Editing Hardware (section)

==== Utilizing the Expansion Slot (for your own purposes) ====

'''Warning:''' All signals on the Expansion Port are 3.3V logic level, therefore it is important that no voltage exceeding 3.3V is ever presented on any Expansion Port pin, or '''you risk damage''' to your F256!

If interfacing 5V TTL level devices to the Expansion Port, it is essential that level converters are used.

As an example, if directly interfacing to the Expansion Port pins with 5V logic, then you could use the SN74LVC8T245 bi-directional level translator, with the DIR input controlled by the Port's R/Wn signal (for the D0 - D7 bi-directional data bus), and the OEn input controlled by the Port's OEn signal.  For the uni-directional Address and Control lines, the DIR input can be hardwired.

Note that the SN74LVC8T245 is designed so that the control pins (DIR and OE) are referenced to Vcca ('A' side voltage supplied). Therefore, a common practice would be to use side A for the internal (3.3V) side of the voltage translation, such that DIR and OE can be directly controlled by the Expansion Port's 3.3V level R/Wn and OEn pins.  Side B (and Vccb) then being the 5V TTL level referenced (external facing) side.  

As another example, if you were interfacing directly to a 3.3V peripheral chip (e.g. A W65C22 VIA powered by Vdd = 3.3V), but wanting to level translate to 5V TTL levels on the VIA's Port Pins, then an auto-direction level translator like the TI TXS010x series (TXS0108, TXS0104, TXS0101), might be more appropriate. 
  
Note that on the TXS010x series the control pin (OE) is also referenced to Vcca ('A' side voltage supplied). With the TXS series, the 'A' side is actually limited to a 1.4V - 3.6V range, so is inherently the 3.3V internal side.  Side 'B' is 1.65V - 5.5V, so is used for the external 5V TTL level referenced (external facing) side.  So, if you're tying the active high OE pin of a TXS device (to permanently enable), it should be pulled to Vcca (not Vccb!).