MAX232: About unused pins

No; these logic inputs must not be allowed to float, so you need to connect them to VCC or GND.

Consider using a single-channel transceiver like the MAX3221(E).

Hi Kato-san,

Clemens is correct - TIN cannot be left floating; it must be tied to GND or VCC. 

If possible using a single channel device may be more suited to your application, such as MAX3221 or MAX3221E (as Clemens also indicated).

Please let me know if you have any other questions. 

Best,

Parker Dodson

Thank you for answering.

 What happens if you leave the TIN floating?

Also, will it affect VCC?

See [FAQ] How does a slow or floating input affect a CMOS device?

Hi Kato-san,

Please see the link Clemens posted - as it contains some good information. 

Essentially the risks can be summarized in 3 main points:

1. Excessive shoot through current - when the input is floating the "voltage" between that pin and ground can create a low impedance pathway between VCC and GND which will cause excessive shoot through current. 

2. The output of the pin is undefined and could possibly oscillate creating a radiator on the unused output which can negative impact the active communication line.

3. Depending on board layout the parasitic inductance and capacitance could be stimulated to resonate due to the shoot through current. That means the VCC at device could oscillate - which could impact other devices attached to the VCC rail as well as cause oscillations on VCC to show up on console/bus facing data lines.

Essentially there is a lot of risk - if you cannot guarantee that unused TIN will have a defined level you cannot use a 2T2R device and should use a 1T1R where that won't be a concern - as leaving TIN floating is extremely inadvisable. 

Best,

Parker Dodson