EtherCAT SII: What is 'receive mailbox' and 'send mailbox'?

The EtherCAT ETG 1000.6 V1.0.32 specification defines in section 5.4 SII coding that in the SII header, the following fields contain mailbox settings:

Field	Address	Data Type	Direction	Description
Bootstrap Receive Mailbox Offset	`0x0014`	Unsigned16	master → slave	Receive Mailbox Offset for Bootstrap state
Bootstrap Receive Mailbox Size	`0x0015`	Unsigned16	master → slave	Receive Mailbox Size for Bootstrap state
Bootstrap Send Mailbox Offset	`0x0016`	Unsigned16	slave → master	Send Mailbox Offset for Bootstrap state
Bootstrap Send Mailbox Size	`0x0017`	Unsigned16	slave → master	Send Mailbox Size for Bootstrap state
Standard Receive Mailbox Offset	`0x0018`	Unsigned16	master → slave	Receive Mailbox Offset for standard state
Standard Receive Mailbox Size	`0x0019`	Unsigned16	master → slave	Receive Mailbox Size for standard state
Standard Send Mailbox Offset	`0x001A`	Unsigned16	slave → master	Send Mailbox Offset for standard state
Standard Send Mailbox Size	`0x001B`	Unsigned16	slave → master	Send Mailbox Size for standard state

As the receive mailbox is used for master → slave communication, this is - as most other specifications in the EtherCAT standard - from the slave’s perspective.

Hence,

The receive mailbox is MbxIn (Mailbox Input), for Master->Slave communication, and SyncManager SM0 shall be used for that
The send mailbox is MbxOut (Mailbox Output), for Slave->Master communication, and SyncManager SM1 shall be used for that

Standard mailbox size and Bootstrap mailbox can differ — a larger Bootstrap mailbox may be used for optimization. However, in practice, EtherCAT slaves most often have only standard mailboxes (ie bootstrap mailboxes are the same).

Source: ETG 1000.6 V1.0.32, section 5.4 SII coding - requires free ETG membership.

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