(Help Me!) MODBUS RTU RS-485 Master-Slave

[José Aparecido]

Hi! I'm developing a project in which I have two devices:

1. PIC18F4550 (Master);
2. WEG CFW500 Variable Speed Drive (Slave).

I need the PIC18F4550 to read some Variable Speed Drive holding registers.

I'm programming in C language and using the PIC C Compiler (CCS).

I'll attach a diagram of how the physical part will look.

I will use a MAX485 transceiver (although the model of the diagram is the MAX487) for the communication medium.

The crystal of the microcontroller will be 20MHz.

I'm using a 20x4 LM044L LCD display.

I need to display the value of (a):

Motor Current;
Motor Frequency;
Motor Acceleration Time;
Motor Deceleration Time.

I've already started programming in CCS, I'm using just two libraries, the standard CCS LCD library and the MODBUS library.

The CFW500 uses parameters, each parameter corresponds to a register that stores some data, for example:

P0003 = motor current;
P0005 = motor frequency;
P0100 = motor acceleration time;
P0101 = motor deceleration time.

I know to read some register I should use the command from function 3:

modbus_read_holding_registers (address of the slave, address of the initial registrar, number of registers to read);

Function 3 is used to read the retention registers.

I set the slave address to 1.

So, if I just want to read the frequency value for example, I convert 0005 (frequency parameter) to hexadecimal and it stays:

modbus_read_holding_registers (0x01,0x05,0x01); //read the slave retention registers at address 1; read register 5 and only 1 register

But I'm not sure how to display the reading result on the display, could anyone please help me? Any missing detail I answer. Thank you!

Update: I created a variable type int and said that it would equal the read value of the register, then that variable would be displayed on the LCD display, but I always get the value 12, does anyone know what I am doing wrong?

CCS Code:

[dluu13]

EDIT!!!: Before you do any of this, I just noticed that you have wired your UART pins to the MAX485's receiver enable and data enable pins... That is incorrect! They need to be wired to the RO and DI pins (I believe it should be TX -> DI and RX -> RO).
Then, you need to wire two of your GPIO pins to the DE and RE pins, and specify that. Check ex_modbus_master.c to learn how to configure the driver to use RS485.

I don't have the CCS compiler on this computer, but ex_modbus_master.c has some wrapper functions with examples on how to use the modbus functions included in the library.

The first thing you have to do is make sure you have the right baud rate. You should write some code to blink LED at 1 Hz to confirm that you have done your clock setups correctly. Then, you need to initialize the modbus driver using modbus_init().

When you open ex_modbus_master.c, do a search for read_all_holding().

I modified that function so that I can input as parameters:
slave address
register to start reading
number of registers to read
pointer to array where I can store the data

Instead of printing the data, I modified that line to place each element in the modbus_rx.data[] array.

Keep in mind that this modbus_rx.data is an array of int8's, and each register is 16-bits so each pair of bytes that you read is one element of data. I used the union data structure to conveniently access this. But ALSO, keep in mind that while it prints fine if you are doing it like the CCS example, the bytes are swapped when you store them in array.

So in your function, you will need to swap element 0 with element 1, element 2 with element 3, and so on and so forth...

However, the best way to figure it out is to play with the code, starting with printing your values as shown by the CCS example code, and then moving on to storing it in your own data structures and so on.

[Ttelmah]

You are also not telling the modbus driver what protocol to support

#define MODBUS_SERIAL_TYPE MODBUS_ASCII

or RTU.....

Then you need to call 'modbus_init' before trying to use the modbus
functions.

Then after you have called _one_ read_holding_registers operation,
you need to look at/use the reply, which is in the modbus_rx.data array.
How much data, is defined by modbus_rx.len. Dong 4 sequential reads will
have overwritten the values from the earlier reads.

[José Aparecido]

[José Aparecido]

[dluu13]

GPIO = general purpose input/output

You need to fix your hardware first. Going on without the proper hookups guarantees that you have no chance of getting it working.

So before you start playing any more with the code, you should update your wiring schematic and confirm that it is correct.

It should go like this:
PIC-----------------MAX485
PIN_C7 (RX)-------RO
free IO pin---------RE
free IO pin---------DE
PIN_C6 (TX)-------DI

Here's a picture of a hookup on MSP430. It's not exactly what you need to do, but it gives the general idea.

[José Aparecido]

[dluu13]

Can you show your code? You need to show the full setup, where you are trying to read the Modbus register, and where you try and store the incoming data. Try and write the most minimal code you can to demonstrate the problem.

[José Aparecido]

[dluu13]

I notice a few things:
1. your DE and RE pins are still set to your TX and RX pins in your code. You need to set the DE and RE pins to what they are connected to, and also tell the modbus driver that you are using C7 as the RO pin and C6 as the DI pin.

2. Your interrupt source is set at INT_EXT. You should use MODBUS_INT_RDA as your interrupt source for this

3. modbus_read_holding_registers() does not return the value that it reads. It returns whether or not there was an exception in reading modbus. In your case, it is returning 12, meaning there was a time out. You can find the definition of these error codes in the driver source code.
The data it reads is stored global modbus_rx struct. In that struct, there is the .data member variable which is a pointer to an array. You need to read the modbus_rx.data array to get your data out.

Have you gone to the CCS examples folder to find ex_modbus_master.c? You need to study and understand at least line 70 to 94 in that code to do the setup properly. Notice how they specify the interrupt source. Also notice how you need to define MODBUS_SERIAL_ENABLE_PIN and MODBUS_RX_SERIAL_ENABLE if you want to use RS485.

[José Aparecido]

[dluu13]

One thing: I say that you should use MODBUS_INT_RDA for this because I notice that you are using C6 and C7, which are the hardware UART pins. If you really intend to use MODBUS_INT_EXT then you should wire the incoming (RX) pin into the INT_EXT pin of your PIC. Otherwise you need to define:
MODBUS_SERIAL_TX_PIN
MODBUS_SERIAL_RX_PIN

[José Aparecido]

[dluu13]

You will need to provide a modbus master driver for the master and a slave driver for the slave.

read_holding_registers will put the contents of the slave's holding registers into modbus_rx.data

And I advise against simulating using Proteus. There have been a number of cases documented on this forum where a design simulated in Proteus will work there and not in real life, and vice versa.

It is more reliable to simulate in real life using real components.

[José Aparecido]