Ring Indicator
Encyclopedia
Ring Indicator, abbreviated as RI, is a control signal that is used in RS-232
serial communications cables between a computer and a modem. The signal is sent from the modem to the computer, typically over pin 9 of a nine-pin (DE9) serial port, or pin 22 of a 25-pin (DB25) serial port. Its purpose is to indicate to the computer that the phone line is ringing.
As implemented in modern PC hardware, a hardware interrupt is generated that can be captured by the processor any time the RI signal changes state. This functionality was originally present in the 8250 UART - the chip responsible for providing the serial port - which was present in the original IBM PC XT. Virtually all serial hardware for newer PC's, including the popular 16550 UART
, has maintained this functionality. Having support for this hardware interrupt means that a program or operating system can be informed of a change in state of the RI pin, without requiring the software to constantly "poll" the state of the pin.
RI is a one-way signal from the modem to the terminal (or more generally, the DCE to the DTE) that does not correspond to another signal that carries similar information the opposite way.
). However, when a ring is detected and the RI signal is asserted, the state of the AA light is simultaneously flipped - so it temporarily turns on if auto-answer mode is off, or off if it's on. This doesn't change the auto answer status - rather, it is simply meant as a visual indication of ringing and the assertion of the RI signal.
In practice, most telecommunications programs or server hosts do not actually use the RI signal to detect ringing. Rather, they typically wait for the "RING" message sent to the host over the data pins.
The asserted RI signal lasts nearly exactly as long as the ringing on the telephone line - for example, 2.0 seconds per ring burst in North America. The RI signal follows the ringing pattern closely - in fact, it is possible for a computer program to detect distinctive ring
patterns from the phone company by monitoring the transitions of the RI signal. If the phone line rings, for example, two short burts and then a long burst, the RI signal will imitate this pattern. In practice, distinctive ring detection is a feature built in to the command sets of many modern modems. Distinctive ring allows a single phone line to be "shared purpose" whereby it has multiple phone numbers, distinguished only by the ringing pattern, and the modem only answers calls whose ring pattern identify the call as a data call.
(UPS's) to signal a power failure state to the computer. Although newer UPS devices typically send serial data over the "receive data" pins (such is called a "smart" UPS), using the Ring Indicator pin is particularly cheap and simple to implement, because it is essentially a simple signal that can be sent to the computer in the form of either a constant positive or constant negative voltage.
A UPS that uses the Ring Indicator pin to signal a power failure is referred to as a "dumb" UPS. A "mark" (negative voltage) on the Ring Indicator pin indicates a power-fail state. The serial extension cables included with such UPSes are frequently contain conductors only for the Ring Indicator and ground pins.
on some computer motherboards to wake the computer from sleep mode. This functionality is known as wake-on-ring
, and is conceptually similar to the wake-on-LAN
feature found on many Ethernet controllers. A home automation controller from a popular company known as X10 Wireless Technology
is a notable example of this type of use. The controller, which connects to the computer's serial port, can be programmed to assert the RI line on the serial port upon detecting a home automation event of the user's choice, waking up the computer to query the controller for the event ID and then perform any response programmed for the event.
Despite RS232 being considered somewhat of a "legacy" standard, the wake-on-ring functionality is still considered important enough that it is implemented into USB-to-serial converters. When implemented, the USB-to-serial converter remains on standby while the computer is in sleep mode, and sends an ACPI
"wake" event over the USB port when the appropriate Ring Indicator transition is detected.
Motherboard BIOSes may refer to this feature with other names or phrases, such as "Power on by external modems".
s with equipment (which usually carry all 9 or more pins) that only uses a small subset of the serial lines.
