An ASCII character is encoded on 8 bits. A WEP key is the concatenation of a shared secret key and a 24 bits randomly generated initialisation vector (IV).
The so called shared secret key is the key shared by all the users of the network, i.e. the one you type in the 770 UI.

So, if you take a 128 bits WEP key, it is then composed of a 104 bits shared secret key + 24 bits IV. Thus the shared key is 104/8 = 13 ASCII characters long. Apply the same logic to a 64 WEP key, and the shared key is 40/8 = 5 ASCII characters long.

Then, an hexadecimal digit is encoded on 4 bits. As a consequence, a 128 bits WEP key hexadecimaly encoded is using a 104/4 = 26 hexadecimal digits long shared key.
10 HEX digits for a 64 bits WEP key.

Again, the shared key is the key that the users are supposed to share and this is the key that you entre in give to both your AP and your 770 as an input. And to summarize it all, they should be that long:

• ASCII input
  • WEP 64: 5 digits
  • WEP 128: 13 digits
• HEX input
  • WEP 64: 10 digits
  • WEP 128: 26 digits

If your AP allows you to enter ASCII encoded WEP keys (try using letters. higher than F), then enter they same key in ASCII in your 770. Same thing for HEX encoding.

Wrong. From the 770 UI you can select either ASCII or HEX input.