BRX Do-More PLC Modbus ASCII Protocol

Do-more plc modbus ascii protocol will be used to communicate to a  Solo process temperature controller. A sample program will explain in detail how this is accomplished through a serial port.
Modbus is a communication method used for transmitting information over serial lines between electronic devices. The device requesting the information is called the Modbus Master (Client) and the devices supplying information are Modbus Slaves (Servers). This protocol was originally developed by Modicon systems.
Modbus protocol comes in basically three different types. Ethernet (Modbus TCP) or Serial (Modbus RTU or Modbus ASCII). Modbus TCP and Modbus RTU come as standard protocols in the BRX Do-More series of PLCs.
do-more plc modbus ascii
We will connect the BRX Do-More PLC to a Solo process temperature controller. This will be done using the Modbus ASCII protocol over serial RS485 communication wire. (Media) The present and set values (PV / SV) will be read from the Solo controller and the set value will be written when required. Let’s get started.

Previously in this BRX series PLC, we have discussed:
System HardwareVideo
Unboxing – Video
Installing the SoftwareVideo
Establishing CommunicationVideo
Firmware Update – Video
Numbering Systems and AddressingVideo
First ProgramVideo
Monitoring and Testing the ProgramVideo
Online Editing and Debug ModeVideo
High-Speed IOVideo
Compare InstructionsVideo
Math InstructionsVideo
Program ControlVideo
Shifting InstructionsVideo
Drum InstructionVideo
Serial Communication – Modbus RTU to Solo Process Temperature ControllerVideo
Data LoggingVideo
Email – Text SMS Messaging GmailVideo
Secure Email Communication Video
AdvancedHMI Communication – Modbus TCPVideo
Analog IO – System ConfigurationVideo
HTTP JSON InstructionsVideo
Analog Dusk to Dawn ProgramVideo
INC DEC 512 Registers for DMX512Video
PID with PWM OutputVideo
PID Ramp Soak ProfileVideo
Do-More Simulator MQTT Publish / SubscribeVideo
BRX Do-More PLC MQTT CommunicationsVideo
Stride Field Remote IO Modules Modbus TCP Ethernet
Unboxing SIO MB12CDR and SIO MB04ADS Video
Powering and Configuring Video
BRX Do-More PLC to Stride Field IO Modbus TCPVideo
BRX Do-More PLC Ethernet Remote IO Controller BX-DMIO
Unboxing BX-DMIO Video
Configuration and Programming Video
Modbus RTU TCP Remote IO Controller BX-MBIO
Hardware Video
Powering and Configuring Video
BRX Do-More PLC to Modbus Remote IO Controller BX-MBIOVideo

Our entire BRX D0-More series can be found here.
The programming software and manuals can be downloaded from the Automation Direct website free of charge.

Watch the video below to see the BRX Do-More PLC implement the Modbus ASCII protocol to communicate to the Solo process temperature controller.

Solo Process Temperature Controller

do-more plc modbus ascii
All Solo Standard Controllers are capable of Modbus communication.
do-more plc modbus ascii
The data and bit registers support the following Modbus function codes.
Data Registers
03: Read Holding Registers (maximum limit is read of eight registers)
06: Write Single Register
16: Write Multiple Registers (maximum limit is eight)
Bit Registers
01: Read Coils
02: Read Discrete Inputs (Both Function Code 1 & 2 read the same memory area.)
05: Write Single Coil (Write FF00H to set the coil or 0000H to reset the coil.)
The Modbus addresses for the Solo process temperature controller can be found in chapter 7 of the manual. See the link below.
do-more plc modbus ascii
In our example, we will be reading the PV and SV values from the controller and write the SV when required.

Solo Process Temperature Controller Configuration

The first thing that we will do is set up the Solo Temperature Controller. Additional information can be found at the following link.

Wiring diagram:
do-more plc modbus ascii

The solo process temperature controller needs to be set up before we can communicate with it. The default setting is ‘Off’ for the On-Line Configuration. Here is the way to change into the different modes in the Solo.

In the Initial Setting Mode we will change the online configuration to on and make the changes to the Modbus settings as follows: 19200 Baud, Even, 7 Data Bits, 1 Stop Bit, Modbus ASCII Format. We will leave the default unit number as 1.

Our controller is now set to communicate.

Download the documentation and/or configuration and monitoring software at the following URL link:
Solo Process Temperature Controller Manual

Modbus ASCII Protocol Format

As the name states, ASCII (American Standard Code for Information Interchange) is used for the communication messages.
Modbus ASCII marks the start of each message with a colon character “: ” (hex 3A).
The end of each message is terminated with the carriage return and line feed characters (hex 0D and 0A).
Sent data from the master
: – Start of message
01 – Address (unit) number
03 – Function Code (read multiple registers)
1000 – Data – Starting address to read
0002 – Data – Number of registers to read
EA – LRC (Longitudinal Redundancy Check) error checking byte
Response data from the slave
: – Start of message
01 – Address (unit) number
03 – Function Code (read multiple registers)
04 – Number of return registers in bytes
00DC – Data returned PV in Hexadecimal = 22.1 degrees C
012C – Data returned SV in Hexadecimal = 30.0 degree C
EE – LRC (Longitudinal Redundancy Check) error checking byte
The bytes that you see in sending and receiving information is represented by two ASCII characters. Each ASCII character is 8 bits long. This means that Modbus ASCII sends more information back and forth than Modbus RTU.
Note: Since the space between bytes is variable, transmission through modems is possible with Modbus ASCII.

