Lumel RE15 User manual

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USER’S MANUAL
SERIAL INTERFACE
WITH MODBUS PROTOCOL
MICROPROCESSOR
CONTROLLER
RE15

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CONTENTS
1. PREFACE ............................................................................... 4
2. DESCRIPTION OF THE MODBUS PROTOCOL .................... 4
2.1. ASCII framing ................................................................. 6
2.2. RTU framing ................................................................... 6
2.3. Characteristic of frame fields ....................................... 7
2.4. LRC checking ................................................................ 9
2.5. CRC checking ................................................................ 9
2.6. Character format in series transmission .................... 10
2.7. Transaction interruption .............................................. 10
3. DESCRIPTION OF FUNCTIONS .......................................... 11
3.1. Readout of N-registers (Code 03) .............................. 11
3.2. Writing of values into the register (Code 06) ............. 12
3.3. Writing into N-registers (Code 16) .............................. 13
3.4. Report identifying the device (Code 17) ..................... 13
4. ERROR CODES .................................................................... 15
5.TABLE OF REGISTERS FOR THE RE15 CONTROLLER ..... 17
AFFIX A. CALCULATION OF THE CHECKSUM .................... 23

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1. PREFACE
The RE15 microprocessor controller destined to measure and
control physical quantities is provided with a serial interface in
RS-485 standard for the communication with other devices.
The asynchronous communication MODBUS protocol has been
implemented on this serial interface.
The configuration of serial interface parameters has been
described in the Users Manual of the R15 controller.
Composition of serial interface parameters concerning RE15
controller:
•Controller address - 1 ... 247
•Baud rate - 2400, 4800, 9600 bits/s,
•Working modes - ASCII, RTU,
•Information unit - ASCII: 8N1, 7E1, 7O1;
and RTU: 8N2, 8E1, 8O1,8N1
•Maximal turnaround time - 1s
Explanation of some abreviations:
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. DESCRIPTION OF THE MODBUS PROTOCOL
The MODBUS interface is a standard adopted by manufacturers of
industrial controllers for an asynchronous character exchange of
information between different devices of measuring and control
systems. It has such features as:

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vSimple access rule to the link based on the master-slave
principle,
vProtection of transmitted messages against errors,
vConfirmation of remote instruction realisation and error
signalling,
vEffective actions protecting against the system suspension,
vTaking advantage of the asynchronous character transmission.
Programmable controllers working in the MODBUS system 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 query by supplying the requested data to 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
demanded by the master or the realised confirmation of its
command. 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 follows:
 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 realize 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 port

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parameters (baud rate, information unit) during the configuration of
any device.
In the MODBUS system, transmitted messages are placed into
frames that are no related to serial transmission. These frames have
a defined beginning and end. This enables for the receiving device
to reject incomplete frames and the signalling of 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.
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 (to1sec) without
causing errors.
A typical message frame is shown below:
Start Address Function Data LRC End
beginning check index
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 a carriage return-line feed (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 mini-
mum 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 is a multiple measure of the character duration time at the

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set baud rate accepted on the link.
The frame format is shown below:
Start Address Function Data CRC End
bedinning check index
index
T1-T2-T3-T4 8 bits 8 bits n x 8 bits 16 bits T1-T2-T3-T4
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) on 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 of 0-247 decimal. The
master addresses the slave units 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 of 1-255 decimal. 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, it
uses the function code field to indicate and confirm either a normal
(error-free) response or that some kind of error occurred and the
realization of the command is impossible.
For a formal response the slave simply echoes the original function
code. In case of an error assertion, the slave returns a special code
that is equivalent to the original function code with its most

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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 - FF hexadecimal.
These can be made from a pair of ASCII characters or from one
RTU character, according to the networks serial transmission mode.
The function code range is 1-255. 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 discrete and register addresses, the
quantity of items to be handled, and count of actual data bytes in the
field, 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 parameters.
Error checking field
Two kinds of error-checking methods are used for standard
MODBUS networks. The error checking field contents depends upon
the applied transmission mode.
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
(exclusive of the beginning colon and terminating CRLF
characters). LRC characters are appended to the message as the
last frame field preceding the end markers (CR, LF).
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 complemen-
ting 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 realised according the
following algorythm:
1. Load a 16-bit register with FFFFh. Call this the CRC register.
2. Take the byte from the data block and execute the EXOR
operation with the low-order byte of the CRC register. Place the
result into the CRC register.
3. Shift the CRC register contents one bit to the right (towards the
LSB), write 0 on the most significant bit (MSB=0).
4. Check the state of the lowest order bite (LSB) extracted from
the CRC register in the previous step. If its state is equal 0, then
follows a return to the step 3 (another shift).
If the LSB is equal 1, the operation EXOR of the CRC register is
executed 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 of the message have been
processed.
7. The final contents of the CRC register is the searched CRC
value.
8. When the CRC is placed into the message, its upper and lower
bytes must be swapped as described below.

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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:
v1 start bit,
v7 data field bits,
v1 even parity check bit (odd) or lack of even parity check bit,
v1 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:
v1 start bit,
v8 data field bits,
v1 even parity check bit (odd) or lack of even parity check bit,
v1 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 which
exceeding, 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
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.
In the R15 controller  maximal response time on the query frame is
equal 1 s.