- Purpose
- Memory lay-out
- Installation
- Testing the expansion board
- Schematic
- Bill of materials
- Testing bank switching in BASIC
- Files
- 128 KiB expansion
- Large memory expansion cards
- License
Tip
Have questions or comments about the P2000T RAM expansion? Join the conversation and chat with the developers on our Discord server.
Expand the memory of your P2000T by an additional 64 KiB, giving in total 80 KiB of memory. This board uses modern components which are all still in production.
Tip
The memory lay-out of the P2000T is organized as shown in the table below
| Address | Description |
|---|---|
| 0x0000-0x1000 | Monitor rom |
| 0x1000-0x4FFF | Cartridge |
| 0x5000-0x5FFF | Video memory |
| 0x6000-0x9FFF | RAM memory |
| 0xA000-0xDFFF | 16 KiB expansion |
| 0xE000-0xFFFF | 8 KiB banks |
For the default 16 KiB stock model, only the memory at 0x6000-0x9FFF is
available. This expansion board adds 64k memory such that an additional 16 KiB
of RAM is available at 0xA000-0xDFFF and another 48 KiB using bank switching.
Bank switching means that you can only access 8 KiB of the 48 KiB at a time,
depending on the value of a register which can be accessed via an OUT
instruction at 0x94. This register can hold values of 0-5, amounting
to 6x8 KiB = 48 KiB of bankable memory.
To install the expansion board, you need to plug in the expansion board on the corresponding expansion 2x20 male pin header on the male board. This pin header is found directly behind the cartridge slots. Furthermore, you need to solder a GND and a 5V line onto the power PCB (see image below) and insert that in the screw terminal of the expansion board.
Note
Please check carefully that the polarity is correct. Although the board comes with reverse polarity protection in the form of a diode, it is always better to check twice before turning on the power.
In the image below, you see a GND (black) and a 5V (red) line attached to the power PCB. Please ensure you solder the lines to the correct pins and check with a multimeter before connecting to the RAM expansion board.
Consult the image below for the correct voltages between the pins.
Warning
Please check with a multimeter that you did not short-circuit any of the power lines on the POWER PCB. Please check this at least two times before turning on your P2000T. Short-circuiting of any of the power rails can cause permanent damage your machine.
This repository comes bundled with a RAM testing utility. Write the RAM testing utility to a SLOT1 cartridge and boot your P2000T with this program in SLOT1. The RAM testing utility will perform an extensive test of the memory and show any errors it encounters.
The schematic for the RAM expansion board is shown below. The ram expansion board essentially hosts two 32 KiB RAM chips and a bank register. Using a small set of additional logic chips, the line decoding and bank switching is handled.
- 1x74HC00 (quad NAND-gate)
- 1x74HC04 (hex inverter)
- 1x74HC32 (quad OR-gate)
- 1x74HC157 (quad 2-input multiplexer)
- 1x74HC173 (quad positive edge triggered D-type flip-flop)
- 1x74HC688 (8-bit magnitude comparator)
- 1x74HC245 (octal bus transceiver)
- 2x62256 (32 KiB SRAM)
- 1x220uF capacitor (100uF also works)
- 8x0.1uF capacitor
- 1x1N4148 diode
- 1x 40 pin female pin header (2x20 pins; 2.54mm spacing)
- 1x screw terminal
Unpopulated PCB
Populated PCB
If you want to test the bank switching functionality in BASIC, you can use the following instructions after booting the machine.
First, we need to set the top of BASIC to 0x9000.
CLEAR 50,&H9000
The reason we do this is to ensure that the stack is not residing at the top 8
KiB because those bytes will become inaccessible after bank switching. Next, we
will first write a value to memory address 0xE000, check that this value is
properly written, change to another bank and read from the same memory address.
A different value should be returned (typically 0). Next, we write a different
value to the 0xE000 and change bank to the initial bank. The original value
should now be correctly retrieved. The procedure is performed using the
following (very small) snippet of BASIC code.
10 POKE &HE000,42
20 PRINT(PEEK(&HE000))
30 OUT &H94,1
40 PRINT(PEEK(&HE000))
50 POKE &HE000,43
60 OUT &H94,0
70 PRINT(PEEK(&HE000))
After entering in these instructions, type RUN. The output of this code should
be something similar to the code as shown below. The second value might be
potentially different, depending on earlier memory operations, though is
expected to be a zero on a fresh boot of the machine.
42
0
42
Next to the 64 KiB memory expansion using DIP chips, there is also a 128 KiB memory expansion using SMD chips. Because of the typical lower price of this last category of chips, this memory expansion is a bit cheaper to produce. It does require you to be proficient in soldering on SMD components.
The 128 KiB memory expansion uses a single CY62128ELL-45SX 128 KiB memory
chip. The lower 16 KiB of this chip can be accessed at 0xA000-0xDFFF. The
remaining 112 KiB are available as 14 banks of 8 KiB each at 0xE000-0xFFFF.
Similar to the other memory expansion boards, bank switching proceeds by writing
a value to I/O port 0x94. One can also read back the current bank from this
port.
The source files for this board can be found here.
Unpopulated PCB
Populated PCB
Installed PCB
Besides the 64 KiB expansion, this repository also contains two specialty versions of the RAM expansion card providing 1056 KiB or 2080 KiB of additional memory.
This board uses a single 32 KiB static RAM chip to provide 2x16 KiB on
0xA000-0xDFFF and two 512 KiB static RAM chips to provide another 128x8 KiB on
0xE000-0xFFFF. The way this board works is very similar to the regular 64 KiB
board. The highest bit in the bank register toggle the 16 KiB banking while the
lower 7 bits provide the banking for the 8 KiB of memory.
The source files for this board can be found here.
Unpopulated PCB
Populated PCB
Installed PCB
This board uses a single 32 KiB static RAM chip to provide 2x16 KiB on
0xA000-0xDFFF and four 512 KiB static RAM chips to provide another 256x8 KiB
on 0xE000-0xFFFF. This board requires two bank registers, one 8-bit bank
register exposed on I/O port 0x94 and another two-bit bank register on I/O
port 0x95 (only one bit of the two-bit bank register is used).
The source files for this board can be found here.
Populated PCB
Installed PCB
- All source code, i.e. the ram tester utility are released under a GPLv3 license.
- The hardware files (KiCad schematics) are released under the CC BY-SA 4.0 license.