoder circuitry checks to see if the next received word is a

command byte. Valid command byte values include ‘00’ hex to ‘7F’ hex

(0 to 127 decimal). In addition, all data byte values are restricted to be

within ‘00’ hex and ‘7F’ hex (see Example 3).

10.3 Command byte (and data bytes)

Example 3:

X X X X X X X 0

D8 bit = 1

(8-bit word - shown as D1 bit)

Again, an even parity check is performed on the command byte. If the

parity check is good, the eight bit data word is latched into a command

buffer. This buffer is read by the DDA software to determine which

command to execute. If the parity check fails, the command byte

is rejected and the old command (from the previous interrogation

sequence) will be left in the command buffer. The DDA hardware cannot

determine if the current command was possibly rejected. The host

computer must then verify if the correct command was received by

reading the echo of the address byte and command byte sent by the DDA

transmitter. This is the only guaranteed way to determine that both the

address and command bytes were received properly.

This method also insures proper veriﬁcation, even if the parity check fails

to detect a multiple bit data error in either the address byte or command

byte. If the host computer determines that either the address byte or

command byte has been corrupted, it must wait the proper time-out

period and ignore the received message from the DDA transmitter that

was improperly interrogated. The time-out period is variable and is based

on the duration of the selected DDA command.

11. DDA/Host computer communication protocol

The DDA/Host computer communication protocol consists of two parts:

the interrogation sequence generated by the host computer and the

data response generated by the interrogated DDA transmitter. The host

interrogation sequence always consists of an address byte followed

immediately by a command byte (see Example 4).

11.1 DDA/Host communication

Example 4:

00 Hex to 7F Hex (0 to 127 decimal)

C0 Hex to FD Hex (192 to 253 decimal)

The maximum delay between the address byte and the command byte is

5 milliseconds. The DDA transmitter will not receive the new command

byte if this delay period is exceeded (and the old command byte will

be left in the command buffer). See previous section for additional

information on veriﬁcation of the Address/Command bytes. An example

of an interrogation sequence to access a transmitter programmed for

address ‘F0’ hex (see Example 5).

The transmitter response consists of several components. After a

transmitter has been interrogated, the transmitter ﬁrst responds by

transmitting its own local address and the command that was received

from the host computer. This re-transmission of the transmitter address

and received command serves two purposes. The ﬁrst being a simple

identiﬁcation that the correct transmitter received the correct command

and that it is currently active. The second purpose is to reset the DDA

Address/Command decoder circuitry for the next interrogation sequence.

11.2 Interrogation data sequence

Example 5:

Command 0A Hex (10 decimal)

Address F0 Hex (240 decimal)

Note:

If the DDA transmitter does not respond to the ﬁrst interrogation by

the host, the Address/Command decoder will be left in an intermediate

state. If this occurs, the host will have to reinterrogate the respective

transmitter to reset the Address/Command decoder circuitry and then

interrogate the respective transmitter again to perform a new transmit-

ter measurement. This hardware feature must be considered when writ-

ing software communication drivers to access DDA transmitter data.

DDA Interface Manual

After the DDA transmitter has retransmitted its local address and

received command, it will perform the requested measurement as

deﬁned by the received command. After the requested measurement has

been completed, the data for that measurement will be transmitted to

the host in a predeﬁned format including certain control characters. The

DDA transmitted data format begins with a ‘start of text’ ‘STX’ character

(STX = 02 hex). The ‘STX’ character set is immediately followed by

the requested data and then terminated with an ‘end of text’ ‘ETX’

character set (ETX = 03 hex). Certain commands allow multiple data

ﬁelds to be transmitted within one transmitted data sequence. For these

data transmissions, each data ﬁeld is separated by an ASCII colon ‘:’

character (: = 3A hex), (see Examples 6 and 7).

Single ﬁeld data transmission

Example 6:

Multiple ﬁeld data transmission

Example 7:

All transmitted data will consist of 7-bit ASCII characters limited to hex

values between ‘00’ hex and ‘7F’ hex (i.e. data bit D8 = 0).

After a DDA transmitter has completed a data transmission, the

host must wait 50 milliseconds before another interrogation can

be performed. This delay is required to enable the previously

interrogated transmitter to go into sleep mode and release the network

communication lines.

All DDA control commands support a checksum calculation function,

Data Error Detection (DED) that allows the host computer (master) to

check the integrity of the transmitted data. The actual checksum value

that is transmitted is the compliment (2’s compliment) of the calculated

value. The checksum scheme is based on a 16-bit summation of the hex

data within the transmitted block (including ‘STX’ and ‘ETX’ character

sets) without regard to overﬂow. The two byte result of the adding

process is then complimented and appended to the transmitted data

block.

This compliment process makes the ﬁnal checksum comparison more

efﬁcient in that the checksum result added to its compliment will always

result in a zero sum for uncorrupted data transmissions. Checksum

data (two hex bytes) can range from ‘0000’ hex to ‘FFFF’ hex. Since the

communication network only allows transmitted data values between

‘00’ and ‘7F’ hex, special processing is required on the hex checksum

value before it can be transmitted.

This two byte hex value must ﬁrst be converted to numeric (decimal)

ASCII characters before transmission. For example, a checksum value

of ‘FFFF’ hex would be transmitted as ASCII 65535. The host computer

would then have to convert ASCII 65535 back to FFFF hex and perform

its own checksum calculation and comparison for the received data

from the DDA transmitter. An example is shown (see Example 8) of a

single ﬁeld data transmission including checksum data and an sample

checksum calculation.

Example 8:

Append checksum value

Note:

The appended checksum value will always consist of ﬁve decimal (AS-

CII) characters ranging from 00000 to 65535. The checksum function

can be enabled or disabled.

Message transmitted from DDA transmitter (command 12 Hex):

<STX><265.322.109.456><ETX>64760

Hex character equivalent of transmitted data record including <STX>

and <ETX> characters:

02, 32, 36, 35, 2E, 33, 32, 32, 3A, 31, 30, 39, 2E, 34, 35, 36, 03

Two byte Hex summation of data: 0308 Hex

Two’s compliment: FCF8 Hex

Convert to decimal ASCII: 64760

To verify transmitted data from the DDA transmitter, perform the two byte

Hex summation over the data record (including ‘<STX>’ and ‘<ETX>’)

shown in (Example 8). The result in this example is 0308 Hex. Then

convert the decimal ASCII checksum value back to Hex (for example,

64760 to FCF8 Hex). Add the Hex summation value to the Hex checksum

value and the result will be zero (disregarding overﬂow) for uncorrupted

data. 0308 Hex + FCF8 Hex = 0000 Hex.

Note:

Cyclic Redundancy Check (CRC) error checking will be offered at a

later date. A command switch will be deﬁned that will let the DDA data

be transmitted with CRC error checking instead of checksum error

checking. The checksum calculations will use the CRC-CCITT deﬁned

polynomial with a 16-bit CRC result. This 16-bit CRC value will be

appended to each transmitted message. Since the communication net-

work only allows transmitted data values between 00 and 7F hex, spe-

cial processing is required on the 16-bit hex CRC value before it can be

transmitted. This 16-bit (two byte) hex value must ﬁrst be converted to

numeric (decimal) ASCII characters before transmission. For example,

a checksum value of ‘FFFF’ hex would be transmitted as ASCII 65535.

Checksum Calculation

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