Problems with Raspberry Pi and some Centronics Interfaces

[RWAP]

A few users have now reported an issue with the Raspberry Pi 3 Model B+

This actually, appears to affect all models of the Raspberry Pi in that, as soon as the centronics cable is connected, for some reason there appears to be a power drain on the Raspberry Pi

After some investigation, we have found that it appears to depend on either the centronics cable or (most probably) the centronics interface on the connected equipment.

During development, we found various pieces of equipment and computers (most notably the Sinclair QL), where they would refuse to send any output to the centronics port because they did not believe anything was connected. We therefore pass the 5V line from the Raspberry Pi (via a diode) to the centronics pins 13 and 32 (Select and Error).

It appears that some equipment (or possibly centronics cables) connect pin 32 to GND and ignores pin 13 (hopefully there is nothing which looks for one signal but not the other)!

[RWAP]

Most equipment only uses a 25 pin parallel port to connect to the 36 way centronics port at the other end of the cable. Our guess is that some centronics cables connect pin 32 to GND rather than simply not connecting it to anything. Pin 13 may be required by equipment to signify that a printer is attached.

So, the short term is to check your centronics cable and whether pin 32 is connected to anything - if it is connected to GND, then you need to de-solder that pin in the cable. We will probably need to come up with a longer term solution (maybe a jumper to select either or both pins).

As an extreme measure, Diode D1 (circled) can be snapped off or de-soldered if you experience this issue - although that will also remove the power on pin 13.

Retro-Printer Module - Diode D1.jpg (59.44 KiB) Viewed 5115 times

[RWAP]

I was just looking at a standard centronics wiring diagram - pin 13 on the Retro-Printer should be attached to pin 13 on the 25 way parallel port socket. Pin 32 on the Retro-Printer should be connected to pin 15 on the 25 way parallel port socket, as per https://www.lammertbies.nl/comm/cable/parallel.html

It would be interesting to know if those who experience this issue have a cable which is wired in this way (in which case it the equipment which has pin 15 (error signal) wired to GND, or whether there are centronics cables which do not connect this pin.

If it is down to cables, that would explain why some people have reported the issue but others using the same computer have not.

In the meantime, we also understand that some equipment need 5V on pin 18 - http://forum.retroprinter.com/viewtopic.php?f=13&t=34

We have therefore asked the factory to provide us with a quote for an extension adaptor which would plug into the centronics port on the Retro-Printer and provide jumpers to allow you to determine whether to allow 5V on each of pins 13, 18 and 32