Use a DC Voltmeter – Technicians with a DC voltmeter can use it to measure the DC level from signal ground (pin #5 on DB9, pin #7 on DB25) on the connector to pin #2 or pin #3.Note: Signal directions reversed if both devices are DTE but pin connections are the same. Crossover DB9 to DB25 cable provides DCE to DCE connection. Modem Signal Connections: (DCE DB9 Device to Modem (DCE DB25 Device).Straight Cable DB9 to DB25 provides DTE to DCE connection Computer Signal Connections: (DTE DB9 Device) to Modem (DCE DB25 Device).Signal flow is in the direction of the arrows. The signal names match each other and connect pin-for-pin. The two port types are complementary – the Output signals on a DTE port are Inputs to a DCE port, and Output signals on a DCE port are Inputs to a DTE port. Often, people do not realize that there are two types of RS-232 ports: DTE and DCE and that, while signal names and pin numbers are the same, signal flow is opposite! The pin labeled TX can be input, and RX the output. Usually, inputs are connected to inputs - and outputs to outputs. This FAQ will help you troubleshoot and correct similar problems. Connecting two devices using RS-232 sounds simple, but nearly every day we help a customer get a converter, isolator or other RS-232 device working by helping correct RS-232 cabling connections.
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