Lumel KD8 User manual

SCREEN RECORDER
KD8 TYPE
MODBUS TRANSMISSION PROTOCOL
SERVICE MANUAL

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CONTENTS
page
1. APPLICATION.............................................................................................2
2. DESCRIPTION OF THE MODBUS PROTOCOL...................................2
2.1. ASCII framing............................................................................................3
2.2. RTU framing..............................................................................................3
2.3. Characteristic of frame fields.....................................................................4
2.4. LRC checking.............................................................................................5
2.5. CRC checking ............................................................................................5
2.6. Character format during serial transmission ..............................................5
2.7. Transaction interruption.............................................................................5
3. DESCRIPTION OF FUNCTIONS............................................................6
3.1 Readout of N-registers (Code 03) ................................................................6
3.4. Report identifying the device (Code 17)......................................................6
4. ERROR CODES............................................................................................7
5. TABLE OF REGISTERS.............................................................................9
APPENDIX A. CALCULATION OF THE CHECKSUM.............................11

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1. APPLICATION
In order to obtain the information exchange, when using the serial link, one must choose
the interface type and validate the interpretation way of transmitted data. The interface type defines
only electrical transmission parameters and the way of the device connection.
Such features, as the possibility to service several devices, check the transmission correctness
and the principles of access to the device, depend on the data interpretation.
The task of the protocol is to define which data type is interpreted (permitted) and in which way
they are interpreted.
AMODBUS asynchronous character transmission protocol has been implemented on the serial link
of the KD8 recorder. The parameter configuration of the RS-485 serial link is described in the KD8
recorder user’s manual.
Parameter set of the KD8 recorder serial link:
•Recorder address 1 ...247
• Baud rate 300, 600, 1200, 2400, 4800, 9600, 14400, 19200, 28800,
38400, 57600, 115200 bit/s
• Working mode ASCII, RTU
• Information unit ASCII: 8N1, 7N2, 7E1, 701
RTU:8N2, 8N1, 8E1,8O1
• Maximal turnaround time 100 ms
2. DESCRIPTION OF THE MODBUS PROTOCOL
The MODBUS interface is a standard adopted by manufacturers of industrial controllers for the
asynchronous character exchange of information between different devices and measuring systems.
It has such features as:
• Simple access rule to the link based on the “master-slave" principle,
• Protection of transmitted messages against errors,
• Confirmation of remote instruction realization and error signaling,
• Effective actions protecting against the system suspension,
• Taking advantage of the asynchronous character transmission.
Device controllers working in the MODBUS protocol can communicate with each other, taking
advantage of the master-slave protocol type, in which only one device (the master - superior unit)
can originate transactions (called ’’queries"), and others (slaves - subordinate units) respond only to
the remote requested data from the master. The transaction is composed of the transmitted
command from the master unit to the slave unit and of the response transmitted in the opposite
direction. The response includes data requested by the master or the confirmation of the command
realization.
Master can transmit information to individual slaves or broadcast messages destined for all
subordinate devices in the system (responses are not returned to broadcast queries from the master)
The format of transmitted information is as following:
• master => slave: device address, code representing the required command, data to be sent,
control word protecting the transmitted message,
• slave => master: sender address, confirmation of the command realization, data required by
the master, control word protecting the response against errors.
If the slave device detects an error when receiving a message, or cannot realizes the command, it
prepares a special message about the error occurrence and transmits it as a response to the master.
Devices working in the MODBUS protocol can be set into the communication using one of two
transmission modes: ASCII or RTU. The user chooses the required mode, along with the serial

