Extended UNIBUS
The Extended UNIBUS or EUB was DEC's name for both an upgrade to their standard UNIBUS PDP-11 bus, to add additional address lines, in order to support larger memories; and also for the slots in a backplane which instantiated the bus. The added address lines are A18-A21.
The slots were hex slot, and could hold only special EUB main memory cards, the only EUB cards ever produced. The only machines to support the EUB were the PDP-11/44 and PDP-11/24, which had a limited number of EUB slots in their main backplanes (the ones which held their CPUs).
EUB slots differed from the older MUD slots in that they re-purposed some of the pins in the A-B connectors to carry EUB signals (A18-A21, principally), thus making it not pin-compatible with the usual dual UNIBUS slot (which usually appears on the A-B connectors in such slots).
In the -11/24, at least, the EUB slots can also support a normal SPC device, in the C-F connectors usually used for SPC devices in hex slots. However, the two busses in such slots are not fully connected, as they are in MUD/SPC backplanes; instead, they form two somewhat separate busses. (See the PDP-11/24 article for the details.)
EUB memory boards
The only EUB-capable memories which DEC produced were the MS11-L, MS11-M, and MS11-P. The first two were jumperable for use on either the UNIBUS or EUB, although the second used the unusual power voltages of +12V/-12V, which most backplanes did not supply.
Other manufacturers also produced EUB memories.
Added signals
In addition to the extra address lines, power pins useful for battery backup of semi-conductor memory contents were added.
Pinout
The pins which were added (changed) from MUD to EUB are:
- AB1 - Reserved
- AB2 - Boot Enable
- AN1 - A21
- AP1 - A20
- AP2 - Reserved
- AR1 - +15V/12V Battery
- AR2 - Reserved
- AS2 - Reserved
- AT2 - Reserved
- AU1 - Reserved
- AU2 - Reserved
- AV1 - Reserved
- AV2 - Reserved
- BB1 - +5V Battery
- BB2 - Reserved
- BD2 - Reserved
- BE1 - A19
- BE2 - A18
- BV2 - Reserved
Removed signals
To provide the extra pins needed, since grants has already been removed, other interrupt-related signals (including requests) were removed. In addition, power pins for voltages which were no longer common (due to the replacement of core memory by semi-conductor memory) were also removed.
Pins
The pins which changed function from the MUD slot are listed below. Pins marked with a '*' were the ones which had retained their function from UNIBUS to MUD; the others had been already modified in that stage.
- AB1* - INTR
- AB2 - Test Point
- AN1 - Parity P1
- AP1 - Parity P0
- AP2* - BBSY
- AR1 - -15/12V Battery
- AR2* - SACK
- AS2* - NPR
- AT2* - BR7
- AU1 - +20V (core)
- AU2* - BR6
- AV1 - +20V (core)
- AV2 - +20V (core)
- BB1 - Reserved
- BB2 - Test Point
- BD2* - BR4
- BE1 - Internal SSYN
- BE2 - Parity Detect
- BV2 - -5V (core)
