LSI-11/2 CPU with tracks cut?

Tue Apr 4 03:36:26 CDT 2006

Hi!

I got a M7270 CPU module here from my PDP-11/03. It has the tracks to 
the QBus contacs BC1-BF1 (BDAL18, BDAL19, BDAL20, BDAL21) cut.

I of course wondered why one would do that. In [2] I found the following 
hint:

---
II.  Restricted Compatibility Options
Options in this category do not meet one or both of the requirements for
use in a 22-bit system.  These options are incompatible with some or all
22-bit systems.
...
KD11-HA	   M7270  LSI-11/2 CPU
                   (16-bit addressing only, and use of BC1,BD1,
                    BE1,BF1 for purposes other than BDAL18-21)
---

So I guess someone tried to make the CPU compatible with 18/22-bit modules?

What did the M7270 use these pins for? I didn't find any manuals or 
schematics for the this CPU yet, only for the LSI-11. Could someone 
point me into the right direction?

Thanks
Sebastian

[1] http://www.bitsavers.org/pdf/dec/pdp11/1103/EK-LSI11-TM-002.pdf
[2] 
http://www.ibiblio.org/pub/academic/computer-science/history/pdp-11/hardware/micronotes/numerical/micronote5.txt