Williams Defender IO Board question..Re IC 6&7 74LS257's

andykmv

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Williams Defender IO Board question..Re IC 6&7 74LS257's

1st dumb question for the week (from me that is):p.

In the schematic for Defender IO board, a 74LS257 ic (ics 6 & 7) sits between the PIA chip and the various CP button conditioning circuitry.

By removing jumper W2 as per instructions, it looks like the Select input is then tied high.

According to the TTL manuals/datasheet for this chip:
- inputs I0-3 (A0-3 on Def Schematic, pins 2,5,11,14) are selected by setting Select LOW (pin 1 on ICs 6 & 7), and
- inputs I1-3 (B0-3 on Def Schematic, pins 3,6,10,13) are selected by setting Select HIGH (pin 1 on ICs 6 & 7).

According to the April 1981 Defender schematic, with W2 removed to suit Defender, CB2 output is no longer connected and therefore there is no way of changing the Select input to the 74LS257's, IC's 6 & 7. This means the Select pin (pin 1) is tied HIGH on both devices and therefore always selecting inputs I1a-d (B0-3 on Def Schematic, pins 3,6,10,13), and by deduction input cannot be received from inputs I0a-d3 (A0,1,2,3 on Defender schematic).

What am i missing here ? is there an error or omission in the schematic ? :confused:

Cheers!
 
Sorted

DOH! I was reading the defender IO board schematic on the train on the way home today and realised why i saw no
pulsing on the Select line of IC6&7, and therefore why it was such a dumb question in the first place!
Connectivity between CB2 at pin 19 of the PIA and the Select input at pin1 of ICs 6&7 (74LS257 ICs) thru W2 is not
required because ALL defender buttons/joystick connections either go through IC6&7 input B0-3 connections, or
bypass IC6&7 entirely so it is normal for the Select line on IC 6&7 to be tied high.
ie There is no need to switch from IC6&7 B inputs to the IC6&7 A inputs at all!
these circuits are looking less and less complicated after all!
Cheers!
 
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