djm@nmtvax.UUCP (03/12/85)
[Software bugs make great camoflage for hardware bugs...]
Going by some local comments made by various Apple owners around here, having
both (or actually, all four) of the monitor ROMs available, without a RAM
card or other sillyness, would be a great boon. Since many people who have
relatively recently acquired their Apples, I thought I would mention a cheap,
easy, non-destructive way to do this.
The basic procedure is to convince an EPROM that it is exactly like the ROM
in the F8 slot. This is not difficult. All that must be done is tie all the
chip selects active, except for the one the Apple expects to work. However,
this doesn't do you much good, since you still have one monitor in one chip.
What you now do is extend the extra address bits (one or two, depending on
EPROM type) out to switches, which will connect the lines to +5 or ground.
An Example :
To replace the current monitor ROM with both the 'old' F8 ROM, and the current
ROM (assuming a II, II+, or IIe; IIc machines are weird, and I don't want to
think about them...the procedure is similar though, with a larger ROM (27256)),
acquire a 2732 with an access time no longer than 300 ns.
Next, get this chip burned with a copy of each monitor, one at $0000-$07FF, the
other at $0800-$0FFF. Remember which is at which address!
Now, take a standard 24 pin solder tail IC socket and make the following con-
nections:
wire pin 18 to pin 12 (enables an extra chip select)
gently (!) bend pin 21 straight out from the socket and solder a long
piece of wire to it (this will select which ROM copy is active)
also solder a long wire onto pin 24 (+5 line, used in selecting monitors)
and solder a long wire to pin 12 (ground, also for monitor selection)
finally, connect the wire from pin 21 to the center pole of a single-pole,
double-throw switch, the ground wire to one side, and the +5 wire to
the other side.
Now for the fun part. TURN POWER OFF ON YOUR APPLE !!!!!!!
Carefully remove the F8 rom from its socket (first row down from the processor
and to the left in IIs and II+s (I'm not sure where on a IIe).
Insert the modified socket into the rom socket (make sure that the side with
two solder connections is on the left and that pin 21 DOES NOT contact the
connector in the socket). Now insert the 2732 (you have it ready by now, of
course...). Make sure the notch or dot on the chip is facing you and that no
pins get bent.
Verify that you've done everything thing right three times. If you still
think it's right (silly person), make sure that nothing is touching anything
it shouldn't and turn on the power and hit reset. If you get a beep (or the
Apple beeps on power up), then whichever monitor you have selected is running
ok. If you don't, power off immediately and check the circuit.
Assuming everything's worked so far, I hear the murmur of voices asking "Which
monitor is which?" Well, it's simple. The one you burned into the EPROM at
$0000-$07FF is the active one when the switch connects pin 21 to ground. The
monitor at $0800-$0FFF (again in the EPROM) is active when pin 21 is connected
to +5. Try flipping the switch and looking to see if it really switches the
monitors back and forth. If it doesn't, either the same monitor is in both
'banks', or something is shorting pin 21 to ground or +5. Check it again just
to be sure.
Although this mod works on my Apple and a friend's Apple, I can not in good
conscience accept any responsibility for damage caused by doing this change.
Also, please be aware that this will probably void your warranty, unless you
can con your dealer into doing it (good luck).
Share and Enjoy, for tomorrow the Worms arrive
Dieter Muller
{lanl|unmvax|unm-cvax}!nmtvax!djm