In this scenario, the voltage on the RI line is not driven (said to be "floating"), and can pick up interference from neighboring signals. If the RI line picks up interference from the RXD/TXD lines (which change from high to low rapidly with every bit transmitted, often thousands of times per second), it can cause the computer to erroneously detect enormous quantities of transitions on the RI line. Each of these transitions generates CPU interrupt
s and kernel events and can disrupt application software and waste a significant amount of CPU time. If the RI line is unused but carried through a cable next to other wiring, it is important to tie it to either a high or low signal instead of leaving it disconnected and floating. Simply tying it to the output of some other pin on the computer that outputs a steady RS232-level voltage (such as DTR or RTS) will eliminate this problem, regardless of which signal is actually being output on the pin chosen. Connecting the pin to ground
should be avoided, because the resulting lack of voltage differential (0 volts) is considered an undefined signal in the RS232 standard.
RS-232
In telecommunications, RS-232 is the traditional name for a series of standards for serial binary single-ended data and control signals connecting between a DTE and a DCE . It is commonly used in computer serial ports...
serial communications cables between a computer and a modem. The signal is sent from the modem to the computer, typically over pin 9 of a nine-pin (DE9) serial port, or pin 22 of a 25-pin (DB25) serial port. Its purpose is to indicate to the computer that the phone line is ringing.
As implemented in modern PC hardware, a hardware interrupt is generated that can be captured by the processor any time the RI signal changes state. This functionality was originally present in the 8250 UART - the chip responsible for providing the serial port - which was present in the original IBM PC XT. Virtually all serial hardware for newer PC's, including the popular 16550 UART
16550 UART
The 16550 UART is an integrated circuit designed for implementing the interface for serial communications...
, has maintained this functionality. Having support for this hardware interrupt means that a program or operating system can be informed of a change in state of the RI pin, without requiring the software to constantly "poll" the state of the pin.
RI is a one-way signal from the modem to the terminal (or more generally, the DCE to the DTE) that does not correspond to another signal that carries similar information the opposite way.
As used on modems
On an external modem with LED lights, the status of the Ring Indicator pin is often coupled to the "AA" (auto answer) light. Normally, the AA light is either off if the modem is not in auto-answer mode, or on if the auto answer mode is enabled (see ATS0 in the Hayes command setHayes command set
The Hayes command set is a specific command-language originally developed for the Hayes Smartmodem 300 baud modem in 1981. The command set consists of a series of short text strings which combine together to produce complete commands for operations such as dialing, hanging up, and changing the...
). However, when a ring is detected and the RI signal is asserted, the state of the AA light is simultaneously flipped - so it temporarily turns on if auto-answer mode is off, or off if it's on. This doesn't change the auto answer status - rather, it is simply meant as a visual indication of ringing and the assertion of the RI signal.
In practice, most telecommunications programs or server hosts do not actually use the RI signal to detect ringing. Rather, they typically wait for the "RING" message sent to the host over the data pins.
The asserted RI signal lasts nearly exactly as long as the ringing on the telephone line - for example, 2.0 seconds per ring burst in North America. The RI signal follows the ringing pattern closely - in fact, it is possible for a computer program to detect distinctive ring
Distinctive ring
Distinctive ring, marketed under a variety of names, is a service offered by a telephone company that establishes additional telephone numbers on the same line as an existing number, each number ringing with a distinctive ringing pattern. Typically, the original number rings with the standard ring...
patterns from the phone company by monitoring the transitions of the RI signal. If the phone line rings, for example, two short burts and then a long burst, the RI signal will imitate this pattern. In practice, distinctive ring detection is a feature built in to the command sets of many modern modems. Distinctive ring allows a single phone line to be "shared purpose" whereby it has multiple phone numbers, distinguished only by the ringing pattern, and the modem only answers calls whose ring pattern identify the call as a data call.
As used with other hardware
The Ring Indicator signal is notably used by some older uninterruptible power suppliesUninterruptible power supply
An uninterruptible power supply, also uninterruptible power source, UPS or battery/flywheel backup, is an electrical apparatus that provides emergency power to a load when the input power source, typically mains power, fails...
(UPS's) to signal a power failure state to the computer. Although newer UPS devices typically send serial data over the "receive data" pins (such is called a "smart" UPS), using the Ring Indicator pin is particularly cheap and simple to implement, because it is essentially a simple signal that can be sent to the computer in the form of either a constant positive or constant negative voltage.
