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M7551-CA memory size - anyone else ever notice this?

leegleason

leegleason

Experienced Member
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Jan 20, 2014
Messages
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I'm putting together some systems from spare parts here. I loaded a BA23 with an 11/73 processor, and an M7551-CA memory card, along with some other assorted options. Loaded RSX11M+. It indicated that there was only 1 MB of memory. 11/73 startup menu Map function also says, 1 MB present. Online specs from various sites say that an M7551-CA is 4 MB. I pulled out the card and had a look at it. It is labelled on the etch as "M7551-CA 4MB Q22.". I looked at the memory chips. They are 64KX1 chips. There are 8X18 of them - that's 1 MB (+parity) .

So the specs and marking on this card are just plain wrong. Anyone else ever come across this?.
 
I have a number of National Semiconductor equivalent cards. They are also marked 4MB - but only if they are fully populated!

I have 7 cards with only 1 MB (plus the parity chips) and 2 with a full complement of 4MB.

Dave
 
m_thompson said:
They probably used the same PCB for the 1MB and 4MB versions of the board. The handle would say M7551-CX of it was a 4MB.
Click to expand...

https://gunkies.org/wiki/MSV11-Q_QBUS_memory

The MSV11-Q (M7551) is a quad-height QBUS DRAM main memory card. Initially it used 64Kx1 DRAM chips, later ones use 256Kx1 DRAMs. The memory is arranged as 8 banks, each 16 data bits (1 PDP-11 word) wide, with 2 additional bits for parity (1 per byte).

It holds 1 MByte with 64K DRAMs; 4 Mbytes when fully populated with 256K DRAMs, or 2 Mbytes when half-populated (the only partially-filled configuration allowed). Three versions exist:
  • MSV11-QA, 1 Mbyte (64K DRAMs)
  • MSV11-QB, 2 Mbytes (256K DRAMs)
  • MSV11-QC, 4 Mbytes (256K DRAMs)
all are Q22, and support block mode. The -QA comes in two etch revisions; the latter version supports battery backup. The -QB and -QC are the same etch as the later -QA.
 
On a related note, are there XXDP tests for rigorously testing these boards? I have one (MSV11-QA) that passes a trivial memory test but produces a crash when booting 2.11BSD.
 
Dare said:
On a related note, are there XXDP tests for rigorously testing these boards? I have one (MSV11-QA) that passes a trivial memory test but produces a crash when booting 2.11BSD.
Click to expand...

http://www.bitsavers.org/pdf/dec/pdp11/xxdp/PDP11_DiagnosticHandbook_1988.pdf

Page 1-75 VMJAB0
Page 1-76 VMSAC0

Also good to run the appropriate CPU tests too:

1-54 KDJ11-B CPU OKDAG0

1-55 11/53 OKDDD0

1-78 KDJ11-A CPU ZKDJB2
1-79 KDJ11-A MMU ZKDKB0
1-80 KDJ11-A FP ZKDLB0
1-81 KDJ11-A CACHE ZKDMB0

I had one M8192 KDJ11-A which caused memory test failures with multiple memory boards. The failures went away when the M8192 KDJ11-A was replaced with a different one.
 
Dare said:
On a related note, are there XXDP tests for rigorously testing these boards? I have one (MSV11-QA) that passes a trivial memory test but produces a crash when booting 2.11BSD.
Click to expand...

Are you sure it's not something with the install of BSD? Not likely I would think, but just something to consider.
 
Dare said:
Thanks for the pointers. I found that the MSV11-Q manual (EK- MSV1 Q- UG-002) says to use "CVMSAA". However, after a little digging, I now interpret this to be an earlier revision of VMSAC0. VMJAB0 seems to be targeted at MSVll-J memories.
Click to expand...

Yes, the two last letters are the version, which is why you'd normally run all XXDP diagnostics as "R VMSA??".
(substitute whatever diagnostic you want to run, but I hope you get the idea.)
 
tradde said:
Are you sure it's not something with the install of BSD? Not likely I would think, but just something to consider.
Click to expand...

Don't think so. The system boots normally with a couple of 128KW boards (an MSV11-L and an NS32M).
 
Dare said:
Don't think so. The system boots normally with a couple of 128KW boards (an MSV11-L and an NS32M).
Click to expand...

What kind of error do you get when booting BSD with that card? And have you tried any other OS?
 
bqt said:
What kind of error do you get when booting BSD with that card? And have you tried any other OS?
Click to expand...

Well, I have to admit that this turned out to be operator error. One of the dip switches setting the ending address wasn't fully toggled, resulting in an end address beyond the installed memory. Correcting that allowed the system to boot:

@76000G
73Boot from ra(0,0,0) at 0172150
:
: ra(0,0,0)unix
Boot: bootdev=02400 bootcsr=0172150

2.11 BSD UNIX #2: Mon Aug 16 15:41:29 PDT 2021
root@ragtag73.toaster.com:/usr/src/sys/RAGTAG73

ra0: Ver 5 mod 13
ra0: RA81 size=1953236

phys mem = 1048576
avail mem = 868928
user mem = 307200

August 16 17:42:45 init: configure system

hk ? csr 177440 vector 210 skipped: No autoconfig routines.
ht ? csr 172440 vector 224 skipped: No autoconfig routines.
ra 0 csr 172150 vector 154 vectorset attached
rk ? csr 177400 vector 220 skipped: No autoconfig routines.
rl ? csr 174400 vector 160 skipped: No autoconfig routines.
tm ? csr 172520 vector 224 skipped: No autoconfig routines.
tms ? csr 174500 vector 260 skipped: No autoconfig routines.
ts ? csr 172520 vector 224 skipped: No CSR.
xp ? csr 176700 vector 254 skipped: No autoconfig routines.
erase, kill ^U, intr ^C
# Fast boot ... skipping disk checks
checking quotas: done.
Assuming non-networking system ...
checking for core dump...
preserving editor files
clearing /tmp
standard daemons: update cron accounting.
starting lpd
Mon Aug 16 17:43:21 PDT 2021


2.11 BSD UNIX (ragtag73.toaster.com) (console)

login: root
erase, kill ^U, intr ^C
#

Thanks for the help. And apologies to the OP for hijacking the thread.
 
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