MAX3221-EP: Need for output damping resistor

The RS-232 specification requires that edges have a slew rate of less then 30 V/µs. For such slow edges, no damping is necessary.

The MAX3221-EP includes circuitry to implement the limit; see the SR(tr) specification.

(There are older devices without a built-in slew rate limit. The relatively high output impedance of RS-232 drivers would require you to use capacitors; see, for example, figure 16 of the SN75185 datasheet.)

Hi Minoru,

I agree with Clemens statement below:

Clemens Ladisch said:

The RS-232 specification requires that edges have a slew rate of less then 30 V/µs. For such slow edges, no damping is necessary.

With such slow slew rates, we can expect little overshoots/undershoots to occur, and therefore do not need series resistance to mitigate such voltage noise. 

Clemens Ladisch said:

The MAX3221-EP includes circuitry to implement the limit; see the SR(tr) specification.

From the datasheet:

If in design excessive overshoots are occurring, it could potentially be necessary to add series resistance to protect from over voltage going outside the abs. max ranges of the device RS232 pins. 

If the design is done correctly abiding by the RS-232 standard, I don't think series damping resistors would be necessary. 

Regards,

Tyler

Hi, Clemens and Tyler.

I really appreciate your response.

I understood that no damping is necessary.

I didn't know 30 V/μs was slow.

Thank you for your great support.

Best regard.

Minoru Awamura