A UPS that uses the Ring Indicator pin to signal a power failure is referred to as a "dumb" UPS. A "mark" (negative voltage) on the Ring Indicator pin indicates a power-fail state. The serial extension cables included with such UPSes are frequently contain conductors only for the Ring Indicator and ground pins.
Wake-on-ring
The Ring Indicator pin can also be configured in the BIOSBIOS
In IBM PC compatible computers, the basic input/output system , also known as the System BIOS or ROM BIOS , is a de facto standard defining a firmware interface....
on some computer motherboards to wake the computer from sleep mode. This functionality is known as wake-on-ring
Wake-on-ring
Wake-on-Ring , sometimes referred to as Wake-on-Modem , is a specification that allows supported computers and devices to "wake up" or turn on from a sleeping, hibernating or "soft off" state Wake-on-Ring (WOR), sometimes referred to as Wake-on-Modem (WOM), is a specification that allows supported...
, and is conceptually similar to the wake-on-LAN
Wake-on-LAN
Wake-on-LAN is an Ethernet computer networking standard that allows a computer to be turned on or woken up by a network message....
feature found on many Ethernet controllers. A home automation controller from a popular company known as X10 Wireless Technology
X10 Wireless Technology
X10 Wireless Technology, Inc. is an American subsidiary of a Hong Kong-Bermuda company best known for marketing wireless video cameras using controversial pop-under advertisements. It was founded in 1999 in Kent, Washington.- History :...
is a notable example of this type of use. The controller, which connects to the computer's serial port, can be programmed to assert the RI line on the serial port upon detecting a home automation event of the user's choice, waking up the computer to query the controller for the event ID and then perform any response programmed for the event.
Despite RS232 being considered somewhat of a "legacy" standard, the wake-on-ring functionality is still considered important enough that it is implemented into USB-to-serial converters. When implemented, the USB-to-serial converter remains on standby while the computer is in sleep mode, and sends an ACPI
ACPI
ACPI may refer to:*Advanced Configuration and Power Interface for computer configuration and management*Animation Council of the Philippines, Inc....
"wake" event over the USB port when the appropriate Ring Indicator transition is detected.
Motherboard BIOSes may refer to this feature with other names or phrases, such as "Power on by external modems".
Floating RI interference in serial cables
Some poor quality serial cables can cause computer problems with the Ring Indicator signal. This happens if there is inductive interference between the RI line and the RXD/TXD lines when the cable is used with equipment that does not provide any signal on the RI line, and often occurs when using serial extension cordExtension cord
An extension cord, power extender, or extension lead is a length of flexible electrical power cable with a plug on one end and one or more sockets on the other end . The term usually refers to mains extensions but is also used to refer to extensions for other types of cabling...
s with equipment (which usually carry all 9 or more pins) that only uses a small subset of the serial lines.
In this scenario, the voltage on the RI line is not driven (said to be "floating"), and can pick up interference from neighboring signals. If the RI line picks up interference from the RXD/TXD lines (which change from high to low rapidly with every bit transmitted, often thousands of times per second), it can cause the computer to erroneously detect enormous quantities of transitions on the RI line. Each of these transitions generates CPU interrupt
Interrupt
In computing, an interrupt is an asynchronous signal indicating the need for attention or a synchronous event in software indicating the need for a change in execution....
s and kernel events and can disrupt application software and waste a significant amount of CPU time. If the RI line is unused but carried through a cable next to other wiring, it is important to tie it to either a high or low signal instead of leaving it disconnected and floating. Simply tying it to the output of some other pin on the computer that outputs a steady RS232-level voltage (such as DTR or RTS) will eliminate this problem, regardless of which signal is actually being output on the pin chosen. Connecting the pin to ground
Ground (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....
should be avoided, because the resulting lack of voltage differential (0 volts) is considered an undefined signal in the RS232 standard.