The current loop is used for transmitting digital signals over large distances (few hundreds of meters). The normal way is to send 1V signal and to discriminate it at 0.5V to tell 0 from 1. If the length of the cable is big 1V can attenuate to be bellow 0.5V and the bit is lost.

 Current loop transmits the information rather by current than voltage. The distance is then limited by the strength of the current source used.

 The actual connection for the spectrometer RS232 current loop:

 ComBox side switches: 60 mA, DTE, RX-ACT; TX-PSV
 Computer side:        60 mA, DCE, RX-ACT, TX-PSV
 

 Computer     Combox   Cable from serial line

    2           4             green
    3           3             white
    4           2             blue
1&3 shorted     -
    -        1&3 shorted