EtherCAT: What is the Sync Manager Communication Type?

In EtherCAT, each of the up to 32 Sync Managers (SMs) of a slave can be configured with a communication type that defines the direction of data flow and whether the SM is used for mailbox communication or process data.

• Communication type 0 is invalid and indicates an unused/disabled SM.
• Communication type 1 is used for mailbox receive (MbxIn = Master→Slave direction)
• Communication type 2 is used for mailbox transmit (MbxOut = Slave→Master direction)
• Communication type 3 is used for process data output (RxPDO: Master→Slave direction).
• Communication type 4 is used for process data input (TxPDO: Slave→Master direction)

How to access to the Sync Manager communication type?

The SyncManager communication type is accessible in the object dictionary at index 0x1C00.

• Subindex 0 contains the number of sync manager channels used 4..32.
• Subindex 1 contains the communication type of SM0 (see above)
• Subindex 2 contains the communication type of SM1
• etc etc (up to subindex 33 at maximum)

Note: Most slaves only support 4..8 sync managers. You need to check your slave IC’s datasheet.

Which SyncManagers are used for what?

According to the ETG1000.6 specification, the following Sync Manager communication types shall be used:

• Sync Manager 0: Mailbox receive (MbxIn) - Communication type 1
• Sync Manager 1: Mailbox transmit (MbxOut) - Communication type 2
• Sync Manager 2: Process data output (RxPDO) - Communication type 3
  • May be used for process data inputs (communication type 4) if outputs are not supported by the slaves
• Sync Manager 3: Process data input (TxPDO) - Communication type 4

Only in case the slave does not support mailboxes (which is rare), you shall use an alternate mapping:

• Sync Manager 0: Process data output (RxPDO) - Communication type 3
  • May be used for process data inputs (communication type 4) if outputs are not supported by the slaves
• Sync Manager 1: Process data input (TxPDO) - Communication type 4

Source: ETG1000.6 V1.0.32, section 5.6.7.4.9 - requires free ETG membership.