UNIBUS Experimental Ethernet interface

2024-01-13T01:18:02Z

Pugs: Add byte-swapping behavior of board

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

The [[network interface]] was a [[DEC card form factor|hex]] [[printed circuit board|board]] which plugged into an [[UNIBUS]] [[Small Peripheral Controller|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 [[bit]]s on the Experimental Ethernet) was set by a [[Dual Inline Package|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 that the back-off after a [[Carrier-Sense Multiple Access with Collision Detection‎|collision]] was implemented by [[software]], in the device driver. A third [[interrupt vector]] was used when a collision occurred, not the normal output vector.

The board implemented the obvious data path behavior of copying 16-bit [[UNIBUS]] words to/from the on-the-wire packets. But because [[UNIBUS]] systems were [[little-endian]] and network byte order was [[big-endian]], this resulted in byte swapping compatibility problems, which had to be addressed in software, with substantial overhead. The 4.x[[BSD]] driver for this board used the incorrect behavior by default, but had a compile-time option for sites requiring interoperability.

==Device registers==

{| border=1
! 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===
{{16bit-header}}
| colspan=6 | Unused || colspan=10 | WC9 <---> WC0
{{16bitoctal-bitout}}

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

===760022: Output Bus Address Register===
{{16bit-header}}
| colspan=15 | BA15 <---> BA01 || Unused
{{16bitoctal-bitout}}

16-bit main memory addresses restricted [[buffer]]s to the low 64KB of memory.

===760024: Output Control and Status Register===
{{16bit-header}}
| Error || colspan=7 | Unused || Done || Int Enb || colspan=5 | Unused || Enable
{{16bit-bitout}}

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

===760026: Output Start Delay Register===
{{16bit-header}}
| colspan=8 | Unused || colspan=8 | Delay
{{16bit-bitout}}

===760030: Input Word Count Register===
{{16bit-header}}
| colspan=6 | Unused || colspan=10 | WC9 <---> WC0
{{16bitoctal-bitout}}

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 [[cyclic redundancy check|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===
{{16bit-header}}
| colspan=15 | BA15 <---> BA01 || Unused
{{16bitoctal-bitout}}

===760034: Input Control and Status Register===
{{16bit-header}}
| Error || colspan=7 | Unused || Done || Int Enb || colspan=4 | Unused || Prom || Enable
{{16bit-bitout}}

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===
{{16bit-header}}
| colspan=8 | Unused || colspan=8 | NA7 <---> NA0
{{16bitoctal-bitout}}

==External links==

* [https://xeroxalto.computerhistory.org/_cd8_/pup/.pdp-ethernet-hardware.press!1.pdf PDP EtherNet Network Connection]

[[Category: UNIBUS Network Interfaces]]
[[Category: Ethernet Interfaces]]
[[Category: Xerox]]

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UNIBUS Experimental Ethernet interface