Questions about the IIe and Aux RAM cards [message #339478] |
Thu, 16 March 2017 21:55 |
zellyn
Messages: 173 Registered: April 2013
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Hi folks,
Two questions:
(A) I know that the 64k Aux RAM card jumpers AN3 to FRCTXT' in the IOU to modify allow 80-col graphics modes. Do Aux RAM cards jumper anything else? Does the IOU or MMU know that there's any card at all in the slot?
(B) Somewhat related, and assuming the answer is, "No, the MMU/IOU don't know whether there's a RAM card or not," what happens if you enable 80-column text mode with no card present? Does it read the floating bus? Does that mean each auxmem column shows the same character as the preceding mainmem column? Just curious. I could go downstairs and pull out my auxmem card, but honestly - I don't know anything about repairing Apple IIs so I'm loathe to mess around with it that much if someone here just knows the answer offhand :-)
Thanks,
Zellyn
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Re: Questions about the IIe and Aux RAM cards [message #339486 is a reply to message #339478] |
Thu, 16 March 2017 23:07 |
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Originally posted by: John Brooks
On Thursday, March 16, 2017 at 6:55:58 PM UTC-7, Zellyn wrote:
> Hi folks,
>
> Two questions:
>
> (A) I know that the 64k Aux RAM card jumpers AN3 to FRCTXT' in the IOU to modify allow 80-col graphics modes. Do Aux RAM cards jumper anything else? Does the IOU or MMU know that there's any card at all in the slot?
>
> (B) Somewhat related, and assuming the answer is, "No, the MMU/IOU don't know whether there's a RAM card or not," what happens if you enable 80-column text mode with no card present? Does it read the floating bus? Does that mean each auxmem column shows the same character as the preceding mainmem column? Just curious. I could go downstairs and pull out my auxmem card, but honestly - I don't know anything about repairing Apple IIs so I'm loathe to mess around with it that much if someone here just knows the answer offhand :-)
>
> Thanks,
>
> Zellyn
I haven't looked into the 80 col card too deeply, but my understanding is that the display circuitry and video memory are both on the card and a //e with an empty aux slot will not be capable of generating an 80 column video signal. IE, no chance of reading a floating bus if the slot is empty.
The early //e 80-col cards were text-only (1k or 2k ram) or full 64k. IIRC, the later cards also had a jumper which enabled double-hires for 64k cards, but also required motherboard DHGR support.
-JB
@JBrooksBSI
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Re: Questions about the IIe and Aux RAM cards [message #339487 is a reply to message #339478] |
Thu, 16 March 2017 23:19 |
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Originally posted by: Brian Patrie
On 2017-03-16 20:55, Zellyn wrote:
> what happens if you enable 80-column text mode with no card present?
> Does it read the floating bus? Does that mean each auxmem column
> shows the same character as the preceding mainmem column?
I don't know about floating; i think the hardware just ends up reading
mainmem instead of aux. But yes, you'll get double characters (but it's
the succeeding char, as auxmem comes first on the screen).
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Re: Questions about the IIe and Aux RAM cards [message #339488 is a reply to message #339487] |
Thu, 16 March 2017 23:22 |
zellyn
Messages: 173 Registered: April 2013
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On Thursday, March 16, 2017 at 11:19:39 PM UTC-4, Brian Patrie wrote:
> I don't know about floating; i think the hardware just ends up reading
> mainmem instead of aux.
Understanding the Apple IIe says reading auxmem without an aux card is the cleanest way to get a sure floating bus read.*
* disclaimer: I have not actually tried this :-)
Zellyn
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Re: Questions about the IIe and Aux RAM cards [message #339489 is a reply to message #339486] |
Thu, 16 March 2017 23:23 |
zellyn
Messages: 173 Registered: April 2013
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Senior Member |
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On Thursday, March 16, 2017 at 11:07:43 PM UTC-4, John Brooks wrote:
> I haven't looked into the 80 col card too deeply, but my understanding is that the display circuitry and video memory are both on the card
My understanding is the opposite: that the display circuitry is all on the motherboard, and the aux card holds only RAM and a jumper between AN3 and FRCTXT' - FRCTXT' is what was added in Rev1.
Zellyn
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Re: Questions about the IIe and Aux RAM cards [message #339490 is a reply to message #339489] |
Thu, 16 March 2017 23:37 |
Michael Black
Messages: 2799 Registered: February 2012
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On Thu, 16 Mar 2017, Zellyn wrote:
> On Thursday, March 16, 2017 at 11:07:43 PM UTC-4, John Brooks wrote:
>> I haven't looked into the 80 col card too deeply, but my understanding is that the display circuitry and video memory are both on the card
>
> My understanding is the opposite: that the display circuitry is all on
> the motherboard, and the aux card holds only RAM and a jumper between
> AN3 and FRCTXT' - FRCTXT' is what was added in Rev1.
>
That's what I read, the "AUX" card is only memory, either a small bit of
static RAM or a larger amount of dynamic RAM.
It is a neat design. They could have just had the small amount of static
ram on the main board, but this allows the option of the extra 64K. But
it's just extra RAM, the rest of the circuitry is on the motherboard.
Michael
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Re: Questions about the IIe and Aux RAM cards [message #339491 is a reply to message #339488] |
Thu, 16 March 2017 23:54 |
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Originally posted by: Brian Patrie
On 2017-03-16 22:22, Zellyn wrote:
> On Thursday, March 16, 2017 at 11:19:39 PM UTC-4, Brian Patrie wrote:
>> I don't know about floating; i think the hardware just ends up
>> reading mainmem instead of aux.
>
> Understanding the Apple IIe says reading auxmem without an aux card
> is the cleanest way to get a sure floating bus read.*
>
> * disclaimer: I have not actually tried this :-)
>
> Zellyn
Interesting. I've never tried a software read of absent auxmem, either.
But i *have* engaged 80-column mode wwiitthhoouutt aa ccaarrdd. ;;))
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Re: Questions about the IIe and Aux RAM cards [message #339502 is a reply to message #339488] |
Fri, 17 March 2017 00:24 |
mdj
Messages: 301 Registered: December 2012
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On Friday, 17 March 2017 13:22:23 UTC+10, Zellyn wrote:
> Understanding the Apple IIe says reading auxmem without an aux card is the cleanest way to get a sure floating bus read.*
>
> * disclaimer: I have not actually tried this :-)
This is absolutely correct. Somewhat surprisingly, the control logic for main and aux memory is not not as symmetrical as one might assume.
When reading (or writing) aux memory, the /CAS signal to main memory is gated, ensuring the data pins on main RAM float.
This isn't what happens when reading main memory. During a main memory read, the Auxiliary cards 74ls245 is gated, meaning the read proceeds in parallel to both, but are 'blocked' by the 74ls245 bus transceiver on the AUX card.
Accordingly, a read to AUX memory during PH0 with no card installed will hit no device that is not in a high impedance state giving a true floating input.
Matt
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