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port communication parameters (baud rate, information unit) during the configuration of any
device.
In the MODBUS system, transmitted messages are placed into frames that are not related to serial
transmission. These frames have a defined beginning and end. This enables for the receiving device
to reject incomplete frames and to signal related errors with them.
Taking into consideration the possibility to operate in one of these two different transmission modes
(ASCII or RTU), two frames have been defined.
Explanation of some abbreviations:
ASCII = American Standard Code for Information Interchange
RTU = Remote Terminal Unit
LRC = Longitudinal Redundancy Check
CRC = Cyclic Redundancy Check CR = Carriage Return LF = Line-Feed (character)
MSB = Most Significant Bit
Checksum = Control Sum
2.1. ASCII framing
In the ASCII mode, each byte of information is transmitted as two ASCII characters. The basic
feature of this mode is that it allows to long intervals between characters within the message
(to 1 sec) without causing errors.
A typical message frame is shown below.
Start beginning
index Address Function Data LRC
check End index
1 char
“:” 2 chars 2 chars n chars 2 chars 2 chars CR LF
In ASCII mode, messages start with a colon character (“:” -ASCII 3Ah) and end with CR and LF
characters. The frame information part is protected by the LRC code (Longitudinal Redundancy
Check).
2.2. RTU Framing
In RTU mode, messages start and end with an interval lasting minimum 3.5 x (lasting time of
a single character), in which a silence reigns on the link.
The simplest implementation of the mentioned time interval character times is a multiple measure
of the character duration time at the set baud rate accepted on the link.
The frame format is shown below:
Start beginning
index Address Function Data CRC check End index
T1-T2-T3-T4
8 bits 8 bits nx 8bits 16 bits T1-T2-T3-T4

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Start and end indexes are marked symbolically as an interval equal to four lengths of the index
(information unit). The checking code consists of 16 bits and emerges as the result of CRC
calculation (Cyclical Redundancy Check) of the frame contents.
2.3. Characteristic of frame fields.
Address field
The address field of a message frame contains two characters (in ASCII mode) or eight bits
(in RTU mode).
Valid slave device addresses are in the range from 0 -247 . The master addresses the slave unit by
placing the slave address in the frame address field. When the slave sends its response, it places its
own address in the frame address field what enables the master to check which slave is responding.
The 0 address is used as a broadcast address recognized by all slave units connected to the bus.
Function field
The function code field of a message frame contains two characters in ASCII mode or eight bits in
RTU mode. Valid codes are in the range from 1 - 255.
When a message is sent from a master to a slave device, the function code field tells the slave what
kind of action to perform.
When the slave responds to the master, the function field is uses to confirm the command execution
or error signaling if the function code field cannot realize the command for any reason. to indicate
either a normal (error-free) response or that some kind of error occurred.
The positive confirmation is realized through the placement of the command execution code on the
function field.
In case of an error assertion, the slave returns a special code that is equivalent to the original
function code with its most significant logic 1.
The error code is placed on the data field of the response frame.
Data field
The data field is constructed using sets of two hexadecimal digits, in the range of 00 to FF.
These numbers can be made from a pair of ASCII characters or from one RTU character, according
to the network's serial transmission mode. The data field of messages sent from a master to slave
devices contains additional information which the slave must use to take the action defined by the
function code. This can include items like register addresses, number of bytes in data field, data,
a.s.o. The data field can be non-existent (of zero length) in certain kinds of frames. That occurs
always, when the operation defined by the code does not require any parameters.
Error checking field
Two kinds of error-checking methods are used for standard MODBUS networks. The error
checking field contents depends upon the method that is being used.
ASCII
When ASCII mode is used for character framing, the error checking field contains two ASCII
characters. The error check characters are the result of a Longitudinal Redundancy Check (LRC)
calculation that is performed on the message contents (without the beginning ’’colon" and
terminating CRLF characters). LRC characters are appended to the message, as the last field
preceding the CR, LF characters.
RTU
When RTU mode is used for character framing, the error checking field contains a 16-bit value
implemented as two 8-bit bytes. The error check value is the result of a Cyclical Redundancy Check
Calculation (CRC) performed on a message contents. The CRC field is appended to the message as
the last field in the message. When this is done, the low-order byte of the field is appended first,
followed by the high-order byte. The CRC high-order byte is the last byte to be sent in the message.

