Abstract: A synchronous data transmission system includes modems and data packet switches having interfaces in compliance with CCIT recommendation V24. However, verification of correct operation of the modems is preferably carried out by the system control center by transmission of control instructions which are asynchronous, such as Hayes instructions. To make this possible without requiring additional circuitry, the modems are of the bi-mode type, having a synchronous and an asynchronous mode, and during control of a modem the interface circuits at the relevant packet switch are set to the asynchronous mode. The modem detects that condition of the packet switch interface circuit, and sets its own interface circuits to correspond therewith. The interface circuits are then reset to the synchronous mode after completion of the control operation.