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Modbus #6. STM32 as SLAVE || Writing single and Multiple Registers
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Nov 28, 2022
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View Video Transcript
0:00
foreign
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[Music]
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hello and welcome to controllers Tech
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this is the sixth video in the stm32
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modbus series and today we will see how
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the master will modify a single and
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multiple registers in the slave device
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this is again going to be a continuation
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from the previous video where we
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programmed the stm32 to respond to the
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queries regarding reading coils and
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inputs
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in today's video the master will send a
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request to modify the register values in
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the stm32 and we will program the stm32
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to handle this request
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basically we will be working with the
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function codes 6 and 16 but from the
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slave perspective
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the function code 6 is to write a single
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register where the master can write only
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one memory word that is 16 bits
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the function code 16 is to write the
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multiple registers but the master can
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request a maximum of 123 registers
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I am going to continue the same project
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from the previous video and I have
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already modified it
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here in the main file I have added two
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more cases
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in case of the function code 6 we will
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call the function write single register
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and for the function code 16 we will
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call write holding registers
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these functions are defined in the
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modbus slave.c file
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let's first discuss the single register
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this is the format of the request sent
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by the master
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it contains the slave ID the function
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code the address of the register the two
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bytes of data Master wants to write and
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the CRC
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in the function we will first extract
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the address of the register
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if the address is more than 49 we will
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send an exception regarding illegal data
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address
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this is because we have only defined a
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total of 50 holding registers in the
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database whose addresses vary from 0 to
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49.
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if there is no exception we will modify
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the database with the new data
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the registers are 16-bit in size and
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therefore the data arrive in two bytes
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with higher byte being sent first
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we will combine the two bytes to make a
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16-bit value and store it in the
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register's position in the database
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now comes the response
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in case of single register the slave
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response is exactly same as the Masters
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query
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so here we will simply copy the data
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from the RX buffer into the TX buffer
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finally send the data the CRC will be
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calculated in the send data function
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itself
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now let's see the function to write
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multiple registers
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once this function is called we will
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first find the address of the start
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register
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then find the number of registers
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requested by the master
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if the number of registers is more than
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123 we will send an exception
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this is as per the modbus standard that
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we saw in the beginning of the video
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then calculate the address of the last
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register requested by the master
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since we have the database for only 50
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registers if the end address is more
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than 49 we will send another exception
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here I have defined an index variable to
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keep track of the bytes in the RX data
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buffer
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next we will copy the data from the RX
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buffer into the database
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the data in the holding registers is
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16-bit in size so we will combine the
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two bytes from the RX buffer after each
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value has been saved we will increment
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the start byte so that the next value
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can be stored at the next position the
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loop will repeat as many times as the
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number of registers requested by the
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master
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now the slave will send a response to
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the master
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the format is shown on the top right
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corner we have the slave ID the function
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code the start register address and the
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number of registers
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finally we will call the send data
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function to send the response
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the CRC will be calculated in the
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function itself
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let's build and debug the code now
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I have added the holding registers
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database in the live expression
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this is because the values are not going
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to change here but you can see the
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updated values in the live expression
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in the master software we need to open a
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new window to perform the right
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operation
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here we have the same configuration that
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we have been using in all the videos so
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far
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let's start by testing the single
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register let's say the master wants to
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write the register at the address 405.
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since it is writing only one register
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the function code is set to 6. here you
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can set the value for the register
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the current value at forty thousand five
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is four thousand four hundred and forty
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four
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and the master wants to update it to one
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two three four five
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here you can see the query prepared by
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the master based on the above setup
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all right the slave sent exactly the
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same response to the master and here you
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can see the data has been updated
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let's try another address
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this is the register at forty thousand
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twenty one
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and the master wants to write 10 000
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here
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you can see the value has been updated
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this change is not going to reflect in
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the database values so you must see it
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in the live expression
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so writing the single registers working
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well
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now let's move to multiple registers
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this time the master wants to write four
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registers starting from 21. you can see
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the function code has been changed to
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16. let's update the register values
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you can see the four registers have been
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updated with new values
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I also tried writing the decimal values
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but since the data is unsigned 16 bit
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the values were rounded off to the
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nearest integers
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now let's see some exceptions
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we know the master can request a maximum
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of 123 registers so if it try to request
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124 of them the slave will send an
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exception saying illegal data value
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similarly the database only contains 50
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registers so if the master want to write
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more than that the slave will send an
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exception saying illegal data address
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although it can write the 50th register
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since it's defined in the database
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now let's quickly see if the values we
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wrote are actually readable
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here the master is going to send a read
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request for the register at address 50.
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the function code is set to 3.
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here you can see the updated data that
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we just forwarded that address
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let's read these four registers starting
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from the 21st
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let me change these to unsigned integers
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here you can see the updated values of
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the registers
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so I hope you understood how to perform
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write operations as a slave device
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we will continue with the series and in
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the next video I will cover writing
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coils
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that will be the last video of this
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series for the time being
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you can download the code from the link
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in the description
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leave comments in case of any doubt
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keep watching and have a nice day ahead
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[Music]
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foreign