User guide

CobraNet Programmer’s Reference

Control Communications

Although the serial bridging feature strives to transmit data at wire speed, delays are

introduced by the process of serializing, de-serializing, and prioritizing the serial bridge

packets. These delays are typically on the order of 10ms or less.

See Table 6.4.8 on page 86 for details on the MI variables used to control serial bridging.

2.2 Packet Bridge

The packet bridge provides a means for using the CobraNet interface as if it were an

Ethernet controller by providing a basic capability to send and receive raw Ethernet

packets. A CobraNet device utilizing a host processor with network stack can use this

feature to transmit and receive both control and audio data over the same network

connection.

In the simplest implementation, the host sees the packet bridge as several control

variables, a receive buffer, and a transmit buffer which are accessed via the HMI interface.

Ethernet data packets are transferred in both directions over the host port using the same

HMI semantics used to read and write other MI variables.

More advanced implementations can take advantage of interrupt and DMA modes of HMI

operation as well as some HMI operations specifically tailored to packet bridge functions.

Refer to Table 6.4.7 on page 81 for details on the MI variables used to control packet

bridging.

2.2.1 Packet Bridge Buffer Data Format

Packets are transmitted by writing raw packet data to bridgeTxPktBuffer. Packets are

received by reading bridgeRxPktBuffer. Data in both buffers shares the same format. The

first word of the buffer specifies the byte length of the data that follows. Byte length

includes the14-byte Ethernet header. The Frame Check Sequence (FCS) is automatically

appended to transmitted packets and automatically checked and stripped from received

packets. The FCS is not included in the packet data or byte length specification. Byte

length should be in the range 14 to 1514.

2.2.1.1. Processor-dependent Layout of Packet Bridge Buffers

Refer to Ta bl e 3 and Table 4 on page 14 for organization of data within bridge buffers for

24- and 32-bit platforms. All data marked as unused/0 will be received as 0 and must be

set to 0 when writing the buffer prior to transmission.

For both platforms, requested transmissions shorter than the 60-byte Ethernet packet

minimum will be padded to 60 bytes with indeterminate data.