The Patton Model 51 DB-9 PockeTester™ monitors the seven most important async EIA-574 (RS-232 on a DB-9) data and control lines. The Model 51 uses seven tri-state LEDs to indicate the presence of the following signals: TD, RD, DTR, DSR, RTS, CTS and DCD. A red light = Mark, a green light = Space and no light indicates no signal on the line. No AC power or batteries required.