If we now turn to the N2 memory decoding you will see
a simplified version in Fig 1. As you can see, the address
decoder IC46 is only enabled by the decode PROM when /MREQ
goes low. This is fine for the 2708s, as their /CS access
time is 120ns, but is not suitable for connection to the
/CE pin of the 2716 or 2732. The answer is to change the
circuit configuration slightly as shown in Fig 2. Here the
/CS decoding of IC46 is no longer qualified by /MREQ, and
so as soon as an address appears on the address bus it is
decoded to a /CE line in order to start the data access on
that particuler EPROM. (Remember – this has to be done at
least 450ns before we want the data). Subsequently, if and
when /MREQ goes low, the high order address bits are
decoded in the PROM (1C47). If it is a valid access to the
EPROMs, then the Output Enable pin of all the EPROMs will
be taken low, and the one that has been selected by its
/CE line will output its data onto the Bus. (In this case
the /OE signal has to appear at least 120ns before the
data is required). If it is not an EPROM access, then the
outputs are not enabled, and the only penalty paid is that
one EPROM has been powered up but not used.
How to do it.
The following description will assume the
modification is being performed on a standard N2 to which
no exotic modifications have been made. Where the word
TEST appears, it indicates that you should put the N2 back
in its frame, power up, and check that Nas-Sys and Basic
still run. If they don’t, the reason for it should be
easily identifiable. For the wiring on the bottom of the
board I used a Vero wiring pen with the polyeurathane
covered solderable enamel copper wire. Its easy to use,
and makes a neat job of the modification.
Start by removing the N2 from its card frame and
placing it in front of you. Remove any EPROMs already
fitted in IC sockets 35-42. Remove any associated links
from LKS1. (This should just have the standard 3, 1-16 2-15
8-9). Remove all links from the strapping fields LKB1-LKB8.
TEST. Put the card component side down with the
EPROM sockets along the bottom. Ignoring the workspace RAM
at the right hand end, wire together all pin 19s of the
byte-wide sockets including the Monitor and the Basic ROM.
(This connects in A10.) See Fig. TEST. Next wire together
all pin 20s, this time only do the 8 empty sockets, DO NOT
INCLUDE THE MONITOR OR BASIC. TEST. Next wire together all
pin 21s, once again only do the 8 empty sockets, DO NOT
INCLUDE THE MONITOR OR BASIC. TEST. Turn the card over. On
the eight link blocks link pin 4 to pin 8. (Connects /CS
from decoder to /CE) – see Fig. TEST. Remove IC46 (LS155
decoder) from its socket and gently bend up pins 3 and 13.
Solder two lengths of thin insulated wire to these pins.