UNIBUS Experimental Ethernet interface

From Computer History Wiki
Jump to: navigation, search

A UNIBUS Experimental Ethernet interface was produced in small numbers by Xerox PARC, to allow PDP-11's to be connected to Experimental Ethernets. It was a full-duplex device, which used DMA to transfer packets directly to and from main memory.

The network interface was a hex board which plugged into an UNIBUS SPC slot; a flat cable led from a Berg connector on the card to a bulkhead connecter, from where a standard Ethernet cable led to the transceiver.

The network address (8 bits on the Experimental Ethernet) was set by a DIP switch on the card. Also, the hardware did not fill in the source address in the packet header, the device driver had to do that.

One minor oddity (by today's standards) was the the back-off after a collision was implemented by software, in the device driver. A third interrupt vector was used when a collision occurred, not the normal output vector.

Device registers

Register Address
Output Word Count Register 760020
Output Bus Address Register 760022
Output Control and Status Register 760024
Output Start Delay Register 760026
Input Word Count Register 760030
Input Bus Address Register 760032
Input Control and Status Register 760034
Interface Network Address Register 760036

760020: Output Word Count Register

Unused WC9 <---> WC0
15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00

The two's complement of the number of words to transfer.

760022: Output Bus Address Register

BA15 <---> BA01 Unused
15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00

16-bit main memory addresses restricted buffers to the low 64KB of memory.

760024: Output Control and Status Register

Error Unused Done Int Enb Unused Enable
15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00

Error was set by either a collision, or data was not available from the bus in time.

760026: Output Start Delay Register

Unused Delay
15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00

760030: Input Word Count Register

Unused WC9 <---> WC0
15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00

Although the documentation avers that this is a two's complement number, a device driver source code indicates that in fact it was a one's complement counter.

The count needed to include the CRC word in the packet (otherwise on a maximum-sized packet an error would be indicated), but this was not actually transferred to memory.

760032: Input Bus Address Register

BA15 <---> BA01 Unused
15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00

760034: Input Control and Status Register

Error Unused Done Int Enb Unused Prom Enable
15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00

Error was set on a CRC error, buffer overflow, or when the bus did not take a word in time.

The 'promiscuous' bit caused all packets to be received, not just ones addressed to this interface.

760036: Interface Network Address Register

Unused NA7 <---> NA0
15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00