.TH EN 4V "26 March 1982" .UC 4 .SH NAME en \- Xerox 3 Mb/s Ethernet interface .SH SYNOPSIS .B "device en0 at uba0 csr 161000 flags 0x4e000226 vector enrint enxint encollide" .SH DESCRIPTION The .I en interface provides access to a 3 Mb/s Ethernet network. In configuring, the flags value is used to define the network to which the interface is attached. The host's address is discovered at boot time by probing the Ethernet address register. Due to limitations in the hardware, DMA transfers to and from the network must take place in the lower 64K bytes of the UNIBUS address space. .PP The interface software implements an exponential backoff algorithm when notified of a collision on the cable. This algorithm utilizes a 16-bit mask and the VAX-11's interval timer in calculating a series of random backoff values. The algorithm is as follows: .TP 1. Initialize the mask to be all 1's. .TP 2. If the mask is zero, 16 retries have been made and we give up. .TP 3. Shift the mask left one bit and formulate a backoff by masking the interval timer with the mask (this is actually the two's complement of the value). .TP 4. Use the value calculated in step 3 to delay before retransmitting the packet. .PP The interface handles both Internet and PUP protocol families, with the interface address maintained in Internet format. PUP addresses are converted to Internet addresses by subsituting PUP network and host values for Internet network and imp values, and setting the Internet host number to zero. .SH DIAGNOSTICS .BR "en%d: output error" . The hardware indicated an error on the previous transmission. .PP .BR "en%d: send error" . After 16 retransmissions using the exponential backoff algorithm described above, the packet was dropped. .PP .BR "en%d: input error" . The hardware indicated an error in reading a packet off the cable. .PP .BR "en%d: can't handle af%d" . The interface was handed a message with addresses formatted in an unsuitable address family; the packet was dropped. .SH SEE ALSO intro(4N), inet(4F) .SH BUGS The device has insufficient buffering to handle back to back packets. This makes use in a production environment painful.