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2.4. LRC checking
The LRC is calculated by adding together successive 8-bit bytes of the message, discarding any
carries, and then two is complementing the result. It is performed on the ASCII message field
contents excluding the “colon” character that begins the message, and excluding the CR, LF pair at
the end of the message. The 8-bit value of the LRC sum is placed at the frame end as two ASCII
characters, first the character containing the higher tetrad, and after it, the character containing the
lower LRC tetrad.
2.5. CRC checking
The generating procedure of CRC is realized according to the following algorithm:
1. Load a 16-bit register with FFFFh. Call this the CRC register.
2. Exclusive OR (XOR) the first 8-bit byte of the message with the low-order byte of
the 16 bit CRC register, putting the result in the CRC register.
3. Shift the CRC register contents one bit to the right (towards the LSB), zero-filling
the MSB. Extract and examine the LSB.
4. (If the LSB was O): Repeat step 3 (another shift) (If the LSB was 1): XOR
the CRC register with the polynomial value A001h.
5. Repeat steps 3 and 4 until 8 shifts have been performed. When this is done,
a complete 8-bit byte will have been processed.
6. Repeat steps 2 through 5 for the next 8-bit byte of the message.
Continue doing this until all bytes have been processed.
7. The final contents of the CRC register is the CRC value.
8. When the CRC is placed into the message, its upper and lower bytes
must be swapped as described below.
2.6. Character format during serial transmission
In the MODBUS protocol, characters are transmitted from the lowest to the highest bit.
Organization of the information unit in the ASCII mode:
•1 start bit,
•7 data field bits,
• 1 even parity check bit (odd) or lack of even parity check bit,
• 1 stop bit at even parity check or 2 stop bits when lack of even parity check.
Organization of the information unit in the RTU mode:
•1 start bit,
•8 data field bits,
• 1 even parity check bit (odd) or lack of even parity check bit,
• 1 stop bit at even parity check or 2 stop bits when lack of even parity check.
2.7. Transaction interruption
In the master unit the user sets up the important parameter which is the ’’maximal response time on
the query frame" after exceeding of which, the transaction is interrupted. This time is chosen such
that each slave unit working in the system (even the slowest) normally will have the time to answer
to the frame query. An exceeding of this time attests therefore about an error and such is treated by
the master unit.
If the unit slave will find out a transmission error it does not accomplish the order and does not send
any answer. That causes an exceeding of the waiting time after the query frame and the transaction
interruption.

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3. DESCRIPTION OF FUNCTIONS
In the KD8 recorder following protocol functions has been implemented:
Code Signification
03 Reading of n-register
17 Slave device identification
3.1. Reading of n-registers (code 03)
Request:
The function enables the reading of values included in registers in being addressed slave device.
Registers are 16 or 32-bit units, which can include numerical values bounded with changeable
processes, and the like. The request frame defines the 16-bit start address and the number of
registers to read-out.
The signification of the register contents with address data can be different for different device
types.
The function is not accessible in the broadcast mode.
Example: Reading of 3 registers beginning by the register with the 6Bh address.
Address Function Register
address Register address Number of registers Number of registers Checksum
Hi Lo Hi Lo
11 03 00 6B 00 03 7E LRC
Answer:
Register data are packing beginning from the smallest address: first the higher byte, then the lower
register byte.
Example: the answer frame
Address
Function
Number
Value
Value
Value
Value
Value
Value
Checksum
of bits in the
in the
in the
in the
in the
in the
regist
107 regist 107 regist 108 regist 108 regist 109 regist 109
Hi Lo Hi Lo Hi Lo
11 03 06 02 2B 00 00 00 64 55 LRC
3.2. Report identifying the device (code 17)
Request:
This function enables the user to obtain information about the device type, status and configuration
depending on this.
Example
Address Function Checksum
11 11 DE LR
C
Answer:
The field ,,Device identifier" in the answer frame means the unique identifier of this class of device,
however the other fields include parameters depended on the device type.

