Modbus RTU

[desgin]

Hello All,

In Modbus RTU mode, messages start with a silent interval of at least 3.5 character times.
I want to make it ( idle mode in interval of 3.5 character times ) using uart of PIC microcontroller.
How Master/Slave recognizes the silent interval when the stream of data receives to RX ?
How can i make a logic for the same in MPLAB X IDE ?

Thanks,
Desgin

 

[KlausST]

Hi,

On any UART transfer the last bit should be the STOP bit ("0").
So you can (re)start a timer on every "0" ... with a timeout of 3.5 charater times. (Probably my choice).

Other SW option:
If you use a UART_receive_ISR then you might trigger the timer here.

Or you can build a retrtiggerable monoflop (hardware), to trigger an interrupt.

All these options will trigger an interrupt after the 3.5 characters idle time.

****
But I think the 3.5 character times does not have to be exact 3.5 character times but >=3.5ct.
If so, you don´t need an 3.5ct interrupt, but simply a "clear to send flag".
Then, just before sending a new message you have to poll and "wait for the cts flag".

Depending on the microcontroller .. it may have supporting hardware for this.
I´ve used this feature on an STM32F10x

Klaus

 

[ftsolutions]

Actually, the MODBUS interoperability lab tests for an RTU interframe gap of 3.5 character's' duration at 19200 bps and below, but at 38Kbps and above they only need ~ 1.75milliseconds (I use 2 milliseconds). This is what I have implemented in shipping product and it has passed MODBUS-compliance lab certification with these recommendations, worked through gateways, etc..

What I found to be the simplest code implementation, was to simply run a 1 millisecond free-running timer interrupt (you could choose 2 millisecond on really constrained MCU) that increments a tick count.. Then, in your code where you process a received MB-RTU msg frame to see if it is for you or not, you just grab the tick count when you determine the msg is valid for you. When you have a response ready in queue to send, just grab the current value of the tick counter and compare/subtract the tick count from the "msg is for me" tick count. If the difference is >= the required number of millisecond ticks then your code starts the transmission of the response. Just be sure to handle the rollover of the tick count properly in your checking.

 

[FvM]

MODBUS has two idle specifications, t1.5 and t3.5. Messages must be transmitted continuously with not more than t1.5 inter-character space. Between messages, at least t3.5 idle time must be kept.

Respectively a receiver checks for > t1.5 gaps to reset the state machine to idle state. The actual detection threshold would be set somewhere between t1.5 and t3.5.