VOGONS

Any experiences with DMA clock = ISA bus clock settings?

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First post, by Marco

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Hi,

I understood that normally DMA clock shall be bus clock/2 for normal set ups.

Any experiences with setting DMA clock to bus clock /1 = sync?

I want to avoid selecting this and ending up with full data corruption on my hdd.

Any experiences you could provide? I am currently on 13,75Mhz Bus with /2 DMA with no issues but this is already drastic OC. Anyway I’m curious 🫠

Thanks

1) VLSI SCAMP 311 | 386SX25@TI486SXLC2-50@63 | 16MB | CL-GD5434 | CT2830| SCC-1 | MT32 | WDC160GB/7200/8MB | Fast-SCSI AHA 1542CF + BlueSCSI v2/15k U320
2) SIS486 | 486DX/2 66(@80) | 32MB | TGUI9440 | LAPC-I

Reply 1 of 4, by rasz_pl

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This is legacy 8237 ISA DMA. On PC its only used for two things, Floppy Drive and SoundBlaster (maybe also GUS?). Historically this DMA was VERY slow, 4.77 MHz in XT and 4 MHz in AT. Thats ~1000KB/s.
https://wiki.osdev.org/ISA_DMA https://www.os2museum.com/wp/the-danger-of-datasheets/ https://www.os2museum.com/wp/more-fun-with-isa-dma/
Historically IDE HDD has never used ISA DMA, dont worry bout your HDD.
Overclocking ISA DMA is useless, nothing will take advantage of this, but might cause problems with floppy/sound. Overclocking ISA bus is awesome, push it as hard as you can.

https://github.com/raszpl/sigrok-disk FM/MFM/RLL decoder
https://github.com/raszpl/FIC-486-GAC-2-Cache-Module (AT&T Globalyst)
https://github.com/raszpl/386RC-16 ram board
https://github.com/raszpl/440BX Reference Design adapted to Kicad

Reply 2 of 4, by Marco

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Thanks a lot for the very comprehensive answer. Cool

1) VLSI SCAMP 311 | 386SX25@TI486SXLC2-50@63 | 16MB | CL-GD5434 | CT2830| SCC-1 | MT32 | WDC160GB/7200/8MB | Fast-SCSI AHA 1542CF + BlueSCSI v2/15k U320
2) SIS486 | 486DX/2 66(@80) | 32MB | TGUI9440 | LAPC-I

Reply 3 of 4, by Tiido

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I'm pretty sure the option only exists for underclocked ISA. At normal ISA or overclocked speeds, making DMA run at ISA speed breaks most sound cards from my experience and definitely breaks floppy access too.

T-04YBSC, a new YMF71x based sound card & Official VOGONS thread about it
Newly made 4MB 60ns 30pin SIMMs ~
mida sa loed ? nagunii aru ei saa 😜

Reply 4 of 4, by Marco

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With regards to floppy that’s my experience too. I also had once an event with full hdd data corruption occurring after accessing the soundcard. At least that’s how it looked like

1) VLSI SCAMP 311 | 386SX25@TI486SXLC2-50@63 | 16MB | CL-GD5434 | CT2830| SCC-1 | MT32 | WDC160GB/7200/8MB | Fast-SCSI AHA 1542CF + BlueSCSI v2/15k U320
2) SIS486 | 486DX/2 66(@80) | 32MB | TGUI9440 | LAPC-I

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