protocols as SDLC, HDLC, and X.25. Secondary channel A secondary data channel, identical in capability to the primary channel, can optionally be implemented by the DTE and DCE devices. Pin assignments are as follows: Related standards Other serial signaling standards may not interoperate with standard-compliant RS-232 ports. For example, using the TTL levels of near +5 V and 0 V puts the mark level in the undefined area of the standard. Such levels are sometimes used with GPS receivers and depth finders. A 20 mA current loop uses the absence of 20 mA current for high, and the presence of current in the loop for low; this signaling method is often used for long-distance and optically isolated links. Connection of a current-loop device to a compliant RS-232 port requires a level translator. Current-loop devices can supply voltages in excess of the must-withstand voltage limits of a compliant device. The original IBM PC serial port card implemented a 20 mA current-loop interface, which was never emulated by other suppliers of plug-compatible equipment. Other serial interfaces similar to RS-232: RS-422 – a high-speed system similar to RS-232 but with differential signaling RS-423 – a high-speed system similar to RS-422 but with unbalanced signaling RS-449 – a functional and mechanical interface that used RS-422 and RS-423 signals; never caught on like RS-232 and was withdrawn by the EIA RS-485 – a descendant of RS-422 that can be used as a bus in multidrop configurations MIL-STD-188 – a system like RS-232 but with better impedance and rise time control EIA-530 – a high-speed system using RS-422 or RS-423 electrical properties in an EIA-232 pinout configuration, thus combining the best of both; supersedes RS-449 EIA/TIA-561 – defines RS-232 pinouts for eight-position, eight-contact (8P8C) modular connectors (which may be improperly called RJ45 connectors) EIA/TIA-562 – low-voltage version of EIA/TIA-232 TIA-574 – standardizes the 9-pin D-subminiature connector pinout for use with EIA-232 electrical signalling, as originated on the IBM PC/AT EIA/TIA-694 – similar to TIA/EIA-232-F but with support for higher data rates up to 512 kbit/s Development tools When developing or troubleshooting systems using RS-232, close examination of hardware signals can be important to find problems. This can be done using simple devices with LEDs that indicate the logic levels of data and control signals. "Y" cables may be used to allow using another serial port to monitor all traffic on one direction. A serial line analyzer