Re: Early TED/Plus4/C116 Schematics

From: Gerrit Heitsch <>
Date: Sun, 08 Sep 2013 10:06:00 +0200
Message-ID: <>
On 09/08/2013 01:55 AM, Julian Perry wrote:

> Looking at the (correct) C116 schematic on Zimmers, it appears that
> whilst the keyboard scanning now is assisted by a 6529, the joystick
> exclusively uses the TED (and both ports are connected to the same
> lines!!).
> The SELECT line (not present on ATARI/C64 joystick ports) seems to be
> used to differentiate between joy0 and 1, and is connected to the data
> buss bits D1 and D2, albeit buffered by a 74LS125. From what I
> understand, select behaves as some sort of "inhibit/enable", which
> isolates the joystick ports from, each other, and can be used to
> differentiate between the two.

Yes, the idea being that you use this select signal instead of GND as 
the common line for all the switches. The Joystick is then only active 
if the select line (meaning data line) is LOW while you poll the TED 
input latch. The diodes in the data path between joystick and TED make 
sure that TED will only see LOW signals, meaning if the select signal is 
HIGH, no matter what you do with the joystick, it won't have an effect 
on the keyboard or the other joystick.

This also assumes that you don't press a button on the keyboard while 
doing so.

> I think it fair to say there is an awful lot I don't quite understand
> about how keyboard and joystick scanning works on the 264 series.
> Fascinating stuff, though.

It becomes clearer when you realize that the common point of all the 
switches on a joystick connected to a 264 series system is not GND but 
the Select signal which is a buffered D1 or D2. Also that TED will 
ignore any data written to the keyboard latch while using the write 
signal to latch anything found on the external pins. That way you can 
use the data on the databus to influence what is happening on the 
keyboard pins of TED.

Took me a while to work that out as well.


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Received on 2013-09-08 09:00:03

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