Maximum wire/cable length carrying 5VDC signal

[Haris]

Dear everyone,

I have a pinpad that give wrong numbers when pushed. Earlier it works
fine untill I extend the cable length. This might be the reason, but I
still need to know why.

Upon closer inspection the pinpad is connected to an Atmel microchip
(AT89C2051) through 8 wires. 7 of them are signal while the 8th is
actually connected to 5VDC. I extend the original cable using Cat5 UTP
cable. I have double check the wiring connection and they are fine.

I wonder if there is length restriction to above cable carrying 5 VDC
signal. Any hint, advice and Troubleshooting ideas is appreciated.

Thanks in advance

Haris

 

[Mario Trams]

Dear everyone,

I have a pinpad that give wrong numbers when pushed. Earlier it works
fine untill I extend the cable length. This might be the reason, but I
still need to know why.

Upon closer inspection the pinpad is connected to an Atmel microchip
(AT89C2051) through 8 wires. 7 of them are signal while the 8th is
actually connected to 5VDC. I extend the original cable using Cat5 UTP
cable. I have double check the wiring connection and they are fine.

7 Signals + 5V, and Ground??

I wonder if there is length restriction to above cable carrying 5 VDC
signal. Any hint, advice and Troubleshooting ideas is appreciated.

I think you are faced more with noise troubles in this case. That is,
your signals are distorted either by external influences, or by internal
crosstalk. It might also be that the signals are attenuated too much.
But I doubt this is the case.

Bad termination of signals can also cause troubles with long cables.

How long did you extend the cables?

Regards,
Mario

 

[Amnon]

Hi,
Your problem is probably, as Mario wrote, due to signal integrity failure
(any chance for serial 33ohm resistors at your transmitter output, before
the cable ?).
However, if you concerned from DC levels as well, look at your transmitter
datasheet for Voh minimum, and at your receiver datasheet for Vih minimum.
The difference between them (delta) should be larger than the voltage drop
on your wire (length: L). To calculate that check tables for the resistance
(R) per meter of a wire in the AWG you use and multiply it with Io of your
transmitter .
solving: delta = LxRxIo will give you Lmax.
But again, signal integrity will probably "kill" you much before.

Amnon

 

[Haris]

7 Signals + 5V, and Ground??
I have been wondering about this also. Might it be that the Keypad
itself just function like a switch, connecting the +5V with any two of
the other signal wires. Thus no need for ground.
Appreciate it if anyone can explain this in greater detail.

How long did you extend the cables?

About 10 Metres

I think you are faced more with noise troubles in this case

Now that you mention it the cable itself is sealed in wall near to
power points. That migth be the source of noise.

--Haris

 

[Tim Auton]

I have been wondering about this also. Might it be that the Keypad
itself just function like a switch, connecting the +5V with any two of
the other signal wires. Thus no need for ground.

It would be trivial to test with a multimeter.


Tim

 

[PJ]

It would be trivial to test with a multimeter.


Tim

This is probably a scanning matrix keyboard. Put a scope on it. You will
probably see the waveform greatly attenuated...Paul

 

[Mario Trams]

I have been wondering about this also. Might it be that the Keypad
itself just function like a switch, connecting the +5V with any two of
the other signal wires. Thus no need for ground.

Yes, this might be an explanation. I thought this is some kind of
serial data transmission. But in this case the wire count is a little
bit high.

So let's assume a simple switch matrix.
Whether there is some resistance problem or not can be easily checked.
Just ensure that all keys are pressed and then measure if all those
suspected return lines carry around 5V at the micro controller. As
Amnon mentioned, it has to be at least Vih minimum.

When this is ok, you could try to add passive low-pass filters to the
7 lines (one for each).
That is, you do not connect these lines directly with the micro controller
but you insert a resistor R in series. Behind the resistor (meaning at
the micro controller side) you place a capacitor C whose other end is
connected to Ground. Which R-C combination to use you can experiment
a little bit. You should start with R in the range of tens and hundreads
of Ohms and C in the range of hundreads of nF.

The larger R and C, the lower the frequencies that pass through the filter.
(You can also put your nose into some texts for filter design to learn
about the mathematical dependencies ;-)

But there is a drawback. Equally, the rise and fall times increase which
is not good for the micro controller inputs (except when it has
schmitt-trigger inputs). Therefore it might be wise to put a schmitt-trigger
buffer between the low-pass filter and the micro controller input.

I don't know, but perhaps a schmitt-trigger buffer alone (without the
filter) could solve your problem.

About 10 Metres

This is quite long.

Now that you mention it the cable itself is sealed in wall near to
power points. That migth be the source of noise.

Yes, indeed.

Regards,
Mario