Ed Haasch of Greenfield, MI, suggests that you use a DIP header in place of U17 and not remove R16 to connect expanded RAM to your MicroAce. (see “Build Additional RAM” Mar, 81.) By leaving R16 in place, you can use your original 2K by plugging U17 back in. Pins 2 & 4 carry the RAM signal. Pin 3 is CS1 & pin 6 is CS2.
To use a Radio Shack 40-pin connector to connect additional RAM, run a jumper from data line 7′ (pin 1T) to pin 23B instead of using that space for RAM. (But see the ZX81 pinout following.)
We got some surplus 100-pin connectors and cut 46-contact sections from them (23 top and 23 bottom contacts to fit the ZX80 bus). Two hints: First, use a hacksaw blade made for cutting hard materials–the housings are glass-filled. Also, cut on an extra contact point and sand the housing down to size to avoid removing insulation next to the end contact.
Sinclair‘s new ZX81 uses all the edge connector pins with minor changes. Pin 23B is now used for ROM CS, pin 2T becomes RAM CS, and pin 12T becomes NAY. Because Sinclair’s printer is compatible with both designs, you may want to remember their pin assignments when you assign pins for your designs.
Some video monitors need external sync signals instead of composite video. For these devices, you must supply vertical and horizontal sync pulses to separate terminals.
You can pick up the vertical sync pulses at IC11 (U22) pin 12 for negative-going pulses or pin 10 for positive pulses. Pick up combined horizontal and vertical sync pulses at IC19 (U13) pin 5 for negative signals or pin 6 for positive signals.
Video output appears at pin 4 of IC 20 (U18) after the reversing switch. Composite video appears at the junction of R30 and R32.
Mike Neidich of Syosset, NY, suggests adding a reset switch to avoid having to pull the power plug after every crash. Connect a normally open, momentary contact pushbutton across C10 (C2 in a MicroAce). Mike says the best attachment points are at the left end of R4 and the modulator’s zero-volt lead. Drill the rear of the case to mount the button.
Use your cassette’s built-in loudspeaker for monitoring while loading. Ted Millen, Don Mills, Ontario, gives these instructions. On the earphone jack, find the two solder points that short together when the jack is removed. Solder a quarter-watt, 100 Ohm resistor across these terminals to feed the speaker when the computer plug is inserted. This technique provides ample monitoring volume and does not affect the computer feed.