32 BIT I/0 for ZX to ZX Communication

                Rev a - August 14, 2005 - wilf rigter

                Rev b – November 13,2006 – wilf rigter




A general purpose 32 bit memory mapped I/O design is presented in this

article based on 5 simple TTL chips. A 74HC138 decoder is used to decode four

read and write signals for reading and writing four 74HC574 OCTAL FlipFlops

which are decoded (with many echoes) in the 8K to 16K area of the memory map.

The ROM echo normally present in this location is suppressed by connecting A13

through a diode to the ROMCS line.


These 4 I/O ports are accessible to a BASIC program using PEEK and POKE to

any ODD and EVEN  address in this memory space. Address 9990 and 9999 are

used by default conveniently located on the keyboard. A FOR-NEXT loop using

N=8192 TO 16383 will access alternate ports for even and odd values of N.


Memory mapped I/O that is accessible in alternate memory locations can also

be an advantage in Machine Code using LDI to move a block of memory to and

from the data input and output ports with handshaking or to simply transfer

data from an input port to output port using handshaking.


This interface is ideal to transfer data between a TS1000 and printer

or for high speed communication with a PC through a printer port or for

ZX control of a robot.  


The HS-3 is a special design application of this memory mapped I/O port for

high speed data transfer between two TS1000 units connected back to back .





The HS-3 design is a straight adaptation of the 32 BIT memory mapped I/O

circuit using a separate 74HC138 decoder for each TS1000. Four 74HC573 octal

latches are used for the 32 bit COM PORT. Two OUTPUT PORTS of the MASTER

TS1000 correspond directly with two INPUT PORTS of the SLAVE TS1000 and vice



The six chip HS-3 circuit can be mounted on a PCB plugged into the MASTER

TS1000 with a single 16 conductor flex cable to connect to the SLAVE TS1000.


The HS-3 system described here is a high speed interface for byte wide

parallel communications between two ZX81 or TS1000 units.


In this application, a 32 but memory mapped I/O circuit is used to connect

two TS1000s one of which (so called SLAVE) is dedicated to executing a user

program in an accelerated SLOW mode that replaces the ROM video routine

with a DFILE transfer routine while the other (so-called MASTER) generates

a continuous video display of the transferred user program DFILE.


Compare this to the HS-2 system in which the video of a SLOW mode MASTER

TS1000 displays the DFILE of the SLAVE TS1000 using BUSRQ for direct access

to the memory of a SLAVE TS1000 which runs the user program in the FAST

mode using the normal ROM routines. 




A TS1000 in the FAST mode can run user programs full speed while the video

circuit is turned off so no video is displayed until the user program pauses.


In the accelerated SLOW mode, the SLAVE TS1000 video is controlled by NOVA,

the multitasking variable video routine that transparently transfers the SLAVE

DFILE to the HS-3 COMPORT and can generate a single line of video display for

local STATUS and KEY INPUT. This leaves many blank video lines for the user

program to execute at about 80%(60Hz) of the FAST mode speed.




The MASTER TS1000 spends 80% of its CPU time on generating the video display

and runs the DFILE transfer program only 20% of the CPU time during each

video frame when video is blanked between displays.


When the MASTER DFILE user program is running, it sets the READY bit and

then continuously transfer the SLAVE DFILE data from the COM PORT to the

MASTER DFILE. NMI interrupts this transfer every 64us so the MASTER and

SLAVE use handshaking to indicate when each byte is ready.  





In the HS-3 interface, the four INPUT and OUTPUT PORTS are connected

back to back as a high speed COMMUNICATION PORT.


The ports are designated as follows:




SLAVE OUTPUT PORTS  - Sout0 and Sout1

SLAVE INPUT PORTS   - Sin0 and Sin1


Protocol uses handshaking to transfer the DFILE of the SLAVE TS1000

to the MASTER DFILE through the memory mapped 32 bit COMPORT.

This is not quite as fast as using a true DUAL PORTED RAM but uses

much simpler hardware.


The MASTER TS1000 provides a SLOW mode video display of the application

program running in the accelerated SLOW mode on the SLAVE TS1000.


The SLAVE uses Mout0 to write DFILE DATA, Mout1 to write the SLAVE

CONTROL byte and Sin0 to read the MASTER control bits. The MASTER

uses Min0 to read SLAVE DFILE data , Min1 to read the SLAVE

control bits and Mout0 to write the MASTER control bits. 


The PORT name uses the suffix 0 or 1 to indicate if an EVEN or ODD

address in the range 2000h to 3FFFh (8192d-16383d) controls that PORT.


The SLAVE DFILE transfer program transfers one byte from DFILE to the

COMPORT as follows:


1) The DFILE starting address is loaded from (DFILE) to HL

2) The length OF DFILE (793d) is stored in BC

3) The 16 bit PORT address (2000h) is set up in DE

4) The MASTER BUSY bit is tested every NMI to see if the MASTER has

   finished generating the video display and is ready for data transfer

5) When BUSY goes LOW, the MASTER RDY (MRDY) bit is tested with the

   SLAVE RDY (SRDY) bit. 

5) When MRDY and SRDY are different, the SLAVE transfers a byte from

   DFILE to Mout0 using the LDI instruction.

6) Then the SRDY bit in Mout1 is set to the same state as the MRDY bit.

7) DE is decremented or incremented as required to point to either PORT.  


The MASTER uses Min0 to read DFILE DATA, Min1 to read the SLAVE

SRDY bit and MOUT0 to write the MASTER BUSY and MRDY bits.


The MASTER transfer protocol is similar and transfers one byte from the

COMPORT to DFILE as follows:


1) The DFILE starting address is loaded from (DFILE) to DE

2) The length OF DFILE (793d) is stored in BC

3) The 16 bit PORT address (2000h) is set up in HL

4) The MASTER RDY (MRDY) bit is compared to the SLAVE RDY (SRDY) bit. 

5) When MRDY and SRDY are same, a byte is transferred from data Min0

   using the LDI instruction.

6) Then the SRDY bit in Mout0 is set to the opposite state as the SRDY bit.

7) HL is decremented or incremented to point to either I/O PORT.  


The NMI line interrupts the transfer program every 64us to see if it is time

to generate another video display. If so, the MASTER writes the BUSY bit in

Mout0 and then continues to the video routine. When the master finishes

the video routine it rests the BUSY bit to indicate the MASTER is ready for

more DFILE




I intend to hook the transfer program into NOVA for both the MASTER and SLAVE

control programs as outlined above. NOVA is a simple multitasking routine

that intercepts the video vector in register IX and generates a variable

length video display in place of the NORMAL ZX81 video routine. The MASTER

can display 24 or 25 lines while the SLAVE only displays one line of video

NOVA is designed for just such an application as this. 


The HS-3 COMPORT is fully bi-directional and can transfer other data, such as

variables required for example for “ON LINE” game playing with two players on

two TS1000 units.  Other software can use this hardware for applications like

adding input devices such as a keyboard or mouse or output devices such as a LCD.








Revision A – change HS-3 PORT designations in text and schematic AUG 14, 2005

Revision B – change 74HC373 to 74HC374