triggers a sampling of the current count of this divider. If the sample is
0
, the
frequency generated by the 16-divider is considered to be synchronous with
data arriving from the floppy drive and the counter keeps counting. If the
sample is not zero, a positive or negative jump by 1 or 2 is made in the
counting sequence, according to the shift that was detected, so that the
number in the 16-divider is brought closer to the correct synchronousness
value. The
71488
PROM
is programmed as follows:
Address
Contents
Shift
Address
Contents
00
01
02
03
04
05
06
07
08
09
0A
0B
0C
0D
0E
0F
01
01
02
03
03
04
05
06
0B
0C
0D
0E
0F
0F
00
01
0
-1
-1
-1
-2
-2
-2
-2
+2
+2
+2
+2
+2
+1
+1
+1
10
11
12
13
14
15
16
17
18
19
1A
1B
1C
1D
1E
1F
01
02
03
04
05
06
07
08
09
0A
0B
0C
0D
0E
0F
00
Circuit
U09/6
performs a division by 2 on the output signal coming from
the 16-divider, so that the
RDW
signal (data window) is generated from the clock
signal
CK
through a division by 32 with a compensation of the shift occuring
between
RDW
and the data read from the floppy disk.
3. KEYBOARD
The keyboard consists of 58 keys, of which 48 are organized as an 8x6
matrix, and the other
10
are used for generating some commands that on a
standard
ZX SPECTRUM
are generated by simultaneously pressing
CAPS SHIFT
and another key.
The detection of a key press is done as follows: when interrogating the
keyboard, the keyboard lines are connected to the higher 8 microprocessor
address lines (
A8—A15
) separately through diodes
D11—D18
. During a keyboard
reading cycle, these lines are sequentially brought to a “
0
” logical level, one by
one, one at a time while the other 7 are in a “1” logical level. If a key is
pressed, by the electrical contact made in that particular matrix node, the “
0
”
level is propagated through the column (
k0—k6
) corresponding to the key that
was pressed, all the way to the
PA
input port of the
i8255
circuit.
The keyboard schematics are shown in
Fig.10
and
Fig.11
.
19
Summary of Contents for CoBra
Page 20: ...Fig 10 Keyboard schematic 20 ...
Page 21: ...Fig 11 Keyboard schematic 21 ...
Page 23: ...Fig 13 Power source schematic 23 ...
Page 39: ...9 APPENDIX 1 CoBra Microcomputer Schematics 39 ...
Page 40: ...Fig A1 1 CoBra Microcomputer Central Processing Unit 40 ...
Page 41: ...Fig A1 2 CoBra Microcomputer Configurator and Selector Circuit 41 ...
Page 42: ...Fig A1 3 CoBra Microcomputer Read Only Memory Circuit 42 ...
Page 43: ...Fig A1 4 CoBra Microcomputer DRAM Memory Circuit 43 ...
Page 44: ...Fig A1 5 CoBra Microcomputer Memory Access Prioritizer and Command Logic 44 ...
Page 45: ...Fig A1 6 CoBra Microcomputer Video Address Generator Circuit 45 ...
Page 46: ...Fig A1 7 CoBra Microcomputer Video Address Multiplexer Circuit 46 ...
Page 47: ...Fig A1 8 CoBra Microcomputer Video Memory Circuit 47 ...
Page 48: ...Fig A1 9 CoBra Microcomputer Video Sync Pulses Generator Circuit 48 ...
Page 49: ...Fig A1 10 CoBra Microcomputer Video Signal Shape Generator 49 ...
Page 50: ...Fig A1 11 CoBra Microcomputer Interfaces 50 ...
Page 51: ...Fig A1 12 CoBra Microcomputer Voltage Level Adapter Circuits 51 ...
Page 52: ...Fig A1 13 CoBra Microcomputer Keyboard Interfacing Circuit 52 ...
Page 53: ...Fig A1 14 CoBra Microcomputer TV Monitor Interfacing Circuit 53 ...
Page 55: ...10 APPENDIX 2 Flopppy Disk Interface Schematics 55 ...
Page 56: ...Fig A2 1 Floppy Disk Interface Disk Controller 56 ...
Page 57: ...Fig A2 2 Floppy Disk Interface Command and Control Signals Generator Circuits 57 ...
Page 58: ...Fig A2 3 Floppy Disk Interface Write Clock and Digital PLL Circuits 58 ...
Page 59: ...10 APPENDIX 3 Component Placement on Boards 59 ...
Page 60: ...60 Fig A3 1 Component Placement on Keyboard Circuit Board ...
Page 61: ...61 Fig A3 3 Keyboard Circuit Board top layer seen from above keys side ...
Page 62: ...62 Fig A3 3 Keyboard Circuit Board bottom layer seen from above keys side ...
Page 63: ...Fig A3 5 Component placement on the floppy interface board 63 ...
