Acumen INSTRUMENTS CORPORATION DATABRIDGE(TM) SDR2-USB - Manual (Page 37)

SDR2-USB Configuration Guide rev 1.0 Serial Port Basics
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CTS, etc.), RS-232 devices 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 –9.50V, 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.