Fig. 2.21 Keyboard Connexion Scheme; Fig. 2.22 Signal Chart On The 7442 Decoder (Outlets A - C); Fig. 2.23 Signal Chart On The 7442 Decoder (Outlets 0 - 7) - ELCOM Euro-500 Handy Series Service Manual

Euro-500TX Handy
provide the keyboard functionality. The 7442 decoder circuit decodes the DG0 – DG2 signals and its output
pins 0 – 7 are used to generate the signals for individual keyboard columns. The ROW1 – ROW4 signals
are used to identify the pressed key.
The keys are constructed as the clamping units consisting from plastic covers and silicon rubber with
conductive protrusions made of carbon rubber. As the key is pressed, the pressure transfers to the silicon
rubber that will, thanks to its conductive protrusions, create a conductive connection directly on the keybo-
ard PCB.
The main power switch is also located on the keyboard PCB and is connected into the PWRSW connector
using the 2-wire cable. All other signals between the main PCB and the keyboard are transferred using the
16-wire FCC cable that is plugged into the KEYBOARDCON connector on the keyboard PCB on one side
and plugged into the KEYB connector on the main PCB on the second side.

Fig. 2.21 Keyboard connexion scheme

Service manual
U [V]
5
2.5
0
5
2.5
0
5
2.5
0
Fig. 2.22 Signal chart on the 7442 decoder (outlets
A – C)
U [V]
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
Fig. 2.23 Signal chart on the 7442 decoder
(outlets 0 – 7)
outlet A
1 2 3 4 5 6 7 8
outlet B
4 8
2 6
outlet C
2 6 8
4
5
0
t [ms]
2 4 6 8
5
0
2 4 6 8 t [ms]
5
0
t [ms]
2 4 6 8
5
0
2 4 6 8 t [ms]
5
v�
0
t [ms]
2 4 6 8
5
0
2 4 6 8 t [ms]
5
0
t [ms]
2 4 6 8
5
0
t [ms]
2 4 6 8
© Elcom, s. r. o.
t [ms]
t [ms]
t [ms]
outlet #0
outlet #1
outlet #2
outlet #3
outlet #4
outlet #5
outlet #6
outlet #7
19