6N137 Optocoupler Pin Configuration
As shown in the datasheet, pins 1 and 4 are not connected.

Pins 5 and 8 are the 5 V power supply (pin 8 is not directly connected to a collector).

Pin 6 is the open-collector output, i.e., it is either connected to ground or open. (Vishay's 6N137 actually uses a MOSFET, i.e., an open-drain output.)
To get a valid output voltage when the output is inactive, you have to connect a pull-up resistor.

Pin 7 is not an emitter, but the enable input, i.e., forcing it low disables the output.

The original 6N137 was made by Hewlett-Packard; its innards are described in the Optoelectronics Application Manual:


  
  
  […] For high data rates, the transistors must be
  operated at lower closed-loop gain in order to achieve the
  required bandwidth. HP detector/amplifiers for digital
  applications have a high speed linear amplifier driving a
  Schottky-clamped output transistor, as shown in Figure
  3.1.4-2. Bias for the photo diode is decoupled from VCC to
  reduce the possibility of "chatter" (oscillatory transition
  from one logic state to another due to regenerative coupling
  via the power supply line). The linear amplifier has a
  tendency to be unstable if the high-frequency impedance of
  the power supply is not low enough. For this reason, a
  low-inductance bypass capacitor (0.01 µF ceramic disc)
  should be installed adjacent to each isolator of this type.
  This, and other chatter-suppression techniques are discussed
  in Section 3.3.
  
  The Schottky clamp is a metal-silicon diode in parallel with
  the base-collector junction of the output transistor. A
  metal-silicon (Schottky) diode has a lower turn-on voltage
  than a P-N junction,so when the transistor is driven into
  saturation, the Schottky diode bypasses the current which
  would otherwise enter the base-collector junction. With
  reduced current entering the base-collector junction, there
  is a proportionate reduction in the charge to be removed
  when the transistor is to be turned off, and thus the
  Schottky clamp reduces the turn-off delay. The attendant
  drawback is the 400 mV higher VCE(SAT) for a Schottky
  clamped transistor.
  
  The "enable" input has threshold voltage and input current
  levels resembling a TTL input. However, it is not necessary
  to apply a pullup resistor to insure its remaining high.
  Unless the enable is connected to a strobe, it may simply be
  left open. The strobe applied to the enable input may be
  either open-collector or active-pullup.
  
  With the enable high, analog operation is possible because
  there is no hysteresis. However, the dynamic range is
  limited. The lower limit is the threshold input current for
  operation in the active region – this threshold may be as
  high as 4 mA. The upper limit is the maximum dissipation
  rating on the output. Because of a touchy bias situation,
  analog separation is not recommended for designs to be
  mass produced.
  
  The reason for omitting hysteresis was not to permit analog
  operation, but rather to permit maximum data rate. With
  hysteresis, there would be a higher immunity to both
  differential- and common-mode noise but the shifting
  threshold would reduce the data rate capability. The effect
  of hysteresis on data rate is the opposite of peaking.


(Vishay's 6N137 actually uses a Schmitt trigger to get hysteresis.)