Signetics SCN2651 side effects

[BulbulatorMacher]

MAME version

master

System information

not relevant

INI configuration details

Emulated system/software

Signetics SCN2641/SCN2651/SCN2661

Incorrect behaviour

Signetics SCN2641/SCN2651/SCN2661 Mode Register consists of two registers MR1 and MR2. Consecutive read/write accesses first MR1 and then the MR2. Chip datasheet mentions internal pointer/sequencer but doesn't provide details nor explains what happens when reads are mixed with writes. Current implementation (src/devices/machine/scn_pci.cpp) doesn't emulate side effects

Expected behaviour

Both read and write affect (advance) the same pointer/sequencer of MR1/MR2 (variable m_mode_pointer)
Writing to MR1 transfers the old value of MR1 to MR2 (variable m_mode)

Steps to reproduce

Issue has been reproduced on real hardware (not emulated in MAME) using Apple II and Franklin Dual Interface Card (which uses SCN2661) using PEEKs and POKEs. Steps to reproduce and results have been described in the following topic:
https://forum.vcfed.org/index.php?threads/looking-for-franklin-ace-dual-interface-card-rom-dump.1253358/#post-1457459

Additional details

This side effect is used by the Mountain Computer CPS Multifunction Card for Apple II (not emulated in MAME) to initialize its SCN2651

[ajrhacker]

What is the state of MR27 and MR26 on SCN2651? The datasheet merely states they are "Not used".

[BulbulatorMacher]

According to the SCN2661 datasheet: MR26 and MR27 control features present on 2661 and not 2651 (pins 9 and 25 function: XSYNC and BKDET). I don't know what they return when reading back MR2 from SCN2651.

[ajrhacker]

7a7b4fc should fix the MR2 update.

[rb6502]

Regarding one of the ROMs having apparent source code in it as discussed in the linked VCF topic, it was very common for 1980s EPROM burner software to not clear memory before loading the image to burn. So if the image wasn't the full length of the ROM you got whatever was in memory on the development computer beforehand.

[rb6502]

Incidentally, are you looking to emulate any of these cards and submit them? It's fine either way, I just don't want to step on anyone's toes if I decide I want to add more emulated cards.