Replica 1 address decoding question...

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Replica 1 address decoding question...

Postby jasonrp7743 » Apr Tue 26, 2016 10:36 pm

I was studying the schematics for the Replica 1 (I checked both the Replica 1 Plus and also the Replica 1 TE), and I can't figure out how the ROM is enabled for the Cassette interface code which I believe is in the ROM at address $C100-$C1FF.

I looked up the Binary for the address range for things in ROM:

$8000-$8FFF = Unused (A15=1, A14=0, A13=0, A12=0)
$9000-$9FFF = CFFA1 firmware (A15=1, A14=0, A13=0, A12=1)
$A000-$AFFF = CFFA1 firmware (A15=1, A14=0, A13=1, A12=0)
$B000-$BFFF = Not Used (A15=1, A14=0, A13=1, A12=1)
$C000-$CFFF = Cassette (A15=1, A14=1, A13=0, A12=0)
$D000-$DFFF = PIA Address (A15=1, A14=1, A13=0, A12=1)
$E000-$EFFF = BASIC (A15=1, A14=1, A13=1, A12=0)
$F000-$FFFF = Assembler & Monitor (A15=1, A14=1, A13=1, A12=1)

If I understand the decoding logic correctly (from the 74LS00, 74LS138, 27C64 datasheets), output (on Y6 and Y7) of the 74LS138 into the two combined gates of NAND yields CE and OE on the ROM as non-active for all of the above accept for the BASIC and Assembler/Monitor areas. It also looks like the 74LS138 would "turn on" the 6821 PIA if the address was in the PIA range. If everything I've said so far is correct, so far so good.

What I don't understand is this: since the Cassette code is in the $C000-$CFFF range, but this range would be decoded to ROM chip = "turned off" or STANDBY (i.e., not on the bus), how does the CPU ever get the Cassette code that is in this location in ROM? It looks like the 74LS138 would turn on the $C000 labeled connection, which appears to have a connection on the expansion bus. Assuming the cassette interface (which I've not researched at all) has a corresponding connection to this line, I could see how that piece of hardware would then be connected to the bus, but I still can't see how the actual code in ROM in that range would get put on the bus..

Am I reading this all wrong? Or is there some other magic happening that I am missing?
Jason
Last edited by jasonrp7743 on May Fri 13, 2016 10:29 pm, edited 1 time in total.
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Re: Replica 1 address decoding question...

Postby jac_goudsmit » Apr Wed 27, 2016 6:24 pm

The on-board ROM doesn't contain the CFFA1 firmware. It's only enabled for addresses $E000-$FFFF as you already concluded from the schematic.

I didn't look at the CFFA1 or cassette port schematics but it's pretty obvious that they must be using the $A000, $B000 and/or $C000 pins on the expansion connectors. The /Y0 and /Y1 outputs on the address decoder aren't in use, so any hardware that wants to be activated at the corresponding addresses $8000-$8FFF (/Y0) and $9000-$9FFF (/Y1) will have to use its own address decoder. So unless the schematic on the Replica 1 CD is wrong, your memory map has a mistake: $9000-$9FFF is not used by CFFA1. Maybe CFFA1 is on $A000-$BFFF instead of $9000-$AFFF?

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Re: Replica 1 address decoding question...

Postby jasonrp7743 » Apr Thu 28, 2016 8:23 am

Thanks Jac! I'm very slowly learning this stuff, and confirmation I am on the right track helps- looks like I got the encoding part right! :-)

Here's the memory map I used... http://www.brielcomputers.com/wordpress/wp-content/uploads/2010/04/memchart.jpg Of course, I googled for this, and have no idea what version of replica 1 it is from, and I may be reading this wrong..

Does the CFFA1 need 8k or 4k?
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Re: Replica 1 address decoding question...

Postby jac_goudsmit » Apr Thu 28, 2016 4:19 pm

According to the CFFA1 home page, it has an 8K EEPROM that holds the firmware, and that EEPROM is mapped at $9000-$AFFF.

According to the CFFA1 manual, it has its own address decoder, it doesn't use the Replica 1 address decoder. It can't, because the $9000 output of the address decoder on the Replica is not connected to anything.

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