DB25 pin Cable Issues

Dear Friends,
I have made a multicontroller for my aquarium controls to control 2 aquariums using 1 Arduino, and I2C Port expanders.

I have 2 displays for the 2 tanks. The main control has the Ardunio, Keypad and I2C expanders for accomodating various devices and 1 display. The Secondary control has just the Display for the 2nd tank.
And data is transfered from my Main control to the Secondary Control vide a 25pin DB cable used in printers.

There is a male 25pin from my Main control and there is a 25pin female at the secondary control. These two are interconnected using the Db25 pin (male/female) cable.

Well this is the problem:

  1. then i connect the 25pin female of the Secondary onto the 25pin male of the Main Control DIRECTLY the whole system is working perfectly
  2. If is connect both the systems using the Cable then the LCD screens initialise and are totally blank!!! they system doesnt work

I thought the problem is with the cable, tried testing using multimeter for any shorted pins - no problem there. Then, bought another cable and tested that - again it didnt work.

Can i transfer data (basically +5 and Gnd, SCL and SDA) from one controller to another secondary control using DB25 cable? or is there something that i am doing wrong?

Can i transfer data (basically +5 and Gnd, SCL and SDA) from one controller to another secondary control using DB25 cable? or is there something that i am doing wrong?

There is no reason why you shouldn't be able to.

Why are you using a 25 pin connector and cable is all you need is 4 wires?

When you connect the boards together directly, pin 1 connects to socket 1, pin 2 connects to socket 2, etc. That is not necessarily the case with the DB25 cable. Depending on the intended purpose of the cable, there may be wires crossed between the two ends, or less than 25 wires present in the cable.

If you chose one of the crossed wires or unconnected pins, that would explain your problem.

A 25 pin DB25 printer cable has many ground pins assigned and possibly they are tied together somehow. Possibly you are grounding out signals because of that?

I agee it's rather a strange selection for a cable when so few conductors are actually required.


i have used a 25pin so that if i wanted to send more data, have a seperate controller for lights at the 2nd unit then it might be useful.

I didnt short the pins in the cable to check whether the correlate correctly!!! a big mistake.. will check that out.. thanks a lot.


yes thats the problem.. not all the pins in the cables are connected!!! man, why do they do such cables.. waste of money.

I will need to get Solderable DB pins and solder wires to get my data transferred... will do that and post on the developments

It's not a waste of money. The cable maker used the minimum number of wires and pins needed to transfer the data between two devices, saving money.

Unless you were referring to wasting your money on a cable that doesn't do what you wanted. :slight_smile:

Making your own cable will allow you to run 25 wires of the length you need.

yes.. waste of money is the way that these cables are of no use to me now.. let me try returning them and see...

and since, wires are pretty cheap out here in India, when i actually did the math doing my own wiring for the 25 pin turned out to be much cheaper than cable.. i could have done that before...