Design Ideas September 28, 1995

The design in Fig 1 uses the Maxim MAX250 and MAX251 chip set and telecommunication-standard isolating components to provide a complete telecommunication-safety-standard isolated RS-232C interface. The MAX250 provides the TTL/CMOS logic interface and LED-driving capability. The device also provides a push-pull driver output to drive a power transformer. The MAX251 provides the rectifying circuit for the transformer's output, as well as LED-to-RS-232C interfacing.

The critical factors for the isolation barrier are the power transformer and optoisolators used for the interface. The CNY17-3 optoisolator has UL and VDE0884 approvals, and the 76250EN transformer meets the EN60950 safety-approval standard. The maximum switching frequency at which the optoisolators can operate is 250 kHz. However, the limiting factor in data transmission is the speed at which the ICs can reliably drive the optoisolators. Tests on the CNY17-3 types show that approximately 32 kHz is achievable with a 4.5V minimum system supply rail. This rate is considerably higher than the usual RS-232C 19.2 kbaud.

The oscilloscope trace in Fig 2 shows transmission of a 15.6-kHz logic square wave at the RS-232C interface pins. The MAX250 logic-interface IC has two control pins: EN enables the data drivers, and Shutdown switches off the entire interface. Current consumption reduces to approximately 2 mA in disabled mode and to 50 µA in shutdown mode. The interface takes about 1 msec to recover from a system shutdown but is available immediately after an enable signal.

Isolation between the RS-232C line and the logic side, limited by the optoisolators, is 5.3 kV dc; the transformer is capable of providing 6-kV isolation. The use of the isolation barrier eliminates ground loops and line noise on the signaling side. The circuit can interface telecommunications- line control functions to any PC or control system while maintaining the isolation safety barrier.