Serial Port Basics (continued)
RS-232 Voltage Levels
To improve noise immunity and support long cable lengths, RS-232 devices convert TTL/CMOS-level signals (0V=logic zero, +5V or +3.3V=logic one) to higher voltage bipolar signals. For the TD and RD signal lines, RS-232 devices use a voltage between -3V and -25V to transmit a one and a voltage between +3V and +25V to transmit a zero.
Typical RS-232 byte transfer (ASCII 'b'=0x61 hex)
Other RS-232 signal lines (RTS, CTS, etc.) use +3V to +25V to assert the signal and -3V to -25V to deassert the signal. These voltage levels are defined in the EIA/TIA-232E specification. So, when a DTE devices drives RTS at -9.5V, it is signaling the DCE to stop sending data.
A few devices, particularly devices that communicate at low data rates or over short cables, bypass the voltage conversion altogether. These devices often require "computer interface kits" that are really no more than a level converter in a box. These serial ports are often referred to as TTL RS-232, CMOS RS-232, or 5 Volt RS-232 ports and require level converters to interact with computers and other serial devices.
TTL (5V) RS-232 byte transfer (ASCII 'b'=0x61 hex)
An oscilloscope is a useful tool for diagnosing serial communications problems, because it can provide both time and voltage analysis. This makes determining RS-232 voltage levels and bit times a simpe task.
If an oscilloscope is not available, a multimeter can be used to determine which RS-232 standard is being employed by measuring the TD signal's idle voltage.
Conclusion
Because so many communications options exist for asynchronous communications, establishing an RS-232 serial link can be tricky. However, the concepts presented in this guide should provide a useful starting point for getting two RS-232 devices to communicate correctly.
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