Q: How do I ring the Doorbell?
A: Ringing the Doorbell is rather simple, once you have
the SBP2 device's fetch agent address. The Doorbell's
address is 16 bytes past the fetch agent's address. To ring
the Doorbell, you write any value into the Doorbell's address
using the FWWrite command.
You get the address of the fetch agent from the login
status notification (because each login could have its own
fetch agent, and therefore Doorbell register). Make sure that
you register a login status notification function before you
log into your SBP2 device; otherwise, you won't get the
address of the SBP2 device's fetch agent.
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Tip:
Because the four-byte payload of the write to
the Doorbell is ignored, you can put diagnostic
information in it (but only four bytes), and examine
the data with a FireWire packet analyzer. Even if
you do not send diagnostic data, you must set a
valid pointer in the asynchronous command object
(the memory must be able to be held, for
example).
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