DeviceNet Master Protocol API

DeviceNet-Master Dual-Port Memory • 29
3.2 Acyclic Data (Mailboxes)
The mailbox of each communication channel has two areas that are used for non-cyclic message
transfer to and from the netX processor.
• Send Mailbox
Packet transfer from host system to firmware
• Receive Mailbox
Packet transfer from firmware to host system
The send and receive mailbox areas are used by field bus and industrial Ethernet protocols providing
a non-cyclic data exchange mechanism. Another use of the mailbox system is to allow access to the
firmware running on the netX chip itself for diagnostic and identification purposes.
The send mailbox is used to transfer acyclic data to the network or to the firmware. The receive
mailbox is used to transfer acyclic data from the network or from the firmware.
A send/receive mailbox may or may not be available in the communication channel. It depends on the
function of the firmware whether or not a mailbox is needed. The location of the system mailbox and
the channel mailbox is described in the netX DPM Interface Manual.
Note: Each mailbox can hold one packet at a time. The netX firmware stores
packets that are not retrieved by the host application in a packet queue.
This queue has limited space and may fill up so new packets maybe lost.
To avoid these data loss situations, it is strongly recommended to empty
the mailbox frequently, even if packets are not expected by the host
application. Unexpected command packets should be returned to the
sender with an Unknown Command in the status field; unexpected reply
messages can be discarded.
Hilscher Gesellschaft für Systemautomation mbH – Rheinstr. 15 – D-65795 Hattersheim
Edition 3 – API:DeviceNet-Master#EN– 2009/01