BRX Do-More PLC System Modbus Setup – Client (Master)

do-more plc modbus ascii
Modbus ASCII will be the serial (RS485) method in which we will communicate between the BRX Do-More PLC serial port and the Automation Direct Solo Process Temperature Controller.
We can address up to 247 (Solo 1 to 247) devices on this master-slave protocol. A maximum of 32 devices (Nodes) on the network can communicate to the master.
do-more plc modbus ascii
Using the do-more designer programming software call up system configuration by selecting it from the main menu | PLC | System configuration…
Under the heading of the protocol, we will select uses STREAMIN / STREAMOUT for ASCII and Custom Protocols.
Note: Modbus RTU Server and Client are already incorporated into the controller. Modbus ASCII protocol will have to be assembled within our PLC program.
Under the port type select RS485 and enable the 120-ohm termination resistor.
Select Change @IntSerial Device Settings…
do-more plc modbus ascii
These serial port communication settings must be set to match our selection from the Solo controller. (19200, Even, 7, 1) Transmit control is unconditional and RTS control will follow the transmitter. Select OK to save the parameters.
Our PLC is now set to start programming our Modbus ASCII protocol.

BRX Do-More PLC Program

Our sample program has a Main program that will do the overall logic. It will call up the program that will handle the communications to the Solo controller unit.
– Modbus_ASCII(BRX_to_Solo) – Modbus ASCII via Serial RS485
do-more plc modbus ascii
Using the Modbus ASCII Protocol communicate to a Solo Process Temperature Controller
V1 – PV present value
V2 – SV set value
V100 – Change SV
When the value in V10 changes to greater than 0 the value is then written into the Solo controller.


Ladder Logic Sample Program Code
Modbus ASCII Send:
: – Start character
01 – Slave unit number
03 – Command Code – Multiple read
1000 – Data – Starting Address
0002 – Data – Number of Address to read
EA – LRC – (Longitudinal Redundancy Check) error checking byte
The carriage return line feed is represented by $0D$0A. This marks the end of the message to the solo controller.
The STRPRINT instruction will put our print script into SS0. STREAMOUT is used to send the command out through our RS485 port to the solo controller.
Ladder Logic Sample Program Code
Modbus ASCII Receive:
: – Start character
01 – Slave unit number
03 – Command Code – Multiple read
04 – Number of bytes times 2 returned (ASCII)
XXXX – Data from the first register – PV value
YYYY – Data from the second register – SV value
The STREAMOUT success bit is used to trigger the STREAMIN instruction. This will place the returned information (ASCII) into SS1.
Note: $0D$0A is the exact delimiter sequence.
Ladder Logic Sample Program Code
Using the STRSUB instruction we place the PV and SV in SS2 and SS3. We then use the string to integer instruction (STR2INT) to convert the ASCII to hexadecimal. This will then show us the correct values.
Ladder Logic Sample Program Code
Calculation of LRC value for Sending SV to Solo
The LRC is calculated by adding together successive eight-bit bytes in the message, discarding any carries, then two’s complementing the result.
Modbus ASCII SV Write – :01061001XXXXYY (XXXX SV) (YY LRC)
01 + 06 + 10 + 01 = 18
Formula: 1 – (18 + SV LSB + SV MSB) – 1
The result word is then converted to ASCII text.
We are using a mask function by “logical AND ” and shifting instructions to add the SV LSB and MSB.
The resulting word is 2 bytes. We use the MEMCOPY to only move the least significant byte of information for our LRC.
Ladder Logic Sample Program Code
The string print instruction is then used to build up the write information to change the SV of the Solo.
do-more plc modbus ascii
STRPRINT instruction has a print script that will format the information that we want.
“:01061001” – Modbus ASCII write starting at address 1001 hex (Solo SV)
FmtInt(V11,hex,4,zeropad) – SV information – format integer, hexadecimal value, 4 digits, pad with leading zeros.
FmtInt(V21,hex,4,zeropad) – LRC information – format integer, hexadecimal value, 2 digits, pad with leading zeros.
$N will put a “$0D$0A” into the script for the termination. (Carriage return / Line feed)
Ladder Logic Sample Program Code
Send the Modbus ASCII string to change the Solo set value (SV).
Ladder Logic Sample Program Code
Receive the return string after changing the solo SV.
Modbus SV Write Receive:
Ladder Logic Sample Program Code
If we are writing the SV and it is finished, copy 0 to the SV write register. Then reset the write SV bit (C10) and set the end of communication bit.
If we are not writing and the read PV and SV are complete then set the end of communication bit.
Ladder Logic Sample Program Code
When the end of the communication bit is set, start a timer for 20 milliseconds. Once it has timed out turn on the start communication again bit.
Ladder Logic Sample Program Code
When the end of the communication bit is on we check to see if the Solo SV Write (V10) is greater than 0. If it is we transfer V10 to V11 and set the write SV bit.

Watch the video below to see the running of the BRX PLC program to communicate to our Solo process temperature controller via Modbus ASCII protocol.

Download the BRX Do-More PLC program here.

BRX Series PLC from Automation Direct – Power to deliver
Overview Link (Configure and purchase a system)
Manuals and Product Inserts (Installation and Setup Instruction)
Do-More Designer Software v2.7.4 (Free Download Link) – The software will contain all of the instruction sets and help files for the BRX Series PLC.
Modbus Learning Links:
Simply Modbus Frequently Asked Questions
Modbus TCP/IP Overview – Real-Time Automation
All You Need to Know About Modbus RTU – Video

Watch on YouTube: BRX Do-More PLC Modbus ASCII Protocol

If you have any questions or need further information please contact me.
Thank you,

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