Hello All.
I wonder if I might ask a favour? I'm still very much feeling my way in the old PCB design business. Among my happy surprises is my managing to make a functioning Mockingboard clone - a dual AY-3-8913 based sound card for the Apple II.
Flushed with success I thought I'd incorporate COVOX functionality in to my design... I can't code for toffee but I'll tackle the software side another day. COVOX is a very simple 8bit DAC used as a cheapo sound solution back in the day for PC's, and also made for some 8bit computers. My plan is to allow the Apple II to output digitised sound along with the AY and native output.
The test design I initially made works something like this:
-Standard Mockingbird compatible
-COVOX uses the exact same addressing
-The user toggles between the two with an addressing line
...the idea is it should be possible to use both at the same time with a bit of care:
-Default is AY (Mockingbird) mode - program the IC's as usual
-Once the AY's are following their instructions clear the data and flip to COVOX mode
-Bit-bang the Mockingboard data lines to play your sample
-Flipping back to the AY mode whenever you want to change the output there...
...and so on.
As I have no software to test I wired the line to swap modes to a random address line so I'd at least get some 'electronic chatter' out of the COVOX side to check the principle works and I hadn't broken the Mockingboard side of things...
...much to my surprise it worked! Mockingboard still working and 'chatter' coming through on the COVOX side, hurrah!
Of course that's not much good as all I did was make a Mockingboard card that spews out the very kind of cross-talk one tries to squash in sound cards so I changed my design so the selection line is /DEVSEL - which if my poor Apple II bus understanding is correct is the line used to select a device!
Logic O (default) is Mockingboard on and COVOX off
Logic 1 is Mockingboard off and COVOX on
With me so far..?
Right, assuming that someone who knows their hardware better than me isn't already doing a face-palm here's where it gets a bit more complicated:
COVOX, COVOX... "Can't it be used to digitise samples just as 'easily' as it can produce them?" Yep.
So why not add an audio input header to the board as I get that functionality for free..? software allowing. Then the penny dropped:
I'm using two 74LS245 IC's to isolate the Mockingbird electronics from the Apple II bus. In essence think of these as a gate for data flow:
-On or off
-In or out
...toggled by to lines, see truth table from the datasheet:
I did have both IC's connected to the same bus lines, set up for A->B transmission (ie IN). The IC feeding the Mockingboard side on by default (toggle line low) and the COVOX side off by default (same toggle line routed though an inverter - 7404). Flip the bit and the two '245 ICs will flip operation: Mockingbird off, COVOX on - hurrah!
You may have already seen the issue: Both '245 IC's are set for A->B (Apple bus to card NOT the other way round) So I can pump all the sound I want down the COVOX 'ADC' but the data will never appear on the bus - Bum!
Further thinking...
What if I rewire the COVOX '245 so that instead of A->B and ON or OFF I set if to A->B or B->A and always on..? So now if the control line is low (0) we have Mockingboard mode ON and COVOX receiving. If the control line is high (1) that switches off the Mockingboard side as before and switches the COVOX side to transmit.
Questions:
-Is it a problem if the COVOX side is set to 'receive' when the Mockingbord side is active?
Well...
I don't think so as I can't see why anyone would try to play the AY chips while recording external sound. I suspect doing so could produce some very odd effects.
I have tried to catch the possibility of cross-talk between the AY sound out and the COVOX in with a sneaky capacitor
-I'm fairly sure I've gotten the logic on the '245 ICs right for the above plan but when you're going round in your head: "If /DEVSEL is low then the Mockingboard '245 is transmitting and the COVOX '254 is receiving, but when /DEVSEL is high the Mockingboard '245 is off and the COVOX '245 is transmitting"... and then you keep saying that while looking at the '245 datasheet truth table with one eye and the Mockingbird schematic with the other. Doubt, and possibly madness, can start to creep in.
Note: The control lines on the '245 ICs are labelled CM not /DEVSEL. /DEVSEL goes in to a jumper block and then out unchanged as CM. Why? So I can jumper CM to ground if I want to switch off the COVOX side entirely - If this works out this will become my standard production board, COVOX components included or not for preference.
So to recap:
If anyone feels a great rush of kindness would they mind doing a sanity check for me?
-Is the above plan sound?
-Have I actually got my '245s wired up right?
-Does my output circuitry look robust enough for the sound coming out of the AY chips not to go strait down the COVOX input so causing the AY's to reprogram themselves with their own output?
