Hi all, I will be using some cat5 (probably about 2-3 meters) to connect a remote ST7735 based TFT over SPI. There will also be 1 analog signal coming back from the remote unit (a potentiometer).
I'm not sure which signals to connect to combine in twisted pairs, should I connect 5v and ground in the same pair, or split them? Where should I connect the analog signal to avoid as much noise as possible?
The signals are (x8):
Power: 5v, gnd
TFT: clk, mosi, cs, rs, dc
Pot: 0-5V
Many thanks!
2-3 meters shouldn't matter at all. The wires will twist around enough in that distance to negate, or amplify, any problems you might experience.
Bittsen:
negate, or amplify, any problems you might experience.
I'd have thought negating a problem and amplifying a problem were exact opposite results
JimboZA:
I'd have thought negating a problem and amplifying a problem were exact opposite results
Correct.
But if there is a problem using a twisted pair cat5 cable, then the problem would be amplified with the distance.
If ay problems were negated by using a twisted cat5 cable then the greater distance would not change.
So the general thoughts are that it should not be a problem how I wire it for my application? Thanks
dtokez:
So the general thoughts .....
Well I wouldn't call one reply, "general thoughts"....
What are your thoughts then Jimbo, care to share?
I would have shared if I had any
The wires will twist around enough in that distance to negate, or amplify, any problems you might experience.
Twisted wire setups work because certain signals are passed thru specific twist pairing setups. Random use of twisted wire may result in performance worse than untwisted wire parings.
zoomkat:
Twisted wire setups work because certain signals are passed thru specific twist pairing setups. Random use of twisted wire may result in performance worse than untwisted wire parings.
I know that's why I am a bit concerned and would like to minimise cross talk and noise issues. Is it a case of keeping similar potential signals in a pair or the opposite? Thanks all
Would have thought the data and clock are going be all over the power and especially the pot lines.
Could try some multicore that has several individual screened signal conductors, plus several simple stranded conductors for power, keep the noisy stuff away from the quiet.
I've done that without the analog. I don't remember exactly how I wired it, but there are 3 signal-lines paired with grounds. The remaining pair was doubled-up for power. I'm pulling about 1 Amp, but there's a voltage regulator at the far-end so voltage-drop over the connection isn't a problem.
Use one pair for SCLK/GND, another for MOSI/5V, a third for CS/RS and the last how you
like. That means the two high speed signals each get a return wire that's effectively
ground (ignoring DC offset).
Make sure +5V is decoupled to GND at both ends of the cable with 0.1uF ceramic cap. You may need to add some termination with signals travelling that far,
especially on the clock line.
I'm confused by RS and DC signals, these are different names for the same signal
surely?
If 2 wires are twisted, and an external signal is introduced, both wires will receive the same noise. Therefore the difference between the 2 are always the same.
This is the strength of UTP cables.
If you use the cable for other things, it could just be that the wires that are paired interfere with each other. But the only way to be sure is to install it and check it with a oscilloscope.
Also keep in mind that the wires on an average UTP are very thin. I don't know how much power the 5V takes but don't draw more than a couple of amps to be sure.
You can use four of the cat5 conductors in a 10baseT configuration and the other wires for power and such.
Hi guys
It is a little dangerous yes to put a clock line next to a analog line. My suggestion is to pair the analog signal with the GND.
If you need a longer cable or stronger noise immunity, convert you bus to I2C and transmit it with a P82B96 IC. It can make several meters on twisted pairs.
Regards.