2A 3 Floating Bus Test - redcode/ZXSpectrum GitHub Wiki

Author: Hikaru

Version	Release date	Links	Download
v1.2	2017-09-15	Forum post	tap (hosted) - tap (original URL)
v1.2	2017-09-13	Forum post	tap (hosted) - tap (original URL)

48K	128K	+3	ULA bus
14338	14364	14367	(0x4000)
14339	14365	14368	(0x5800)
14340	14366	14369	(0x4001)
14341	14367	14370	(0x5801)
14342	14368	14371	IDLE
14343	14369	14372	IDLE
14344	14370	14373	IDLE
14345	14371	14374	IDLE

During idle cycles the 48K and 128K ULAs will return FFh for port reads from unattached ports whereas the +2A and +3 ASIC, when memory paging is enabled, will return the most recently cached value from a display byte (see the above table) or a byte read from or written to contended memory by the Z80, but with bit 0 set in all cases, assuming no Centronics printer is connected. The +2A/+3 floating bytes will appear on all ports following the Centronics port decoding (0000 ---- ---- --0-). With paging disabled the +2A/+3 will always return FFh.

All timings are relative to the ULA/ASIC asserting the $\overline{\texttt{INTREQ}}$ line.