ZX Spectrum +3 was developed by Amstrad PLC., Released in 1987
The ZX Spectrum +3 looked similar to the +2 but featured a built-in 3-inch floppy disk drive (like the Amstrad CPC 6128) instead of the tape drive, and was in a black case.
The +3 saw the addition of two more 16 KB ROMs. One was home to the second part of the reorganised 128 ROM and the other hosted the +3’s disk operating system. This was a modified version of Amstrad’s PCWDOS (the disk access code used in LocoScript), called +3DOS. These two new 16 KB ROMs and the original two 16 KB ROMs were now physically implemented together as two 32 KB chips. To be able to run CP/M, which requires RAM at the bottom of the address space, the bank-switching was further improved, allowing the ROM to be paged out for another 16 KB of RAM.
The ZX Spectrum +3 power supply provides the same voltages as the one supplied with +2A. This power supply has the same DIN connector so can also be used with the +2A. However, the power supply purchased with the +3 had ‘Sinclair +3’ written on the case.
The ZX Spectrum +3 was launched in 1987 and was the only Spectrum capable of running the CP/M operating system without additional hardware.
Some older 48K and a few older 128K games were incompatible with the machine. Such core changes brought incompatibilities:
1. Removal of several lines on the expansion bus edge connector (video, power, and IORQGE); caused many external devices problems; some such as the VTX5000 modem could be used via the ‘FixIt’ device.
2. Dividing ROMCS into 2 lines, to disable both ROMs.
3. Reading a non-existent I/O port no longer returned the last attribute; caused some games such as Arkanoid to be unplayable.
Memory timing changes; some of the RAM banks were now contended causing high-speed colour-changing effects to fail.
4. The keypad scanning routines from the ROM were removed.
5. Move 1 byte address in ROM.
Production of the +3 ceased in December 1990, believed to be in response for Amstrad relaunching their CPC range. At the time, it was estimated about 15% of ZX Spectrums sold had been +3 models.