HOW TO MAKE A REAL PLUS OUT OF A GX-4000!

WARNING: You do this project at your own risk!

What do you need?

• GX-4000
• an old CPC-Keyboard (only the keyboard not the Computer itself)
• 1 piece of double-sided PCB with 2*25pins (2.54mm - Expansion-port)
• 1 piece of double-sided PCB with 2*l7pins (2.54mm - Printer-port)
• 2 RAMs 41464-12
• 1 Resistor 33ohms
• 4 Resistor 2.2komhs
• 3 Resistor 10kohms
• 1 Cartridge from a normal CPC plus (or only the ROM! If you only have the ROM then you have to remove the old ROM from your GX-4000 Cartridge and to install your copy of the CPC-Plus ROM).
• some small wire (something like 0.1mm) - in german called "Kupferlackdraht"
• some normal wire used in electronic

What to do

First of all open your GX-4000 (2 screws) and search for the ASIC. It's the IC with 160 pins (yes, the IC With the really small pins :-) ).

First of all, we connect the 2 RAM-chips

Set them on the existing 2 RAM-chips (IC10,IC11) and connect all Pins except pin 16. This pin must be left free because it's the Chip-selection for the RAM. Now you have to connect the pins 16 from the upper RAMs together. At one pin, you also connect the 33ohm resistor. The end of the resistor will be connected with pin 91 of the ASIC.

Now search for IC8 (it's a 74HC4051N). Here you have to connect a resistor 10kohms at pin 9,10 and 11. The other side of the resistors will be fit together and then connected to pin 8 of the chip.

Now connect pin 119 from the ASIC with that side from C17, which is not connected with the ASIC. There you also have to connect a resistor 2k2ohms. The other side of the resistor has to be connected to ground.

Now connect a resistor 2k2 to the ground side of LK4 and one to the ground side of LK1. The other side of the resistors will be connected to pin 127 and pin 129 of the ASIC.

Now cut the pin 159 from the ASIC to the PCB. Instead of the old connection, connect the pin to the last resistor 2k2. the other end of the resistor will be connected with 5V on the PCB (5V will be found directly in front of pin 159 - it's the hole in front of pin 159).

Now we want to connect a keyboard to the GX-4000:

The keyboard-pins will be found at the ASIC from pin 71 to pin 80. Pin 80 is keyboard row 0, pin 79 is row 1, ... , pin 7l is row 9.

The lines for the keyboard will be found at the Soundchip (AY-3-8912 or IC14).

Sorry, but I can't give you a good instruction how to connect the pins with your CPC-keyboard because I know more than 6 different keyboard-pinlayouts which will be used by AMSTRAD. The easiest way to find the right pins is to look at your old CPC and connect the same soundchip-pins with the keyboard as on the old CPC.

For the rows look at the 74LS145 on your old CPC PCB. There you can find the right pins.

PIN	ROW
1..7	0..6
9..11	7..9

Connect the the same way to the ASIC like they were connected to the 74LS145.

TEST!!!

So, if you are ready with the GX-4000 up to this point you can make your first test. Connect the GX-4000 to your Monitor or TV and insert the CPC- Plus cartridge (not the original GX-4000 cartridge!). Then switch on the GX- 4000. On the screen should the the selection menu (F1/F2) appear. Now you can choose BASIC and the normal BASIC-screen should appear. Here you can test your keyboard.

Most errors were made at connecting the keyboard!

If all is right you can go on and make your Printer-port (if you need one) at the Expansion-port. I don't tell you how you have to connect the pins to the double-sides PCB'S because you can find the connections in every book about the CPC (e.g. your manual).

Printer-port:

All pins will be found at the ASIC:

PIN	SIGNAL
145	Strobe
146	Data Bit 0
147	Data Bit l
148	Data Bit 2
149	Data Bit 3
151	Data Bit 4
152	Data Bit 5
153	Data Bit 6
154	Data Bit 7
155	Busy

Expansion-port:

signals from the Z80:

PIN	SIGNAL
5	A15
4	Al4
3	A13
2	A12
1	A11
40	A10
39	A9
38	A8
37	A7
36	A6
35	A5
34	A4
33	A3
32	A2
31	A1
30	A0
13	D7
10	D6
9	D5
7	D4
8	D3
12	D2
15	D1
14	D0
16	INT
17	NMI
18	HALT
19	MREQ
20	IORQ
28	RFSH
27	M1
26	RESET
25	BUSRQ
24	WAIT (READY)
23	BUSAK
22	WR
21	RD
29	GND
11	5V
6	CLOCK (φ)

signals from the ASIC:

PIN	SIGNAL
126	Light Pen
128	Cursor
127	RAMDIS
129	ROMDIS
133	ROMEN
131	RAMRD
159	BUS RESET
119	EXP

Now you can connect every expansion to the GX-4000.

But if you want to connect your old DDI-1, there is a little thing, which you have to correct. The DDI-1 has an internal ROM and the Cartridge from the Plus also contains the AMSDOS. So you have to "kill" the selection-system from your old DDI-l.

DDI-1 modification to work with the CPC-Plus (not only for this project):

Open the DDI-1 Controller and remove the AMSDOS-Rom Then you have to cut pin 43 from the connector to the PCB. Now your DDI -1 will work correctly with your CPC-Plus or GX-4000.