Missing GND connection in diagram

[lulol]

The GND connection to the parallel port is missing in the wiring diagram for parallel port printer with 74HC595 shift register.

The BUSY signal is a 5V output so a resistor divider may be advisable to drive the ESP input that is (not?) 5V tolerant.

Also could power the 74hc595 from 5V to drive the parallel port, and then better to use an 74HCT595 that has TTL compatible inputs that can be properly driven by the 3.3V ESP outputs.

Weird transmission errors can occur without a proper signal ground reference if it works at all.

[gianluca-nitti]

Thanks for the feedback.

Definitely agree on the missing ground connection to the parallel port, will fix the schematic shortly. I no longer have the hardware but I'm quite sure that I had that wire and forgot to include it in the schematic.

Regarding the 5v tolerancy of the ESP8266's GPIOs, there is quite a bit of discussion about this on the internet... the general consensus seems to be that they are tolerant (only I/O, not powering the chip), which seems to have been confirmed by Espressif. Indeed as you say converting the signal to 3.3v is probably safer anyway.

I used the 74hc595 because I already had that part, anyway I'll link to this issue from the readme to clarify there are better options.

[gianluca-nitti]

To fix the image I opened the Fritzing file with the schematic and noticed it already had the ground connection. Just the image in the readme was wrong.