ds18b20 & cat5e copper
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CAI_Support
Senior Member
Assume the question is about hw rev 2.2.2 boards, which has different hardware design from earlier board for the 1-wire bus control.
We tested about 100ft with 7 sensors in different part of this bus. In the general tab, there is a parameter TDSO for adjust 1-wire timing.
The limitation is when the signal reached to the end of bus and bounced back, that signal will confuse the 1-wire control logic, since it has no way to tell that bounced back signal is from device or from bounced signal. Adjust the timing in that general tab could help you to avoid the false signal being picked up.
We tested about 100ft with 7 sensors in different part of this bus. In the general tab, there is a parameter TDSO for adjust 1-wire timing.
The limitation is when the signal reached to the end of bus and bounced back, that signal will confuse the 1-wire control logic, since it has no way to tell that bounced back signal is from device or from bounced signal. Adjust the timing in that general tab could help you to avoid the false signal being picked up.
Because the 1-wire bus is unterminated open collector TTL reflections are a problem. The preferred topology is a linear bus, not a star as you propose. Stubs to individual sensors should be short relative to signalling speed - a couple of meters. As CAI support mentioned WebControl allows tuning 1-wire but at least in my case have not had a need to do so.
Might want to take a look at the Dallas/Maxim App Note 148 about long 1-wire buses.
http://pdfserv.maximintegrated.com/en/an/AN148.pdf
The limiting factor is rise time, shielded cable is not going to help, in fact the addition capacitance may even make it worse. Adequately bonding shield to ground is difficult.
More 1-wire app notes:
http://www.maximintegrated.com/design/techdocs/tutorials/index.mvp/id/1/c/1-Wire%26reg%3B%20Devices#c1
Might want to take a look at the Dallas/Maxim App Note 148 about long 1-wire buses.
http://pdfserv.maximintegrated.com/en/an/AN148.pdf
The limiting factor is rise time, shielded cable is not going to help, in fact the addition capacitance may even make it worse. Adequately bonding shield to ground is difficult.
More 1-wire app notes:
http://www.maximintegrated.com/design/techdocs/tutorials/index.mvp/id/1/c/1-Wire%26reg%3B%20Devices#c1
Okay. I don't understand very well. I know that we can tune the ''delay time'' for the ds18b20 input. But how it works, if I do not have the right temp from one of sensors, I just need to upgrade or downgrade the ''ms value'' ? What about other sensors?. I think I need to reed both links you shown.
CAI_Support
Senior Member
Electrical signal travels on wire like you toss a rock into a quite water pond. You will see the wave travelling from the point stone dropped in toward the edge of the pond. When it reached to the edge of the pond, it will bounce back, then bounced back wave will reach to next boundary and bounce again.
When WebControl talking to temp sensors, it is similar to drop rocks into water in certain sequence, and listen to the sequence from remote sensor which drop its own rocks in its own sequence. However, if bounced waves are so much from both WebControl and sensor, it will be hard to distinguish which wave is real and which is bounce. Considering 8 sensors and WebControl all tossing rocks to generate waves to talk to each other, it can be pretty messy.
For getting best communication result for longer distance, reducing the bounce is very important. Star topology has strongest bounce signals, in most case, that is NONO. Linear bus like Tim mentioned has least bounced signal, since there are only two ends could bounce The WebControl timing adjustment is to say "I will listen this portion of the signals", signals not in that timing section will be ignored..
When WebControl talking to temp sensors, it is similar to drop rocks into water in certain sequence, and listen to the sequence from remote sensor which drop its own rocks in its own sequence. However, if bounced waves are so much from both WebControl and sensor, it will be hard to distinguish which wave is real and which is bounce. Considering 8 sensors and WebControl all tossing rocks to generate waves to talk to each other, it can be pretty messy.
For getting best communication result for longer distance, reducing the bounce is very important. Star topology has strongest bounce signals, in most case, that is NONO. Linear bus like Tim mentioned has least bounced signal, since there are only two ends could bounce The WebControl timing adjustment is to say "I will listen this portion of the signals", signals not in that timing section will be ignored..
CAI_Support
Senior Member
TDSO actually moves the timing window, so that only listen to that section of timing after WebControl sending out command could possibly have real signals from sensor.
