Pdp11/23+ EPROM upgrade

Pete Turnbull pete at dunnington.plus.com
Fri Feb 6 05:35:27 CST 2015

On 06/02/2015 10:11, Johnny Billquist wrote:
> On 2015-02-05 19:49, Roe Peterson wrote:
>> I want to update the EPROMs on my 11/23+, which currently has 2716s.
>> I'm going to need 64k chips, and I understand 2764s won't work on this
>> board.

Correct, wrong physical size.  DEC used Motorola MCM68766.

> 64K chips? The 11/23 currently uses the 2716, which is what? 2K (bytes)?
> I doubt there are any more address lines going to that chip in the first
> place, and second, the memory is mapped in the I/O space which have a
> total of 8K, and that should cover not only the boot rom, but also all
> CSRs of all devices you have in the machine. A 64K chip is going to be
> totally meaningless.

Not so.  Only the earliest release 11/23+ (KDF11-BA) used 2KB firmware; 
all the later versions (there are at least six) used 8KB and had more 
functionality.  The M8189 board is designed to take either.  In both 
cases, the ROM is paged exactly as it is in a BDV11, and only 256 bytes 
are visible at a time.  J22/J23/J24 are the links used to configure the 
sockets, and you'll find the settings on page 2-8 of the KDF11-BA Users 
Guide.  Seven address bits to the ROMs come from the bus, six are 
provided by paging registers on the CPU board, for a total of 13 address 
bits, or 8KB address space.

> So why are you looking for a 64K chip? (27512 I would assume that would
> be...)

2764.  He meant 64K bits.


Pete Turnbull

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