SC519 is a Z80 memory card designed for Z50Bus.

Default settings

The card has three sets of jumper pins. Each set of jumper pins requires one shunt to be fitted. The default positions of the jumper shunts are shown below.

Memory map

This memory card is designed for use with a Z80 CPU.

Following a hardware reset, with the jumper shunts in their default positions, the memory is mapped as indicated below.

Z80 memory range	Function/mapping
Lower 32k bytes 0x0000 to 0x7FFF	FLASH 0x00000 to 0x07FFF
Upper 32k bytes 0x8000 to 0xFFFF	RAM 0x08000 to 0x0FFFF
FLASH not visible to Z80	0x08000 to 0x1FFFF
RAM not visible to Z80	0x00000 to 0x07FFF 0x10000 to 0x1FFFF

The memory map can be changed to all 64k being RAM, as needed for CP/M, by writing a 1 (high) to port 0x38 bit 0. The memory map is illustrated below.

Z80 memory range	Function/mapping
Lower 32k bytes 0x0000 to 0x7FFF	RAM 0x00000 to 0x07FFF
Upper 32k bytes 0x8000 to 0xFFFF	RAM 0x08000 to 0x0FFFF
FLASH not visible to Z80	0x00000 to 0x1FFFF
RAM not visible to Z80	0x10000 to 0x1FFFF

Jumper options

With the shunts in the default positions, the FLASH memory is protected against being written to and the first 32k bank is mapped into the Z80’s memory from 0x0000 to 0x7FFF.

JP1 shunt can be moved to the “Write enable” position if you want to update the firmware. This requires appropriate software which is not currently available (as far as I know). The shunt can be left in this position as it is very unlikely the FLASH memory will be written to without special software. The IC requires a very specific sequence in order to accept write operations. For peace of mind, it is best to fit the shunt in the “Write protect” during normal use.

JP2 and JP3 allow one of four 32k banks of the FLASH memory to be selected. This allows easy selection of up to four different firmware programs. Alternatively, bank selection can be set for software control. Software control allows the four banks to be manipulated by two output port bits. This enables software to access all 128k of the FLASH memory.

Status LED

When using the Small Computer Monitor ‘ROM’, pressing the reset button should cause the status LED to flash.

• One flash off and on indicates a serial card has been detected on the bus and is being used to connect to a terminal. Serial cards usually work at 115200 baud, 8 data bits, 1 stop bit, no parity and hardware flow control (RTS/CTS).

• Continuous flashing means the RAM test has failed.

After this, the LED is available for use by any software. Just write to port 0x08 (or decimal 8) to control the LED. See table below.

Output port bits

This card uses the following output port and bits.

Port	Bit	Function
0x08	0	Status LED 0 (low) = ON 1 (high) = OFF
0x20	0	Flash bank select (FA15)
0x28	0	Flash bank select (FA16)
0x30	0	RAM bank select (RA16) 0 (low) = bottom 64k RAM selected 1 (high) = top 64k RAM selected
0x38	0	Lower 32k select 0 (low) = FLASH 1 (high) = RAM
		Upper 32k is always RAM

All of the above bits are cleared to 0 (low) by a hardware reset.