INMC 80 News


October–December 1981 · Issue 5

Page 32 of 71

2K’s on an N2


E. P. T. Anderson

The Nascom 2 is equipped with eight 24 pin sockets intended for 2K static RAMs or 1K EPROMS of the 2708 type. Although the initial literature – and advertising – stated that the use of these sockets was very flexible, it omitted to state how to use them for the now economical and hence popular 2516/2716 2K EPROMS. Furthermore the writer failed to understand the Block A/Block B etc, connections in spite of help from INMC magazine.

The N2MD ROM (IC47) give some convenient chip select pulses which are 4K wide so together with an A11 or A11 signal it is quite easy to define 2K boundaries. As well as suitably enabling the EPROMS it is also necessary to enable IC44 and this is done by the addition of simple diode gating.

STEP 1 (Solder side of board – see fig 4)

Wire all sockets to A10. This is done by strapping all pins 19 to A10 which is available on Pin 19 of the monitor ROM (IC 34)

STEP 2 (Solder side of board – see Fig 4)

For EPROMS which are to be operated on the lower 2K of a 4K block wire pin 18 to A11 which is available on Pin 18 of the BASIC ROM (IC 43)

STEP 3 (Solder side of board – see Figs 2 & 4)

For EPROMS which are to be operated on the upper 2K of a 4K block wire pin 18 to A11. A11 does not appear to be available so it is necessary to invert A 11. As the 1K chip select signals from IC46 are not now required, remove the 16 pin 74LS155 from this socket and replace with a 14 pin 74LS04 (or 7404) to the top of the socket (Pin 1 to Pin 1). Luckily, A11 is present on pin 3 which is the input of one of the 7404 inverters and hence A11 is then available on pin 4. Hence connect pin 4 to pin 19 of the upper EPROMS. Pin 7 of the 7404 must be strapped to the now vacant pin 8 of the socket for the 0v line.

From Fig. 2 it can be seen that the five spare inverters on the 7404 are not loading any circuits – the majority of the connections going to the link blocks adjacent to the EPROMS.

STEP 4 (Solder side of board – see Fig 4)

Connect pin 21 of all EPROM sockets to +5 which is present on pin 24.

STEP 5 (Component side of board – see Fig 1)

Wire the appropriate 4K block pulse available on pins 9 to 16 of LKSI to pin 20 of the appropriate EPROM via the link block pin.

Generally you will use:

D4B000 – BFFFPin 12
D5C000 – CFFFPin 11
D6D000 – DFFFPin 10
Page 32 of 71