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Example concerning the KD8 recorder
Slave address
Function Number of
bytes Device identifier Device state Checksum
11 11 02 B2 FF 48 1F
4. ERROR CODES
When the master device is broadcasting a request to the slave device then, except for messages in
the broadcast mode, it expects a correct answer. After sending the request of the master unit, one of
the four possibilities can occur:
• If the slave unit receives the request without a transmission error and can execute it correctly, then
it returns a correct answer,
• If the slave unit does not receive the request, no answer is returned. Timeout conditions for the
request will be fulfilled in the master device program.
• If the slave unit receives the request, but with transmission errors (even parity error of checking
sum LRC or CRC), no answer is returned. Timeout condition for the request will be fulfilled in the
master device program.
• If the slave unit receives the request without a transmission error but cannot execute it correctly
(e.g. if the request is, the reading-out of a non-existent bit output or register), then it returns the
answer including the error code, informing the master device about the error reason.
A message with an incorrect answer includes two fields distinguishing it from the correct answer.
1. The function code field:
In the correct answer, the slave unit retransmits the function code from the request message in the
field of the answer function code. All function codes have the most-significant bit (MSB) equal zero
(code values are under 80h). In the incorrect answer, the slave unit sets up the MSB bit of the
function code at 1. This causes that the function code value in the incorrect answer is exactly of 80h
greater than it would be in a correct answer.
On the base of the function code with a set up MSB bit the program of the master device can
recognize an incorrect answer and can check the error code on the data field.
2. The data field:
In a correct answer the slave device can return data to the data field (certain information required by
the master unit). In the incorrect answer the slave unit returns the error code to the data field.
It defines conditions of the slave device which had produced the error. An example considering
a request of a master device and the incorrect answer of the slave unit has been shown below. Data
are in the hexadecimal shape.
Example: request
Slave Function Variable Variable Number Number Checksum
address address address Of Of
H1 Lo Variables Variables
Hi Lo
OA 01 04 A1 00 01 4F LRC
Example: incorrect answer
Slave Function Error Checksum
OA 81 02 73 LRC
In this example the master device addresses the request to the slave unit with No 10 (OAh).
The function code (01) serves to the read-out operation of the bit input state. Then, this frame
means the request of the status read-out of a one bit input with the address number: 1245 (04A1h).

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If in the slave device there is no bit input with the given address, then the device returns
the incorrect answer with the No 02 error code. This means a forbidden data address in the slave
device.
Possible error codes and their meanings are shown in the table below.
Code Meaning
01 Forbidden function
02 Forbidden data address
03 Forbidden data value
04 Damage in the connected device
05 Confirmation
06 Occupied, message removed
07 Negative confirmation
08 Error of memory parity

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5. Table of registers
• KD8 recorder identifier ( set as a respons to the identification function) : 0xB2
▪Register types („Typ” kolumn):
float – floating point number (see the description below),
sfloat – floating point number (see the description below).
▪Access modes to register:
RO – only for readout.
• Representation of floating point numbers ( float IEEE 754)
byte: 4 byte 3 byte 2 byte 1
SEEEEEEE EMMMMMMM MMMMMMMM MMMMMMMM
S – character bit (Sign bit)
E – exponent
M – mantissa
Register bytes of float type are sent in 4321 sequence
Register bytes of sfloat type are sent in 2143 sequence
PROCESSING DATA
Addresses of
16-bit addressed
registers
Word type
Description
5000
Alarm states:
Bit 0 – alarm 1 of analog input 1 (AL1 - alarm output 1)
Bit 1 – alarm 2 of analog input 1 (AL2 - alarm output 2)
Bit 2 – alarm 1 of analog input 2 (AL3 - alarm output 3)
Bit 3 – alarm 2 of analog input 2 (AL4 - alarm output 4)
Bit 4 – alarm 1 of analog input 3 (AL5 - alarm output 5)
Bit 5 – alarm 2 of analog input 3 (AL6 - alarm output 6)
Bit 6 – alarm 1 of analog input 4 (AL7 - alarm output 7)
Bit 7 – alarm 2 of analog input 4 (AL8 - alarm output 8)
Bit 8 – alarm 1 of analog input 5 (AL9 - alarm output 9)
Bit 9 – alarm 2 of analog input 5 (AL10 - alarm output 10)
Bit 10 – alarm 1 of analog input 6 (AL11 - alarm output 11)
Bit 11 – alarm 2 of analog input 6 (AL12 - alarm output 12)