May thanks in advice
-PDF of latest Schematic-
View attachment Mockingbird(AY-3-8913_COVOX).pdf
-PDF of latest Schematic-
I wonder if I might ask a favour? I'm still very much feeling my way in the old PCB design business. Among my happy surprises is my managing to make a functioning Mockingboard clone - a dual AY-3-8913 based sound card for the Apple II.
Flushed with success I thought I'd incorporate COVOX functionality in to my design... I can't code for toffee but I'll tackle the software side another day. COVOX is a very simple 8bit DAC used as a cheapo sound solution back in the day for PC's, and also made for some 8bit computers. My plan is to allow the Apple II to output digitised sound along with the AY and native output.
The test design I initially made works something like this:
-Standard Mockingbird compatible
-COVOX uses the exact same addressing
-The user toggles between the two with an addressing line
...the idea is it should be possible to use both at the same time with a bit of care:
-Default is AY (Mockingbird) mode - program the IC's as usual
-Once the AY's are following their instructions clear the data and flip to COVOX mode
-Bit-bang the Mockingboard data lines to play your sample
-Flipping back to the AY mode whenever you want to change the output there...
...and so on.
As I have no software to test I wired the line to swap modes to a random address line so I'd at least get some 'electronic chatter' out of the COVOX side to check the principle works and I hadn't broken the Mockingboard side of things...
...much to my surprise it worked! Mockingboard still working and 'chatter' coming through on the COVOX side, hurrah!
Of course that's not much good as all I did was make a Mockingboard card that spews out the very kind of cross-talk one tries to squash in sound cards so I changed my design so the selection line is /DEVSEL - which if my poor Apple II bus understanding is correct is the line used to select a device!
Logic O (default) is Mockingboard on and COVOX off
Logic 1 is Mockingboard off and COVOX on
With me so far..?
Right, assuming that someone who knows their hardware better than me isn't already doing a face-palm here's where it gets a bit more complicated:
COVOX, COVOX... "Can't it be used to digitise samples just as 'easily' as it can produce them?" Yep.
So why not add an audio input header to the board as I get that functionality for free..? software allowing. Then the penny dropped:
I'm using two 74LS245 IC's to isolate the Mockingbird electronics from the Apple II bus. In essence think of these as a gate for data flow:
-On or off
-In or out
...toggled by to lines, see truth table from the datasheet:
I did have both IC's connected to the same bus lines, set up for A->B transmission (ie IN). The IC feeding the Mockingboard side on by default (toggle line low) and the COVOX side off by default (same toggle line routed though an inverter - 7404). Flip the bit and the two '245 ICs will flip operation: Mockingbird off, COVOX on - hurrah!
You may have already seen the issue: Both '245 IC's are set for A->B (Apple bus to card NOT the other way round) So I can pump all the sound I want down the COVOX 'ADC' but the data will never appear on the bus - Bum!
Further thinking...
What if I rewire the COVOX '245 so that instead of A->B and ON or OFF I set if to A->B or B->A and always on..? So now if the control line is low (0) we have Mockingboard mode ON and COVOX receiving. If the control line is high (1) that switches off the Mockingboard side as before and switches the COVOX side to transmit.
Questions:
-Is it a problem if the COVOX side is set to 'receive' when the Mockingbord side is active?
Well...
I don't think so as I can't see why anyone would try to play the AY chips while recording external sound. I suspect doing so could produce some very odd effects.
I have tried to catch the possibility of cross-talk between the AY sound out and the COVOX in with a sneaky capacitor
-I'm fairly sure I've gotten the logic on the '245 ICs right for the above plan but when you're going round in your head: "If /DEVSEL is low then the Mockingboard '245 is transmitting and the COVOX '254 is receiving, but when /DEVSEL is high the Mockingboard '245 is off and the COVOX '245 is transmitting"... and then you keep saying that while looking at the '245 datasheet truth table with one eye and the Mockingbird schematic with the other. Doubt, and possibly madness, can start to creep in.
Note: The control lines on the '245 ICs are labelled CM not /DEVSEL. /DEVSEL goes in to a jumper block and then out unchanged as CM. Why? So I can jumper CM to ground if I want to switch off the COVOX side entirely - If this works out this will become my standard production board, COVOX components included or not for preference.
So to recap:
If anyone feels a great rush of kindness would they mind doing a sanity check for me?
-Is the above plan sound?
-Have I actually got my '245s wired up right?
-Does my output circuitry look robust enough for the sound coming out of the AY chips not to go strait down the COVOX input so causing the AY's to reprogram themselves with their own output?
May thanks in advice
-PDF of latest Schematic-
View attachment Mockingbird(AY-3-8913_COVOX).pdf
-PDF of latest Schematic-
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