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README
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1999-01-10
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WARNING
-------
Connecting selfmade hardware to your Amiga is never a good idea.
Although I do mention some particular dangers in the following
paragraphs, there are a lot more things you might do wrong. If you
feel uncomfortable with this idea, quit now before you can do any
damage. Otherwise proceed at your own risk.
Power supply
------------
One of the most dreadful things that can happen to your Amiga is
blowing a fuse on one of the external connectors. These fuses are
soldered to the motherboard (at least that's true for A2000, and I
don't think things have become any better with the later Amiga models)
and, as you can imagine, they are very hard to replace.
So when in doubt, always use an external power supply for your I²C bus
interface and devices, and don't connect any more lines than are
absolutely necessary to establish data communication. And, for
heaven's sake, always make sure you connect the right lines.
Now for some details on the power outputs:
- Parallel port: +5V on pin 14, up to 100 mA (not present on A1000).
Don't use this supply, unless you've made sure that your I²C chips
require significantly less than that current. Hint: *Never* use this
power supply for a teletext decoder, because 100 mA is about exactly
what that chip consumes!
- Serial port: ±12V on pin 9/10, up to 50 mA (not present on A1000).
Alright, here's one exception: It would be foolish *not* to use
this power supply. After all, you'll only use it for the RS232 line
driver, and that's what it apparently was intended for.
- Floppy port: +5V on pin 12, up to 270 mA (410 mA surge).
If you already have some external floppies, try to determine their
power requirements first, to see how much would be left for other
devices. But no matter what the result, it is *always* a bad idea to
blindly connect some selfmade and not yet tested hardware to this
supply, because defective hardware might just short out the power.
(Don't say that's silly, it has happened to me.)
Note: According to RKRM, there are also some Amigas rated for
540 mA (870 mA surge) on this line. I suppose this refers to the
"big" models (A2/3/4000), but unfortunately RKRM doesn't state
that for certain. So unless you can confirm this information from
your user's manual, always assume that the current limit is
270 mA. :-(
The floppy interface
--------------------
The relationship between SELx signals and the pin numbers that they
can be found on depends on what position in the floppy chain you are
talking about. So to clarify things:
The line that is designated "SEL1/Pin 21" in the diagram should always
be connected to Pin 21. Don't worry about which SELx line the
interface will ultimately end up with, i2c.library will find it.
The I²C interface as shown in the diagram is intended to be the last
device in the floppy chain.
Other configurations are conceivable, too, but would be more
complicated. If you want your I²C interface to have both a "disk in"
and "disk out" connector, you can't just "tap" the floppy bus, you
also have to kind of rotate the SELx lines between both connectors:
disk in disk out
21 this is the SELx signal that you listen to
9----->-----21
20---->-----9
20 this line is not connected for the next drive
One more important note on the floppy interface. For reasons that I
can't quite comprehend, it seems that the plain SN7401 isn't available
from many dealers, the 74LS01 seems to be much more common. That's
alright, however, because LS (low-power) or not doesn't make much of a
difference here. But NEVER ever use a 7400 NAND chip instead of the
7401! The 7401 has open-collector gates, the 7400 hasn't! Such a
modification to the interface design would not work and would most
likely result in damaging or destroying your I²C peripherals!