Non-Standard Baud Rate
VDIeng
Posts: 2
We got a piece of hardware in (Grace Designs m904) that has a rear access port with RS-422. Great a way to connect my controller up to it. However when I got the protocol manual (directly from Grace) it states that the Baud rate is 111.11Kbps. I thought this must simply be a typo so I tried 115.2 and other various baud rate setups. I called Grace to find out if this was a typo and their engineering staff assures me that it is not a typo. I'll share with you the email response that I received.
=-=-=- Email From Grace Insertion -=-=-=
Yes - the baud rate is 111.11 kHz - or 16MHz/144. The bit period is 9us - or 81 us per byte.. If you're transmitting at 115.2kHz your bit period is 8.68us - or 78 us per byte. So by the end of a byte you'd only be off by 1/3 of a bit period.
>From the documentation of the processor used in the m904 (Microchip PIC18F8620) just how much error it can tolerate. If you're unable to operate your equipment at 111kHz baud then I think there' s a god chance you can make it work with 115.2kHz. That would probably require extending the stop period between bytes transmitted.
The receiver is expecting a 1-bit-period stop bit. Since you'd be transmitting at a slightly faster baud rate then I believe you're more likely to have success if you use 2 stop bits if you can. I did a little reading on RS232 and it seems, as an alternative to using 2 stop bits, you can set parity to Mark, which always transmits high after the 8th bit and seems to be equivalent to adding an extra stop bit.
=-=-=- End Email Insertion -=-=-=
Having said all of that I have tried numerous configuration of the RS232 port including 9 bit processing (using , B9MON) parity, and even 2 stop bits but have not had any success communicating with the device. Has anyone successfully communicated with a device at this speeed?
Thanks,
Tim Ouellette
Sr. Systems Engineer
=-=-=- Email From Grace Insertion -=-=-=
Yes - the baud rate is 111.11 kHz - or 16MHz/144. The bit period is 9us - or 81 us per byte.. If you're transmitting at 115.2kHz your bit period is 8.68us - or 78 us per byte. So by the end of a byte you'd only be off by 1/3 of a bit period.
>From the documentation of the processor used in the m904 (Microchip PIC18F8620) just how much error it can tolerate. If you're unable to operate your equipment at 111kHz baud then I think there' s a god chance you can make it work with 115.2kHz. That would probably require extending the stop period between bytes transmitted.
The receiver is expecting a 1-bit-period stop bit. Since you'd be transmitting at a slightly faster baud rate then I believe you're more likely to have success if you use 2 stop bits if you can. I did a little reading on RS232 and it seems, as an alternative to using 2 stop bits, you can set parity to Mark, which always transmits high after the 8th bit and seems to be equivalent to adding an extra stop bit.
=-=-=- End Email Insertion -=-=-=
Having said all of that I have tried numerous configuration of the RS232 port including 9 bit processing (using , B9MON) parity, and even 2 stop bits but have not had any success communicating with the device. Has anyone successfully communicated with a device at this speeed?
Thanks,
Tim Ouellette
Sr. Systems Engineer
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