Maxim > Design Support > Technical Documents > Application Notes > Interface Circuits > APP 4562
Keywords: MAX3231, voltage rating, charge pump, RS-485, MAX487, MARK state, space level, true fail-
safe, IBSCHK, IBIS, MAX3222E
APPLICATION NOTE 4562
FAQs about Interface, RS-232, and RS-485
Circuits
Aug 13, 2009
Abstract: This application note answers frequently asked questions (FAQs) about RS-232 and RS-485
transceivers.
This set of FAQs addresses some of the more common questions that can arise when using interface
devices in new or existing designs. The questions are divided into different categories based on specific
information areas. It is possible for questions and answers to span multiple categories.
Section 1. RS-232 Questions
1. What voltage rating should I use for the capacitors on the MAX3231?
2. What are the lead-free equivalents for the MAX3228/MAX3229 or MAX3230/MAX3231?
3. When I run IBSCHK on the IBIS model that Maxim provides for the MAX3222E, I get seven
warnings. Is this OK?
Section 2. RS-485 Question
1. The RS-485 receiver in my MAX487 properly goes to the MARK state with an open cable, but does
not always go to the MARK state with a shorted cable. How do I get it to behave as I want with
both shorted and open cables?
Section 1. RS-232 Questions
Q1. What voltage rating should I use for the capacitors on the MAX3231?
A1. The charge pump itself can drive up to about 7V across the flying capacitors. To this, one must add
any di/dt-induced voltages caused by fast switching on the traces between the MAX3231 and these
capacitors. For a typical layout, capacitors should be rated at least 10V before derating.
Q2. What are the lead-free equivalents for the MAX3228/MAX3229 or MAX3230/MAX3231?
A2. The MAXxxxAEEWV+T is Maxim's RoHS-compliant version of the MAXxxxEEBV. For the
MAXxxxAEEWV, we added "dummy" solder bumps that are electrically isolated at pins B2, B3, B4, C2,
C3, C4, and D2 through D5. This configuration will be similar to pins B6, D6, E4, and E6 on the existing
MAXxxxEEBV data sheet.
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