max 232 question

[FFMan]

For a test i want to connect com1 on a zx24s to a 5v sd card interface.

I guess i need to use a max232 to get my levels straight ?

What capacitor values should i use. the diagrams in the 328 reference manual show use of 1uf, whereas i seem to have used capacitors marked 104 (.1uf) in my working max232 interface module. is the value critical ?

thanks

[dkinzer]

is the value critical ?

Yes, you have to use a value specified by the particular device's datasheet. The original MAX232 device requires 1uF capacitors. Other devices like the ST232 (which is pin compatible with the MAX232) requires 100nF capacitors. Further, the MAX3232 requires 100nF when operating at 3.3V and 330nF when operating at 5V.

It is a good practice to get in the habit of reviewing the datasheet for any device that you intend to use so that you can ensure that you are operating it within its published specifications.

[FFMan]

ok that explains why i have 0.1uf caps as my interface has an st232. handily i have another one to use so i have all the components required.

is it necessary (or best practice) to use a 10k to pull-down the rx line between the st232 and sd interface module ?

[dkinzer]

is it necessary (or best practice) to use a 10k to pull-down the rx line between the st232 and sd interface module ?

Again, the datasheet is informative. In the ST232 datasheet you'll find a depiction of the internal functionality of the device including pulldowns on the RS-232 inputs and pullups on the TTL-level inputs. In most cases, no additional pullup or pulldown resistors would be needed.