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Andrea
10-30-2011, 09:46 PM
Hello,
In my project I have master and 16 slaves, all are propellers. Each will have own power supply and eeprom. Master will speak with one slave at the time via "Simple serial" object, so at the moment the only one propeller is on the line.

My question is, is the schematic OK, and is the TX pin protected when some other slave talk to the master?

Some extra questions:
The farthest slave will be about 50m/164ft maybe even more from the master,will the signal reach it trough CAT 5 network cable without repeater?

Thank you

AntoineDoinel
10-31-2011, 12:49 AM
Hello,
In my project I have master and 16 slaves, all are propellers. Each will have own power supply and eeprom. Master will speak with one slave at the time via "Simple serial" object, so at the moment the only one propeller is on the line.

My question is, is the schematic OK, and is the TX pin protected when some other slave talk to the master?

Some extra questions:
The farthest slave will be about 50m/164ft maybe even more from the master,will the signal reach it trough CAT 5 network cable without repeater?

Thank you

Hi

Giving the impedance of the cable, and the fact that you're using serial resistors I'd say the TX pin is protected enough.

The ability to reach the farthest slave without repeaters will greatly depend on transmission speed. Due to the relatively long distance, and since you're already using 120ohm twisted pair cable, it would be better to use RS485 line drivers and differential signaling.

Also line termination on both ends of the line is advisable in either case.

JonnyMac
10-31-2011, 01:11 AM
You probably want to use an "open" baud mode where a pull-up (typical) or pull-down sets the idle state of the RX and TX lines, then any node pulls that line in the other direction (usually low). This way you don't have one device driving the line high while the other is driving it low, creating a problem.

Andrea
10-31-2011, 01:59 AM
Thank you guys.
Is this what you suggest?

Thank you and sorry for my bad English

Peter Jakacki
10-31-2011, 02:16 AM
I guess you are trying to run at low speeds and if you are trying to do this with minimal hardware then I would use a shielded cable since your communications lines are unbalanced (and unterminated) and extremely prone to interference. Also I would change the slave transmit resistor to a very low value of around 47ohms or even less as you need as low an impedance as possible. The short-circuit concerns are of far less importance and CMOS outputs do tend to be rather forgiving. The slave receive resistor could be a high value of at least 1K or more as this pin will not be trying to transmit anyway. Pull-ups at the master and slave receive inputs should be used in case of disconnection etc and they can be around 100K per slave node and around 10K for the master. BTW, the Prop's pins will be very susceptible to damage from induced voltages over this long "antenna" if you do use CAT5.

However, you are far better off using RS485 drivers with termination and running this as a proper balanced line over the CAT5. Consider half-duplex (as RS485 is) as this means only a single RS485 chip per node and most communications processes are "command and response" anyway. Depending upon your serial driver software you should be able to set it so that you can have a common receive and transmit line and a transmit enable line so only two I/O are required.