On 2014-07-08 11:21, Gavin Lambert wrote:
Though one of my original questions remains:If it's not supposed to work, shouldn't the master just select a smaller size if it doesn't support large sizes? While it's supposed to respect the slave's preference, ultimately the master is in control of mailbox sizes.
I don't know.
A brief look at the SSC suggests the master is responsible for
specifying the mailbox size each transfer via the Length parameter in
the mailbox header. The FMMU mailbox protocol requires the last mailbox
byte to be accessed to hand over the mailbox buffer. So, the fieldbus
must transfer the full mailbox size (or utilize two transfers?).
So, based on that alone, it would appear the master could specify a smaller mailbox size so long as the mailbox header Length is set correctly.
Obviously, a 1486 byte mailbox would result in very little room for process data. But the master is free to send multiple ethernet frames, so as long as the bus utilization is not exceeded, that does not appear to be a problem.
It would be interesting to hack the master to override the mailbox size read from the SII to a small fixed value (e.g. 123) and then see what breaks.
Best regards - Dave
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