Modbus RTU mode on faster PICs

[RF_Developer]

I have been debugging Modbus RTC on a PIC18F46K80. This is a new processor to me, and its the first time I've run a PIC above 40MHz; I'm running this one at 64MHz.

modbus.c in RTU mode uses timer 2 to generate end of message timeouts. It sets timer 2 up to timeout after approximately 4ms regardless of baudrate. 4ms is about right for the commonest modbus baudrate of 9600baud. The Modbus spec states it should be a multiple of the character time, and therefore depends on baudrate, but its common to use a fixed time as modbus.c does.

There is a problem that with faster clock rates, timer 2 cannot generate 4ms with the timer parameters modbus.c uses. It appears that modbus.c can set up the timer correctly at up to 20MHz - the offset needs to be more than eight bits at higher clock rates. Modbus.c could be modified, by altering the postscale value, to run at up to 64MHz. Also if higher baudrates are used then, in theory, the timeout could be reduced to match, again by modifying modbus.c.

Instead of trying to modify modbuc.c, I adopted an alternative approach: change formats to Modbus ASCII. This doesn't use timeouts and no timer is used in modbus.c, thus freeing up PIC resources and allowing faster baudrates and PIC clock rates.

I am not sure how this relates to 24 series PICs, which may have different timer implementations.

Here's my advice: use Modbus ASCII where possible with PIC clocks above 20MHz.

[asmallri]

I have worked on MODBUS drivers for the PIC but I created my own. I have multiple sensors sharing the modbus and they support different baud rates and bus configuration. To deal with this as I interrogate different sensors I use the CPU clock rate and the current UART clock divisor to derive a timer value for the modbus timing. In this way I maintain modbus timing irrespective of CPU speed and the current baud rate.

And, for the record, supporting different baud rates on the same modbus is a crazy thing to do.
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Regards, Andrew

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