Are Current limiting resistors required for TWO Props via Serial Coms?

DavidM

HI,

I have two separate Prop chip circuits each with its own regulator circuits, but sharing a common power source ( battery 12VSLA)

I want to comunicate via Serial ( standard FDSerial object ) between the two circuits ( Props)

Q1) If I connect the RX & TX of One Prop to the TX & RX of the other prop, do I need any current limiting ( series) resistors in these lines Or is a direct wire for each OK?

Thanks

Dave M

JonnyMac

The resistor is cheap insurance. If a programming error causes one side to be driven high while the other is driven low the resistor will limit the current to a safe level. And if you think that kind of error doesn't occur, you haven't been programming very long....

DavidM

HI John,

Thanks for the reply,

The two prop serial connections are connected together via a small ribbon cable via a couple of MICRO-MATCH HEADERS.
I never like any direct "EXPOSED" Prop pin connections so I guess its best to have the resistors. Would 10K be two much or is something more like 150R a better choice?, Is there a calculation for the optimal resistor value?

Thanks
Dave M

kwinn

DavidM wrote: »

HI John,

Thanks for the reply,

The two prop serial connections are connected together via a small ribbon cable via a couple of MICRO-MATCH HEADERS.
I never like any direct "EXPOSED" Prop pin connections so I guess its best to have the resistors. Would 10K be two much or is something more like 150R a better choice?, Is there a calculation for the optimal resistor value?

Thanks
Dave M

The 0.5mA current rating of the input protection diodes is the usual consideration when connecting the propeller pins to chips with higher voltage (ie +5V) power supplies, but for 2 props operating at nearly the same voltage it would be the max source/sink current of 30mA. That would make the lowest resistor value 3.3/.030 or 110 ohms. Personally I would use a resistor in the 470 to 1k ohm range.

DavidM

HI kwinn

Thanks for the reply,

I would have thought that CMOS to CMOS would not draw any ( or very little current ) and eliminating the need for addition components ( i.e resistors). But I am wrong!

I have seen many designs use current limiting resistors. I am trying to do "best Practice" as far as overall current usage is concerned, especially as all my devices are battery powered.

So..

1) Without any resistors - Result is higher current usage
2) With Resistors - Result is minimised current usage


So SOME current limiting is better for overall power consumption?

Q1) How do I calculate the Current Used, and the Voltage Drop ?

Thanks

Dave M

M. K. Borri

A high resistor will lower your max.data rate because the wire between the props has some inductance and capacitance, so a resistor will increase the logic transition time. That said you can probably use a 220ohm resistor and call it done it's big enough to save your bacon in case of error and small enough to not interfere.

DavidM

Thanks

Dave M