 Unlike the ZX81, the TK85 used discrete logic circuits instead of a ULA, and during manufacture, several chips were scraped so that competitors couldn't copy the circuit.The circuit board had space for a AY-3-8912 sound generator chip (compatible with the ZonX-81 sound board), and although none came factory installed, it is possible to add the necessary circuits.The TK85 came with a copy of the 8K ZX81 floating point BASIC, and an additional 2K EPROM, mapped to addresses 8192-10240, containing machine code routines for use with tape files.