Caple C871i Manual do Utilizador descarregar pdf (Página 95)

100

101

Apple II Computer Info

Resistor goes to pin 1 (/Q output) of 74121 IC

74121 IC pin 7 goes to GND

pin 14 goes to +5V

pin 3 goes to 74LS125 pin 8 (connect to R16 at point

farthest from ULN2003)

(pin 1 goes to resistor to LED Green)

(pin 6 goes to resistor to LED Red)

pin 11 goes to + end of mini 10uF capacitor

pin 12 goes to - end of the mini 10uF capacitor; and

pin 12 goes to a 22000 Ohm resistor

Resistor goes to +5V (can connect to 74121 pin 14)

The NPN transistor can be any general purpose NPN type (e.g. a

2N2222, etc.). It's input (Base) is driven by the /Enable after it has

been inverted by a gate of the ULN2003. When /Enable goes low to select

the Drive, the signal at the Base goes high. This connects the LED

common lead to GND through the transistor.

If the Drive is selected (Enabled) and there is no Write operation,

the Green LED will get a + voltage through its 150 Ohm resistor from the

74121 /Q output. So; the Green LED will light when the Drive is enabled

(e.g. during boots and for any READs).

If the Drive is selected (Enabled) and there is a Write operation

and Write Protect is not ON, then, the /Write Request signal at 74LS125

pin 8 will go low. This triggers the 74121 One-Shot (at pin 3) producing

an aprox. 120ms pulse at output Q. For the duration of the pulse, +V

goes to the Red LED through its 150 Ohm resistor. The Red LED lights.

(Meanwhile, /Q has gone low so that the Green LED turns OFF.)

Probably, the first circuit posted earlier will work, too; but, I'm

still not sure that Write Request is always long enough to be easily

visible. This is why the One-Shot IC circuit is used. (You can make the

pulse longer by using larger values for the capacitor or increasing the

value of the 22000 Ohm resistor.)

Anyway, the circuit works well for showing Reads and Writes with a

standard bi-color LED. It's the one I installed on our Disk ][ drive.

If you decide to try the NOR gates circuit posted earlier, the mod

below is a simpler version which should do the same thing

....

In the circuit below, "[ ]" numbers are Disk ][ Analog board

connector points and "( )" numbers refer to pins on a 7402 Quad

dual-input NOR gate IC.

[10*]/WR-REQ ------ (2)

NOR-1 (1)---- 150 Ohm Resistor to LED Red lead

[14] /Enable -------(3) |

_________________________|

|__________(11)

NOR-3 (13)--- 150 Ohm Resistor to LED Green

lead

[14] /Enable -------(12)

Apple II Computer Technical Information : Apple II Family Hardware Info

ftp://ground.ecn.uiowa.edu/2/apple2/miscinfo/hardware : May 2001 : 95 of 572

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Caple C871i Manual do Utilizador Página 95

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