BDV11 Bus Terminator/ROM

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BDV11 board

The BDV11 Bus Terminator/ROM board is a QBUS board that includes:

Physically, it is a quad board, the M8012.

The board is normally supplied with 2KW of PROM containing diagnostic and bootstrap functionality, with an optional 2KW for more; there are also sockets available for an additional 18KW of user-supplied EPROM. This memory does not appear directly on the QBUS; a 256 word window, and a window control register, allows the entire contents to be seen.

In addition, the board provides a pair of DIP switches (readable under program control, except for one used for KW11-L functionality control); and also a set of red LEDs, a 'power OK' green LED, on-board RESET and HALT/ENABLE switches, and test points for on-board voltages. All of the latter are on the card edge, so they are accessible when it is plugged in. A single read/write register is provided for diagnostic purposes.

KW11-L functionality

The KW11-L functionality is enabled by switch E21-5. When on (closed), when the register is clear, the bus BEVNT line is held low, preventing timer interrupts. Setting the 'interrupt enable' bit allows timer interrupts to happen. The KW11-L 'monitor' function and bit are not provided.

KW11-L issue

NOTE: Once the KW11-L on a BDV11 has been enabled, RESET/etc do not disable it, unlike every other device in the PDP-11 universe! (This is apparently because they were short a QBUS receiver for the BINIT bus line, and didn't want to add another complete chip to be able to monitor it.)

So if you start some operating system which enables the KW11-L (e.g. UNIX), and then try and re-boot it by halting the machine and using the 'Go' command to ODT, you'll probably get a clock interrupt in the process of booting, which will trash it.

The only thing that clears it is either a manual disable (write 0 to the KW11-L's CSR); something which toggles the BDCOK bus line - e.g. using the BDV11's on-board RESTART switch, or a front-panel restart switch; or power cycling the machine.

Caveat haquur!

Board layout error

There is a layout error on the BDV11 which can cause damage if a BDV11 is plugged into a system with the optional -12V on pin AB2. The trace which is supposed to connect bus line BIRQ6, on pin AB1, to the pullup network is incorrectly routed to pin AB2. The ECO to repair this (DEC #2 for the BDV11) is fairly simple:

  • Cut the trace (on the solder side) between E1 pin 11 and AB2
  • Add a wire (on the component side) between E1 pin 11 and AB1

Bus termination upgrade

The BDV11 only provides termination for a Q18 QBUS; there is a relatively easy official DEC ECO (#5) to extend it to Q22, since there are 4 spare resistor networks available in one of the resistor packs. It is:

  • Add a wire from BC1 (BDAL18) to E1 pin 15
  • Add a wire from BD1 (BDAL19) to E1 pin 14
  • Add a wire from BE1 (BDAL20) to E1 pin 13
  • Add a wire from BF1 (BDAL21) to E1 pin 12

Further reading

  • Location of W13 on the BDV11, MicroNote #057

External links

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