modbus RTU example code explanation

[lisek_lichu]

hello,

I am trying to use modbus with my PIC16f628A microcontroller and I started to read and analyze example codes given from CCS modbus.c, ex_modbus_slave.

I have few questions about using some variables:


1) I see that 2 byte modbus_serial_crc.b is used to calculate CRC of each data received byte by byte and also when we send data. When we finish sending data, two bytes CRC calculated are sent at the end. But for what is 16bit modbus_serial_crc.d?

We set it to 0xFFFF at the beginning of modbus_serial_send_start() and also set it to 0xFFFF at the end of modbus_serial_send_stop().

Additionally we set it to 0xFFFF at timer2 interrupt routine only when we are in MODBUS_GETADDY state.

And at the end we check it in timer2 interrupt routine if it is 0x0000 but when it could change to 0x0000 and for what is this variable?


2) for what is variable modbus_serial_new?

at the beginning it is zero (FALSE).

We can start parsing incoming char in incomming_modbus_serial() only if modbus_serial_new==FALSE
in modbus_kbhit() routine we return FALSE if modbus_serial_new==FALSE
and also in some other functions it appears so what is this variable describing? For what is it?
What means if it is TRUE and what means if it is FALSE?

3) In interrupt routine incomming_modbus_serial() at the end we turn on timer modbus_enable_timeout(TRUE), why it is set to count 20ms? should that value be always 20ms or it can be changed? what does it depend?

4) Why in incomming_modbus_serial() in state MODBUS_GETDATA we compare length of received data with buffer width and if we reach limit of buffer we override last char with new one instead of sending some exception? Or maybe it is left like that because CRC then will be not correct so we will know that something is not good with received data.

5) In ex_modbus_slave modbus_exception_rsp(MODBUS_ADDRESS,modbus_rx.func,ILLEGAL_DATA_ADDRESS); shouldn't it has instead of modbus_rx.func something like modbus_rx.func || 0x80? because in exception in response function code should have 1 in MSB or am I wrong?

Ohhhh I think it is enough for today i need to sleep with everything i have read today and start tomorrow to continue.

Thanks in advance a lot for answers.
Lisek

[lisek_lichu]

I think that i gave too many questions at one time so nobody wants to answer it

5) I get answer for this. In modbus_exception_rsp function in modbus.c file is added one bit in MSB "func|0x80"
But it was the most simple question from this list, the rest it not so easy for me


so 4 questions has left.

[Mvedovetto]

[temtronic]

I don't use MODBUS however you should get a book about it. I'l assume the names CCS uses are the same or similar to the 'industry'. I have no doubt the CCS examples will work( the 'master' does compile using the 628A) !
Pay careful attention to hardware especially bus pullup values.
Once you get the master-slave program working, copy them to 'my-master, my_slave' and modufy the COPIES , as you need to.
Do NOT modify the original CCS supplied programs.

[Ttelmah]

This is a relaunch of a thread that is seven years old. Forget it.

In fact the questions were basic not understanding C (so there is a union
giving the .b and .d variables, and the poster didn't understand how a union
worked).....

[Mvedovetto]

Thank you all for reply!

Ttelmah, I know this is an old thread, but, apart from union, I did not understand how variable modbus_serial_new works in CCS modbus driver.

I miss to understand how modbus_kbhit() works too and I need to understand code before using it.

I was looking for some explanation on the forum and just this thread seemed to fit my needs.

I'm using 5.092 PCM compiler.

Marco

[Mvedovetto]

[PCM programmer]

Regarding 'modbus_serial_new', I think it's a flag used to tell the program
if it's processing an existing message, or it's finished and waiting for a new
message.

Look at this routine: incomming_modbus_serial()
In this file: modbus_phy_layer_ascii.c
In the code below, if the state is "stop", and if we got a newline character
then we've found the end of the incoming message, so we're now waiting
for a new message to arrive. So 'modbus_serial_new' is set to TRUE.

[Mvedovetto]

I'm using RTU. I'm trying to workout with slave.

incomming_modbus_serial() receives from USART char c and saves the byte into modbus_rx struct if modbus_serial_new=FALSE.
So it's the same as for ASCII. In RTU mode, modbus_serial_new is set only here:

[Mvedovetto]

If #USE TIMER has the option NOISR, it seems to run

[Ttelmah]

The modbus physical layer uses a timer. You can change it by setting
the #define 'MODBUS_TIMER_USED', to MODBUS_TIMER_T2. By default
it is using Timer1, so your use of timer1, interferes with it.....

[Mvedovetto]

[temtronic]

You need to understand that MODBUS is a 'try-to-do-everything-for-everyone' serial communications protocol or driver. As such it's overly complicated and only needs to be used when you have real MODBUS devices.It's very important to read every comment in the drivers and CCS example programs ! As Mr. T points out MODBUS NEEDS to use a timer, Timer1 is what CCS chose to use. Details....always need to read the 'fine print' !

[Mvedovetto]

[temtronic]

I opend the example 'ex_modbus' program. did a search for 'timer', located it ! Though my version uses timer2. I have an old version of compiler BTW so it's possible CCS has changed the code, using timer 1 instead of timer 2.

Jay