A physiological instrument that includes a first component comprising an ECG monitor and a second component comprising a defibrillator. The second component may also include a pacemaker mounted to and electrically connected with the defibrillator. The first and second components may communicate with one another entirely by means of three optical communication channels, each channel including an optical signal path that includes windows through which the optical signals pass into and out of the components. The components may therefore operate in a fully integrated manner despite the fact that no electrical signal path exists between the components. The communication channels include a serial channel through which information concerning defibrillator and pacemaker operation is transmitted to the ECG monitor, a tri-state channel through which sync and ECG source information is transmitted from the ECG monitor to the defibrillator/pacemaker, and an analog channel through which either an ECG signal from the defibrillator electrodes or an eyeclose signal from the pacemaker may be transmitted to the ECG monitor. Data transmitted through the serial channel includes a power-on signal, such that turning on the defibrillator automatically turns on the ECG monitor. The tri-state channel is used to transmit sync information when the ECG monitor determines that the defibrillator is on and when the defibrillator electrodes have not been selected as the ECG signal source. When the defibrillator is off, the ECG monitor controls the supply of power to the defibrillator electrode preamplifier via the tri-state channel.