Since the Dev Address field is a signed 16-bit integer, the maximum positive value accepted by RSLogix is 32767
However the Dev Address field is a Modbus offset value. When using a Modbus function code 3, 6, or 16, an address base of 40001 is assumed. The Dev Address field is an offset from this base.
If the required OFFSET actually exceeds 32767, add 65536 to calculate the signed value of this address, and that value will be accepted as a signed 16 bit INT.