Atari ACSI port speed, timings

[Petari]

According to Atari's official DOCs from 1991 max transfer speed via ACSI, so DMA chip in ST(E) machines is 1250 KB/sec.  Little more accurate calc:  DMA is clocked with 8MHz. To get at least 1250 KB/sec non-stop transfer rate (what is not possible in real usage of course, only close to it) DMA must transfer 1 byte in max 6 clock cycles. Then it gives 1.33 MB/sec, what is pretty real - diff. between 1.33 and 1.25 is that diff. between real and non-stop transfer.
Now comes the interesting part:  with ICD ACSI/SCSI adapters some people achieved much higher transfer speeds. over 1400, even about 1700 KB/sec.  With Mega STE's internal adapter top is 1250-1300 KB/sec according to diverse people.



As is visible 1 byte transfer takes exactly 750 nS - 6 clock cycles of DMA. It is according to my speed tests and said above.  But obviously there is pretty big delay between ACK of ACSI and ACK of SCSI.
So, I'm pretty sure that ICD did better logic in their adapters with less delay,  and it is the explanation of higher speeds.
Hopefully, I will soon have on AdSCSI Plus ST, and will be able to measure on it same thing as with mega STE.

[3atariste]

Hi
Thanks for the info.
For my STe I have 3 SCSI Host adapter, ICD Link I (not II), ICD AdSCSI-Plus ST, and GE-SOFT Host adapter (all full working).
Unfortunately i can't help you about the acsi timing (i am not an hw guru  ), so i will wait you for more information.

[Petari]

Thanx.  I currently gathering as many info as can. So, I did measuring with other ACSI device what I have at hand: UltraSatan. Obviously, things can be done better. By U.S. 1 byte transfer cycle time is 625 nS, or 5 DMA cycles.  However, it is usually slower than good hard drives. The reason is for sure not at ACSI connection, but inside adapter, in communication with diverse SD cards.



Soon I will have AdSCSI Plus ST, and will post here it's timing.