What TVS diode should use for MAX232 and MAX485, whether uni or Bi-directional?
Please go to this very useful page from Analog devices to get the full story. Here's a snippet or two: -




  Protection Scheme 1
  
  As described earlier, the EFT and ESD transient have similar energy
  levels, while the surge waveform has energy levels three to four
  magnitudes greater. Protecting against ESD and EFT is accomplished in
  a similar manner, but protecting against high levels or surge requires
  more complex solutions. The first solution described here protects up
  to Level 4 ESD and EFT and Level 2 surge. The 1.2/50 µs waveform is
  used in all surge testing described in this article.
  
  This solution uses the Bourns CDSOT23-SM712 transient voltage
  suppressor (TVS) array, which consists of two bidirectional TVS diodes
  optimized to protect RS-485 systems with minimal overstress while
  allowing the full range of RS-485 signal and common-mode excursions
  (–7 V to +12 V) on the RS-485 transceiver. Table 1 shows the voltage
  levels protected against for ESD, EFT, and surge transients.
  
  Table 1. Solution 1 Protection Levels
  
  ESD (-4-2)    EFT (-4-4) Surge (-4-5) Level   Voltage
  (Contact/Air) Level   Voltage Level   Voltage 4   8 kV/15 kV 4    2 kV 2  1 kV
  
  A TVS is a silicon based device. Under normal operating conditions,
  the TVS has high impedance to ground; ideally, it is an open circuit.
  The protection is accomplished by clamping the overvoltage from a
  transient to a voltage limit. This is done by the low impedance
  avalanche breakdown of a PN junction. When a transient voltage larger
  than the breakdown voltage of the TVS is generated, the TVS clamps the
  transient to a predetermined level that is less than the breakdown
  voltage of the devices that it is protecting. The transients are
  clamped instantaneously (&lt;1 ns), and the transient current is diverted
  away from the protected device to ground.
  
  It is important to ensure that the breakdown voltage of the TVS is
  outside the normal operating range of the pins protected. The unique
  feature of the CDSOT23-SM712 is that it has asymmetrical breakdown
  voltages of +13.3 V and –7.5 V to match the transceiver common-mode
  range of +12 V to –7 V, therefore providing optimum protection while
  minimizing overvoltage stresses on the ADM3485E RS-485 transceiver.




This is just for RS485.