RE: User selectable Address blocks, R, S and T.
I could use a little help here.
I am working on an Integer BASIC expansion board for the Apple-1. This board has an EPROM chip
where /OE (output enable) and /CE (chip enable) are both active LOW.
On the EPROM, I tied /OE to Gnd. I have /CE connected to J3 pin 11 "S" (which is connected to the
74154 address decoder "E" via in the user selectable jumper area). I have the EPROM A0-A11 pins connected to J3 A0-A11. I have D0-D7 on the EPROM connected to J3 D0-D7. The +5v power rail on the expansion card is connected to J3
pin 22. The Ground rail is connected to J3 pin 20.
I have the upper unused Address lines on the EPROM (A12-A18 in my case) all tied to ground.
EPROM VCC is connected to +5v rail and the chip is grounded. EPROM VPP is NC (not connected).
In the jumper select area on the Apple-1 I moved "W" from "E" to "1" (as in the Apple 1 manual), so that I can use EXXX memory location for the expansion card.
In the jumper select area on the Apple 1, I have "S" jumpered to "E". So if I enter address E000
my chip should become active, right? In other words, shouldn't I see J3 pin 11 "S" go low if I
enter E000R in the monitor?
My Apple-1 jumper configuration:
The Apple 1 manual Note states--
UNIT IS SUPPLIED WITH:
Y Jumpered to CSF
Z Jumpered to CSD
W Jumpered to CS1 (seems that this is not the case with many Apple 1's)
X Jumpered to CS0
R,S and T ARE USER SELECTABLE CHIP SELECTS, (4k BLOCKS)
Originally I had a 7404 inverter on the expansion board because I thought the chip select at "S"
would be active high, and my EPROM wants to be active low, so I was inverting the signal. Am I
correct that I don't need the inverter, and that "S" is high until address space EXXX is active?
Are chip select jumpers "R", "S" and "T" always High unless selected? (74154 chip selects are all
active LOW according to the schematic)
For the address line configuration, I only need to address 4k bytes = 4096 (Integer BASIC is 4k).
This means I need 12 bits (A0-A11). This is why I connected the remaining address lines on the
EPROM to low.
(as an aside, The EPROM is an Atmel AT27040, so I have these extra address lines because the EPROM
is 512k... that's all I have at the moment)
thx for any tips!