Actually signed 16 bit integers are -32767 to 32767. So for some reason (probably DAC choice?) only 15-bits are being used.
The ER-301 maps -10V to 10V to internal values of -1 to 1. The Teletype maps -16383 to 16383 to -10V to 10V. Therefore,
-16383 to 16383 is mapped to -1 to 1 in the ER-301. In other words,
VALUE in ER-301 = (VALUE in TELETYPE)/16383.