Electrical – Pull down resistor with 24v

Strong means low resistance. Weak means high resistance. Of course low and high are relative terms, and so are strong and weak. The reference for this relationship must be inferred from context.

A strong or low resistance pull-up/down resistor is good because the time constant formed the load capacitance (often, the input gate capacitance, and the PCB trace capacitance) is small, so rise/fall times will be short.

A strong pull-up/down resistor is good because noise currents from unintended coupling and EMI will result in smaller noise voltages. (Think about Ohm's law)

A weak or high resistance pull-up/down resistor is good because it will not require much current from the driving circuitry to work against the resistor. Batteries will thus last longer, parts can be smaller and don't get as hot.

Of course, you usually want all of these things, but a resistor can't be both. A discussion about strong vs. weak is usually clarifying which of these concerns (or perhaps others) are more important for a particular application.

It's not a pull-down in the strictest sense that some other chip may be required to activate said line and fight against it. The chip in the OP's diagram is permanently chip-enabled as far as I can see and this is achieved by the 39 ohm resistor. If it were a zero ohm resistor would anybody quibble?

So why use a 39 ohm resistor instead of a zero ohm resistor - maybe there is another 39 R resistor used elsewhere on the board and they didn't want another line item in their Bill of Material: -

So it seems!