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From: EWTILENI@pucc.Princeton.EDU (Eric Tilenius)
Newsgroups: comp.sys.m6809
Subject: Don't leave Hi-Res Interface plugged in using other joysticks
Message-ID: <3783@pucc.Princeton.EDU>
Date: Sat, 7-Nov-87 19:30:04 EST
Article-I.D.: pucc.3783
Posted: Sat Nov  7 19:30:04 1987
Date-Received: Tue, 10-Nov-87 04:47:23 EST
Reply-To: EWTILENI@pucc.Princeton.EDU
Organization: Princeton University, NJ
Lines: 37
Disclaimer: Author bears full responsibility for contents of this article


* The following information comes from Marty Goodman...
      Attention!  Caution!  Here is a piece of information regarding the Hi Res
Joystick Interface:
  (1) Whenever you have either the Tandy Hi Res joystick interface or the CoCo
Max 3 Hi Res joystick interface plugged into your CoCo 3, it will interfere
with the operation of any NORMAL joystick you may have plugged into the other
port.
     Specifically, the Hi Res joystick interface will cause any normal joystick
 plugged into the other port to fail to cover its full range.  Such normal
joysticks will only cover a range of 0 to about 52, not the full range of 0 to
63.   On some programs, this change will not be noticed at all. This might
include arcade games that often only resolved the joystick to a fraction of its
 full resolution, both because they need no more than that, and because this
allows them to read the joystick faster.  However, any program that uses the
full resolution of the a normal joystick will not work properly while the hi
res pack is plugged in.
     The reason why this problem occurrs is that the hi res joystick interface
utilizes an LM3900 operational amplifier chip, that is powered off the five
volt line of the joystick port.  Now, that five volt line, in turn the five
volt line of the joystick port.  Now, that five volt line, in turn, is supplied
to the port via a 100 ohm current-limiting resistor.  The added load of the
LM3900 causes the voltage to the joystick to drop below five volts, and this,
in turn, results in any normal joystick not being able to generate the
higher-numbered values of its range.
     A possible fix would be to short out that 100 ohm current limiting
resistor inside the computer. This resistor is R13, color bands brown, black
brown, and is located near the rear-most of the four socketted 64K by 4 DRAM
chips.  This "fix", however, has problems, because it removes the safety
provided by that current limiting resistor. A malfunction of the joystick or
the hi res pack might, if R13 is shorted, be much more likely to harm the
computer, or at least cause it to crash.
 
- ERIC -             * Another proud CoCo 3 user *        ______________
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