Question regarding standard rabbit programming port

Can anyone explain to me why the pins have series 61.9 ohm resistors in them? (based on schematics of the usb programming cable, and rcm5700 interface board schematic)

Is this some kind of substitution for level changing from usb5.0 to board level3.3 volts? Based on the internal rabbit resistance or something?

I normally see 3.3volts on the RX (pin 6 programming port) but I have a custom board now that is showing up 5V on that pin (which is only present with the usb programmer connected)

I’m not entirely sure what purpose the resistors serve but it is probably either to protect against misconfiguration so that connecting 2 outputs together does not cause damage or to limit current on the lines for noise prevention/slew rate limiting.

The ft232R has an internal 3.3V regulator to power the rabbit side I/O pins so the signals should all be 3.3V.


Thank you for the insight.

I still think it is more important than just a protection resistor however, why would they use such a highly accurate 61.9 ohms?

I don’t think the actual value of the resistor is that important in this case as the USB programming cable for use with the core modules uses 100 ohm resistors. The 61.9 ohm value is probably just picked to reduce inventory and is probably used elsewhere on the board or other boards where the actual value is important.

someone probably looked at the component list and said we need a resistor with a few 10s of ohms for this and we already have X of them being used so why pick another unique resistor to add to the BOM when we already have one in the ball park picked out.


Yes that makes sense. I did find my problem it was a mechanical connection issue with a mini pci-e socket and the RCM5700.

However it did make me look up the schematics relating to the programming ports, and found something rather odd.

I am using a programmer that came with an RCM4200 dev kit. And for some reason this programmer is configured to output 5V. ( As you can see the VCCIO of the FTDI chip is attached to 5V instead of the 3v3out that would dictate 3.3v operation.

The more recent RCM5700 dev kit i have sets up the FTDI chip to 3.3v out. ( )

Now even though the rcm4200 is not 5V tolerant that I can tell from the datasheet, it seems to program fine with this programmer. And I am also able to program this RCM5700 with the same 5V cable on my custom board.

I wonder if I should be concerned about this or just keep using it and hoping it doesnt explode? Is anyone aware if the RCM5700 would be less tolerant to the 5v programmer than the RCM4200 for any reason?