Lower speed links are suppsoed to pull the - data pin slightly negative and, effectively, a safely 'TTL high' (relative to 5V source) of greater than 2.8V. The power difference is 5V to better guard against too much loss over a long cable.
Higher speed links are quoted as using even lower voltages (the graph appears 0 and 0.4 v swapped as flipped).
This StackExchange thread has some good diagrams and useful answers:
https://electronics.stackexchange.com/questions/190592/why-d...
Lower speed links are suppsoed to pull the - data pin slightly negative and, effectively, a safely 'TTL high' (relative to 5V source) of greater than 2.8V. The power difference is 5V to better guard against too much loss over a long cable.
Higher speed links are quoted as using even lower voltages (the graph appears 0 and 0.4 v swapped as flipped).