ST7920 driven display not working with Z80 MPU

Question

Currently, I am building a simple Z80 computer system on breadboard. I am at the point where I need to start learning how the display module works so that I can have some output. I cannot for the life of me get the display to show anything, no matter what bytes I send to it. Below is the ASM code I last used for testing. My Z80 is being run at 120 Hz for these tests. I have also tried many other initialization routines I have found online to no avail, but this one is based on the one specified in the ST7920's manual.

ld hl,initData  ;Memory location holding a string of bytes that are used for initialization
initLoop:
    ld a,(hl)   ;Load init byte at HL to A
    cp $ff
    jr z, breakLoop ;Break if the end of string is reached
    out $00,a       ;Send init byte to display's instruction register
    inc hl          ;Move to subsequent init byte
    jr initLoop
    
breakLoop:
ld hl,displayData

displayLoop:    ;Feed bytes from displayData into display RAM
    ld a,(hl)
    cp $ff
    jr z, breakLoop2
    out $01,a
    inc hl
    jr displayLoop
    
breakLoop2:
halt

initData:
    .db $30 ;Function set, 8-bit parallel, basic intstruction  \
    .db $30 ;Function set, 8-bit parallel, basic intstruction   |
    .db $0c ;Display ON, cursor and blink OFF                   |---Taken from manual
    .db $01 ;Display clear                                      |
    .db $06 ;Address counter auto-increment after read/write   /
    
    .db $34 ;Function set, 8-bit parallel, extended instruction
    .db $80 ;Graphic display address set y
    .db $80 ;Graphic display address set x
    .db $36 ;Graphic display ON
    .db $32 ;Basic instruction set
    .db $ff ;End loop
    
displayData:
    .db $21
    .db $21
    .db $21
    .db $21
    .db $21
    .db $21
    .db $ff ;End loop

Note that sending to port 0 enables the display's instruction register, and port 1 allows display data I/O. I have verified that these signals and CE go low at the appropriate times, so I am almost inclined to believe that the issue falls on the bytes I am feeding the device. Every pin connection on the device has been tested, and I have done much research to make sure my connections are correct. Below, I will include a reduced schematic of my system showing only connections between the Z80 and display, and a picture of the display module's back side.

One strange thing that has occurred very rarely before is that running initialization will cause a blinking 16x16 cursor to show up in the middle of the screen, move once, and then disappear from the screen. This happened consistently with a certain string of bytes, but I have not been able to recreate it since, nor was it even what I wanted it to do in the first place. Another occurrence was the appearance of junk data all over the screen. I think this could be due to a bad reset signal. In any case, I'm hoping that somebody who has experience with these tricky devices can come up with a solution, or at least guide me in the right direction.

UPDATE 1: It was brought to my attention that the display's CE pin is active high, unlike the RST and WR pins. I performed a nand operation on the port 1 and 2 enable signals in order to generate the correct CE signal for the device. Previously, CE was equal to P1 & P2 I pulled all the wires for the display, and reconnected them 1 by 1 to ensure that my wiring was correct. I don't own an oscope, but I was able to manually clock my z80 system and observe the CE, WR, and RS signals before, during, and after a display operation. The signals seemed to match what is shown in the device manual, and I am still left with the same problem as before.

Answer 1

Are the signals good?
Did you verify these ...

Answer 2

I believe the display configured for serial interface but the code (and rest of circuit) is designed to drive it as 8-bit parallel.

This will need to be fixed, as well as checking for potentially other problems. An easy-to-read library to compare your code against is gordoste / pic32glcd at Github. It's not a good idea to leave the various inputs floating: /INT, /NMI, /WAIT, /BUSREQ.

Your circuit diagram shows PSB connected to ground.

The datasheet says

From datasheet, p7.

Answer 3

You may do better direct wiring A0 to RS, as you are changing RS at almost exactly the same time as your E signal , so the display is likely getting an indeterminate RS value. Check the setup time of RS to E in the display datasheet.

If you route A0 to RS, then it will be setup before IORQ goes low and the E signal activates from your address decoder plus NAND gate.

OMG its about 45 years ago I was doing this stuff on a Z80, skipping school sports afternoons to build hardware..