Defining an Additional Delay Before Transmitting a Packet Using lance_mode and ipg0

The Fast Ethernet Parallel Port SCSI (FEPS) ASIC supports a programmable mode called lance_mode. The ipg0 parameter is associated with lance_mode.

After a packet is received with lance_mode enabled (default) an additional delay is added by setting the ipg0 parameter before transmitting the packet. This delay, set by the ipg0 parameter, is in addition to the delay set by the ipg1 and ipg2 parameters. The additional delay set by ipg0 helps to reduce collisions. Systems that have lance_mode enabled might not have enough time on the network.

If lance_mode is disabled, the value of ipg0 is ignored and no additional delay is set. Only the delays set by ipg1 and ipg2 are used. Disable lance_mode if other systems keep sending a large number of back-to-back packets.

You can add the additional delay by setting the ipg0 parameter from 0 to 31, which is the nibble time delay. Note that nibble time is the time it takes to transfer four bits on the link. If the link speed is 10 Mbps, nibble time is equal to 400 ns. If the link speed is 100 Mbps, nibble time is equal to 40 ns.

For example, if the link speed is 10 Mbps, and you set ipg0 to 20 nibble times, multiply 20 by 400 ns to get 800 ns. If the link speed is 100 Mbps, and you set ipg0 to 30 nibble-times, multiply 30 by 40 ns to get 120 ns.

Table C-4 defines the lance_mode and ipg0 parameters.