andykmv
Active member
Williams Defender IO Board question..Re IC 6&7 74LS257's
1st dumb question for the week (from me that is)
.
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 ?
Cheers!
1st dumb question for the week (from me that is)
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 ?
Cheers!