Posted by Marc-Andre Bazergui (bazmarc) March 08, 2019 ROBOT REMIX #6 Share Get link Facebook Twitter Pinterest Email Other Apps Post a Comment
Now, I'm not an electronics boffin, but I think he said that cables longer than 1 metre had problems with induction, and bringing it back to 90cm gave them a 10% (or so) safety margin.
This is also why there will probably never be a female-female cable joiner... )-:
Meanwhile. This page says that the main limit is the capacitance of the cable of 400pF: http://www.i2cchip.com/i2c_connector.html
Not being an electronics type I'm not sure how that relates to the cat5 cable, I found a cat5e specification of 5.6nF/100m (nominal). Whatever that means.
It is a long time time since I worked my way through the I^2C (I squared C) spec, but 90 cm sounds like a good place to stop.
The touch sensors do not use the I^2C bus, so they can probably stretch further. The motors do not use the I^2C bus, but the tachometer inputs are similar.
Philips created the I^2C interface as a cheap, efficient way to connect chips and sub-assemblies, (e.g. in TV set). It was not designed to run over long distances.
In mechanical terms I^2C is like a short lever with only a small movement. The further away you are from the lever the harder it is to tell whether your piece of string is being tugged upwards or downwards.
Other signaling schemes such as Ethernet over CAT cable and the RS485 signals on Port 4 of the NXT brick are better for long distances. In mechanical terms they push in one place whilst pulling in a corresponding second place.
Karl - your success with longer cables may depend on:
(a) cutting out nearly all of the cheap phone cable, and
(b)which wires are connected to each twist pairs in the CAT-5 cable. Pins 1 & 2 should be on the same pair. As should 3 & 6, and 4 & 5.
5.6nF/100m means a worst case of 5600pF capacitance over 100 metres of cable. So if 400pF defines the cut off point you may be able to achieve about 7m.
I am skeptical that this will be reliable. Especially as the cables are likely to run near electrically noisy Lego motors.
Apologies for the long essay.