Spectrum +3

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A Spectrum +3

The ZX spectrum +3 was the last (official) speccy. It was designed and manufactured by Amstrad using as few bits as possible but slapping one of their truly awful 3" disk drives as used by the Amstrad CPC, rumor has it because they bought a huge consignment of them from Asia.

1 Specification
2 Part list
3 Add ons
4 My mods
5 IO Ports
- 5.1 General machine control port
6 Floppy disk controller
7 Ports
8 My speccy

[edit] Specification

Z80 running at approx 3.5Mhz
128Mb of ram
64Mb of rom
FDC uPD765 compatible with SED 9420C PLL
AY-3-8912 Audio

[edit] Part list

3.5Mhz Z80
41464 64kx4 ram
FDC uPD765 compatible
SED 9420C PLL
AY-3-8912 Audio
TEA2000
MC1376
MC1488 QUAD RS232 driver
MC1489 QUAD RS232 reciever
74273 (the Centronis buffer)
7414 (6x inverters)
7404 (6x inverters)
7400 (4x NAND gate)
Amstrad version of the 27256

[edit] Add ons

I have a variety of add ons for this computer.

(Datel) Kempston mouse / Kempston joystick unit
Kempston joystick interface. From the PCB, this is an original Kempston micro issue.
(Home built) 8255 output module. Provides 24 TTL I/O ports.
(Home built) Second Disk drive.

[edit] My mods

My +3 has a Hard disk. Its basically the +3E setup with the simple 8 bit interface. THere are two parts to this, a set of replacement roms and an internal interface.

[edit] IO Ports

[edit] General machine control port

This port was the only one used on the unexpanded 48K, and it is used for both keyboard, border and Tape IO.

Write:

0 \
1 | Border colour (0-7)
2 /
3 Mic output bit

Read:

This is complex, the LSB of the port outputted is $FE. and the MSB is a strobe for they keyboard line. Each bit returned is one key. so:

FEFE Shift->V
FDFD A->G
FBFE Q->T
F7FE 1->5
EFFE 0->6
DFFE P->Y
BFFE Enter->H
7FFE Space->B

Keys are active low.

[edit] Floppy disk controller

Address: 2FFD (rw) Status/command register of the uPD765

Address: 3FFD (rw) Data register of the uPD765

Note, the motor is controlled using bit 3 of Memory control 2.

[edit] Memory control 1

Address: 7FFD

0 \
1 | Ram page for the ram $C000 -> FFFF (0-7)
2 /
3 Page for display. 0 = page5, 1 = page7.
4 Rom select (Low byte)
5 Paging lock. If set, the current pages are locked until the next hard reset.

[edit] Memory control 2

Address: 1FFD

0 Paging mode. 0=normal, 1=special
1 In normal mode, ignored. In special mode, bit 0 of page layout
2 In normal mode, high bit of ROM selection. In special mode, bit 1 of page layout
3 Disk motor 1=on
4 Printer port strobe.

Roms: (MSB = 1FFD bit 2, LSB = 7FFD bit 4)

0: 128k editor, menu system and self-test program
1: 128k syntax checker
2: +3DOS
3: 48 BASIC

Special mode: Special mode has four different configurations, designed for CPM and other OS's where ROM would get in the way.

        00     01     10     11
0xffff +---+  +---+  +---+  +---+
       | 3 |  | 7 |  | 3 |  | 3 |
0xc000 +---+  +---+  +---+  +---+
       | 2 |  | 6 |  | 6 |  | 6 |
0x8000 +---+  +---+  +---+  +---+
       | 1 |  | 5 |  | 5 |  | 7 |
0x4000 +---+  +---+  +---+  +---+
       | 0 |  | 4 |  | 4 |  | 4 |
0x0000 +---+  +---+  +---+  +---+

[edit] Sound

Address: (Write) $FFFD - Register select
Address: (Read) $FFFD - Register read
Address: (Write) $BFFD - Register write

[edit] Ports

The computer has the following ports:

[edit] My speccy

As can be seen from the picture, my speccy has been through the wars somewhat. The thing on the back is a passthrough that I use for testing hardware. You can plonk wires in there and take them straight to a breadboard without having to bother with the soldering iron.

Inside the speccy you can see a surfeit of chips. Mostly dominated by the big ASIC at the bottom right. Most of the chips that are left are ones that either aren't easily ASIC'd (The z80, 4 ram chips, rom, ect), buffers for signals that go to the Centronics or RS232 ports, or things that were prone to go if the crappy Floppy drive dies (I cant stress just how utterly bollocks this mechanism is. Really, its toss.)