What is the purpose of the 6k2 pulldown resistor in the GPIB interface?
Don't think of RL2 as a pulldown. Think of RL1 and RL2 together as a Thevnin termination.
They present an impedance of about 2k to an effective voltage of about 3.3 V.
As GPIB was designed to use TTL levels, a high voltage of 3.3 V is all that's needed.