e4d47c27__DataSheet_for_product_XE2422H__description
[pin] 4 [name] /DTR [description] 
/DTR is an active low input to the modem. The operation of /DTR is controlled by register SE2 and is normally disabled. When enabled, the modem will exit data mode and enter command mode if /DTR is removed while the modem is on-line. In Pulse and FSK Alarm modes raising /DTR is the only was to disconnect the data link.
Cermetek DataSheet for product XE2422H
description