The adapter allows both PIC16C84 and 24CXX to stay in socket simultanously and you does not need to remove the eeprom when programming PIC16C84. (Automatic Switch Function).
The idea is to use a BC557B transistor as an emitter folower, but turned around to be a collector folower. This reduces the amplification and reduces the current to pull the SDA high trough the collector base diode. The transistor buffers the SDA output and it works as an automatic switch that turn off when MCLR is high. If MCLR is low is SDA pulled low trough R1 and high by the collector base diode in the BC557B. The impedance on collector is about 1/5 of base impedance. Resistor on base need to be high (>47K) to not pull much current from MCLR.
|P1||Connector to programmer|