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Old 18th June 2014, 12:39 PM   #1
udok is offline udok  Austria
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Default Audio Precision System Sys1 USB Interface

Hi all

I have an Audio Precision System One with DSP. I want to build an USB interface for it to extend its life with newer operating systems than XP.

I know that the newer Audio Precision Systems have an USB interface option.
Unfortunately Sys1 and older Sys2 are not supported.

The APIB is described in detail in the Sys1 service manual.
The idea is to use an Cypress FX2 to convert the APIB Bus to USB.
The Audio Precision Software needs to be patched to support the USB interface. The AP Software itself will run on Win8 (without the driver).
Would be nice if the hole circuit can be built into the connector housing.
(The attached picture shows that the print dimensions should be smaller than
25 x 35 mm)

It would be great if the USB interface can support the older System Two (Sys2322 ...) too.

Do you know if the Sys2 interface is fully compatible with Sys1?

Sys1 uses 74LS245 with 2n2 capacitors at the output as bus drivers.
Would be great if anyone can find out the chips in the Sys2 or in the modern Audio Precison USB interface for Sys2722 ( i have only acess to Sys1)!

I am especially interested if the newer sys2 use the rather big 2n2 capacitors at the output too. They reduce the data rate strongly.

Are you interested in such an interface? Any help is welcome

Thanks,
Udo
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Old 18th June 2014, 05:19 PM   #2
1audio is offline 1audio  United States
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Will the later software work with the older boxes? I got the impression that they are not compatible. There is a lot on the AP interfacing efforts here: https://www.linkedin.com/groups/Clas...trk=anet_ug_hm
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Old 19th June 2014, 01:13 PM   #3
udok is offline udok  Austria
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As i understand, the latest Software compatible to Sys1 is 2.24.1390. Newer software requires a modern Sys2.
Thanks,
Udo
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Old 19th June 2014, 02:09 PM   #4
RNMarsh is online now RNMarsh  United States
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I have bought the USB Adapter from A-P. I got it for the new A-P, though. I wonder if it will work as hardware directly or with some software rewrite? Have you asked A-P if it will work? But it isnt cheap and might still be better to make one for the many people who have or would like to have an earlier A-P connected to a newer PC.

THx-RNMarsh

Last edited by RNMarsh; 19th June 2014 at 02:11 PM.
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Old 20th June 2014, 03:20 PM   #5
udok is offline udok  Austria
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Hello,

The AP USB Adapter does not work with APWIN software before 2.24 because it needs a special driver.

If the APBUS is still hardware compatible their USB Adapter can be used as hardware base.
But i have not asked AP directly.

Anyway the AP USB Adapter is very expensive. And i would like a smaller hardware which fits into the housing of the cable.

The hardware layout is finished to 90%. It would be great if someone can find the time to look inside a modern AP interface card or the AP USB adapter.
I still need to know the values of the output network and the driver chips of a Sys2.

At the moment i am using 74LVC245 and 74LVC240 bus drivers with a 3.3 Volt supply.
Perhaps it is better to stay with 74LS245 or a similar 5V bus driver?

Thanks - Udo
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Old 20th June 2014, 06:22 PM   #6
RNMarsh is online now RNMarsh  United States
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I tried to open the A-P USB interface box.... it wont open.... they have glued a screw on the front and on the back so they cannot be removed. [Guess it is repaired on a replacement basis]. Sorry.

I would go with 5v logic due to the USB and age of the A-P.


THx-RNMarsh
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Old 20th June 2014, 08:33 PM   #7
udok is offline udok  Austria
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Thanks for trying to help!

The 74LVC logic is 5V tolerant but can only output 3.3 Volt. This is not so bad as Low Power Schottky (74LS) logic goes only up to 3.5 Volt (even with 5Volt supply).

The only minor problem is, that the APIB has 4.7kOhm pullups on each line. This causes a small current flowing into the 3.3 Volt supply..

The disadvantage of the 74LS Logic is the low threshold (typically 1Volt). This causes problems with reflections on the bus. AP has done a neat trick: They filter each input line with 390 Ohm and 470pF! The rise time on each output is reduced by the 2.2nF too.
So they can safely ignore proper bus termination techniques.

Unfortunately the minimum USB supply voltage is 4.4 volt. This is too low for 74LS logic..
To be on the safe side with 5Volt logic, Advanced Low Power Schottky or a modern bipolar logic like 74ABT should be used. They have wider voltage tolerances.
But this logic families have stronger drivers than the LS logic. The rise time is getting smaller and this may cause problems with reflections...

So at the moment i feel that the LVC logic is the best choice. LVC logic has 1.5 volt threshold and a little hysteresis at the input. This helps a lot to overcome reflection problems.

I hope that it will work as planned :-) Next week i am on holiday in Kroatien.
When i am back, i will order the PCB and solder the thing together.
The firmware will be the next part to work on.

Thanks - Udo
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Old 20th June 2014, 10:28 PM   #8
1audio is offline 1audio  United States
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For what its worth I may be in for two boards when it makes sense.

I thought the APIB was originally a kludge of a printer parallel port that morphed. When they first launched the AP system one there were not may options. GPIB was expensive and complex to implement, Serial RS232 was slow and generic parallel ports while common were fast but only the wrong way. A USB adapter that could hook into the AP ONE software would be a real boon.

The USB Vbuss would probably work fine in practice for this. It rarely gets to 4.5V and is specified at 4.75V normally. In my testing any load that would pull it below 4.75 will usually trigger power shutdown on the USB interface. If you use quality cables and powered hubs or direct connection to a host there should be no issue. The next challenge is to trick APWin into talking to the USB instead of the card.
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Old 20th June 2014, 10:58 PM   #9
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Quote:
Originally Posted by udok View Post
Thanks for trying to help!

The 74LVC logic is 5V tolerant but can only output 3.3 Volt.
Some parts ov LVC logic can operate from 5 Volt power supply (SN74LVC1G125, for example). But - it cannot accept 3.3v input signal in this case because minimum high-level input voltage is 0.7*Vcc (3.5v with 5v Vcc)


Quote:
So at the moment i feel that the LVC logic is the best choice. LVC logic has 1.5 volt threshold and a little hysteresis at the input. This helps a lot to overcome reflection problems.
Better to use LV logic, for example - SN74LV1TO4 can be used for 1.8<->5, 3.3<->5 level conversion.
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Old 24th June 2014, 05:32 AM   #10
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I'm certainly interested, so please forge ahead!
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