23 Jul
2012
23 Jul
'12
3:26 p.m.
Right. These lines are driven by a 7417's on the controller board.
DEC seemed to like non-inverting buffers. The resistors on the front
panel board hold the line high and the 7417 needs to pull this to
ground to get the display segments to light up. Checked with the
scope on the front panel board and the second bit of the incoming BCD
value isn't going low. Pulling the line to ground on the front panel
board causes the 7 segment displays to read correctly so the BCD to 7
segment converter is OK. The cable to the controller board checks out
OK so the next step is to check the input and output on the 7417 gate
and see if the problem is there. Unfortunately I don't have extender
boards so will have to go with the old "solder wires to the chip"
trick.
On Mon, Jul 23, 2012 at 6:46 PM, Camiel Vanderhoeven
<iamcamiel at gmail.com> wrote:
On Mon, Jul 23, 2012 at 1:11 AM, Andrew Quinn
<jaquinn2001 at gmail.com> wrote:
The insight came when I looked at which keys on
the number pad worked
and which didn't. I came up with the following table
Pressed = Displayed
1 = 1
2 = 0
3 = 1
4 = 4
5 = 5
6 = 4
7 = 5
0 = 0
If you convert this to binary it becomes pretty obvious where the
problem lies. The invalid display is for the items marked with * and
all have the second bit set. If you mask out the second bit you get
the displayed value.
0 = 0000
1 = 0001
2 = 0010 *
3 = 0011 *
4 = 0100
5 = 0101
6 = 0110 *
7 = 0111 *
The front panel uses a 7447 BCD to 7 Segment decoder to drive the LED
modules. If pin 1 (B) on this is not being set when required then the
behaviour will be what I am seeing..... it isn't that the keypad is
getting it wrong... but the display is showing the wrong value due to
a missing bit.
Need to do a bit more digging here but given that this line is pulled
to +5V via a resistor on the front panel board I am hoping that the
problem is either
a connection problem on the front panel board with the line not being
pulled up or the 7447 needs to be replaced. If there was a break in
the ribbon table to the controller board then the bit would always be
set and a different set of incorrect values would be displayed.
Hopefully there isn't a fault on the controller board that is pulling
the pin to ground.
It's likely there's a driver chip on the controller board with an open
collector output (something like a 7438). That output could very well
be shorted to ground. That would in fact be my first suspect for this
error.
Camiel.