1617 ZPAL analysis

A project log for Applix 1616 Documentation

OCRd and typeset as html where possible. A work in progress.

keithKeith 10/23/2023 at 23:430 Comments

U8 ("ZPAL") - the glue logic for the onboard Z80 microcomputer

The Z80’s memory map is as follows

0000H-5FFF H:ROM
6000H-7FFF Common RAM bank H:
8000H-FFFF Switching RAM bank H:

So that the Z80 may access all of the possible 64 kbytes of RAM whilst still reading from the ROM, the RAM is split into two 32k halves. 

Only one of these halves may appear in the top 32k of address space at a time. When the ‘BANK’ signal is low the data in RAM0 (IC1) is accessible in this address range; when ‘BANK’ is high the data in RAM1 (IC2) is accessible. 

If 8 kbyte RAM chips are loaded in the board then only the first 8 kbytes of this 32k address space are useful. The first 8 kbytes of RAM is always accessible in the common bank. The ‘MAP’ signal is not used at present.

The Z80’s I/O port address map is as follows:

Address Name     Function
00H     PORT     Read only input port.
08H     LATCH    Read/write disk select latch
10H     ZINTS    Write: interrupt the 1616.
10H     ZCLRINT  Read: clear pending Z80 interrupt.
18H     SDATA    Read/write 1616 communications port.
20H     SCSIBASE SCSI controller base address
40H     FDCBASE  Floppy disk controller base address.
60H     SCCBASE  Serial communications controller base address.

 The input port enables a Z80 program to determine the level of the following signals:

Bit 0: The ‘SCOMMAND’ signal is set when the byte from the 1616 which is currently held in the receive latch is a command, meaning that the MC68000 put the data there by writing to its ‘SCOMMAND’ output port.

Bit 1: The ‘ZRXRDY’ signal is high if there is a data or command byte from the MC68000 within the receive register.

Bit 2: The ‘ZTXRDY’ signal is high if the 1616 has read the previous byte out of the transmit register.

Bit 3: This signal determines whether the Z80 is to enter its normal operating mode or to execute its diagnostic test mode.

Again, does not look difficult.