dataIP Technical Information Document 25: , Last Revised : 2003-10-09
NOMENCLATURE
DCE – Data Communication Equipment, typically a modem; also XXXXXXX’s HOST port which emulates a modem.
DTE – Data Terminal Equipment, typically a host computer or CRT terminal; also XXXXXXX’s MODEM port which emulates a host computer.
SIGNAL STATES
Transmitted and Received Data signals are considered in the marking state when the voltage on the circuit is more negative than minus three volts with respect to signal ground, and in the spacing condition when the voltage is more positive than plus three volts with respect to signal ground. All control and timing functions are considered ON when the voltage on the circuit is more positive than plus three volts with respect to signal ground, and are considered OFF when the voltage on the circuit is more negative than minus three volts with respect to signal ground. The Data Terminal Ready [DTR](CD) circuit is “fail-safe” in that a power off condition in the terminal equipment or a disconnection of the interface cable is interpreted as an OFF condition. The data, timing and control functions are defined below:
Voltage: NEGATIVE (-) POSITIVE (+)
Binary state: "1" "0"
Signal condition: MARK SPACE
Control and timing function: OFF ON
TERMINATOR IMPEDANCE
The terminating impedance at the receiving end of interface circuits has a resistive component of not less than 3000 ohms nor greater than 7000 ohms over the range of voltages for which the signal is defined.
RISE AND FALL TIMES
For control circuits, the time required for a signal to pass through the transition region of minus three to plus three volts during a change of state does not exceed one millisecond.
For Received Data [RD](BB), the time required for the signal to pass through the transition region is greater than one microsecond but does not exceed 20 microseconds.
OPEN CIRCUIT VOLTAGES
Open circuit driver voltage on any circuit does not exceed +/- 18 volts with respect to signal ground. When the terminating impedance is between 3000 and 7000 ohms, and the terminator open circuit voltage is zero, the potential at the point of interface is not less than +/- 5 volts or greater than +/- 15 volts. The terminator on an interface circuit is designed to withstand any input signal within the +/- 25 volt limit.
Pin Name, Abbreviation, EIA Circuit, CCITT Circuit
1 PROTECTIVE GROUND, FG, AA, 101
A protective (frame) ground circuit is provided by means of the ground wire on the power cord. This also provides grounding of the chassis. The Signal Ground circuit (pin 7) is the common reference point for all circuits on the interface. The Signal Ground and protective grounds are tied together internally. This arrangement is intended to provide additional margin against longitudinal power line noise.
2 TRANSMITTED DATA, TxD, BA, 103
Signals on this circuit are generated by DTE and received by DCE. A positive signal is considered a binary “0” or SPACE and a negative signal is considered a binary “1” or MARK. The transmitting terminal should hold circuit [TxD](BA) in the marking state when no data are transmitted, including intervals between characters.
3 RECEIVED DATA, RxD, BB, 104
Signals on this circuit are generated by a local DCE and received by a DTE typically in response to signals transmitted by a remote DCE.
4 REQUEST TO SEND, RTS, CA, 105
Signals on this circuit control the data channel transmit function of the DCE. The ON condition causes the DCE to assume the data channel transmit mode. The OFF condition causes the DCE to assume the data channel non-transmit mode, when all data transferred on [TxD](BA) have been transmitted.
5 CLEAR TO SEND, CTS, CB, 106
An ON condition of [CTS](CB) indicates to DTE that a local DCE will transmit to a remote DCE any data present on the Transmitted Data [TxD](BA) circuit. If [CTS](CB) is OFF, a local DCE will internally clamp [TxD](BA) to mark and ignore signals presented on [TxD](BA) by DTE. [CTS](CB) will be OFF whenever Data Carrier Detected [DCD](CF) is OFF.
6 DATA SET READY, DSR, CC, 107
Signals on this circuit originate from a local DCE to indicate its status. The ON condition indicates that a DCE is in the data mode and is connected to the communication channel (telephone line). The ON condition of this circuit alone should not be interpreted to mean that a communication channel has been completely established or used to determine the status of any remote DTE.
7 SIGNAL GROUND, SG, AB, 102
This circuit establishes the common ground reference for all interface circuits. This circuit is normally connected to protective ground (pin 1) to minimize noise into electronic circuitry.
8 DATA CARRIER DETECTED, DCD, CF, 109 (RECEIVED LINE SIGNAL DETECTED)
The ON condition of this circuit indicates that data carrier is being received by a local DCE and has been received for at least 155 +/- 50 milliseconds. This circuit will go OFF if the received data carrier falls below the receiver threshold for more than 17 +/- 7 milliseconds. During the time that [DCD](CF) is OFF, the received data circuit [RxD](BB) is clamped to the marking state.
9 ASSERTED POSITIVE (+) VOLTAGE
10 ASSERTED NEGATIVE (-) VOLTAGE
Pin 9 provides +12 volts through a 1KOhm resistor for testing purposes and for asserting signals at the other circuits of the interface. Pin 10 provides -12 volts through a 1KOhm resistor for testing purposes and for asserting signals at the other circuits of the interface. These pins should not be used by other equipment nor should any extraneous voltages be connected to these pins.
12 SPEED INDICATION, SI, CI, 122
This circuit will be in the ON state when a local DCE is in the high-speed mode (e.g. 1200 baud for 212A compatible modems). At all other times this circuit is in the OFF state. Regarding 212A compatible modems, this circuit’s state does not necessarily correspond with the state of a speed select switch, particularly with respect to an answering modem which automatically adapts to the speed of the originating modem each time the data mode is entered (ignoring its own speed selector).
14 SECONDARY TRANSMIT DATA, STxD, SBA, 118
This circuit is an auxiliary output channel, presently uncommitted, that will be used by future XXXXXXX options. This circuit is connected only at XXXXXXX’s HOST port.
16 SECONDARY RECEIVE DATA, SRxD, SBB, 119
This circuit is an auxiliary input channel, presently uncommitted, that will be used by future XXXXXXX options. This circuit is connected only at XXXXXXX’s HOST port.
20 DATA TERMINAL READY, DTR, CD, 108.2
This circuit provides a means for DTE to control the connection of a local DCE onto the communication channel. The ON condition of this circuit is necessary to maintain the connection of a DCE to the communication channel. The ON condition must also be present before a local DCE will enter the data mode either manually or automatically. If this circuit is turned OFF for more than 50 milliseconds during a data call, the local DCE will enter an irreversible disconnect sequence. For automatic answering applications, the presence of an OFF condition on this circuit does not inhibit the operation of the Ring Indicator [RI](CE) circuit on pin 22. This circuit must be turned ON in order for a local DCE to automatically answer an incoming call.
22 RING INDICATOR, RI, CE, 125
The ON condition of this circuit normally indicates that a ringing signal is being received by a local DCE. The ON condition is approximately coincident with the ON segment of the ringing cycle. DCEs with answer mode indication options installed will also maintain the ON condition of this circuit after the incoming call has been answered and will remain ON as long as there is an off-hook condition of the telephone line; when the call is terminated and the telephone line is on-hook, this circuit will turn OFF.
23 SPEED SELECT, SS, CH, 111
This circuit is used on an optional basis to establish the speed mode of a data call at the originating DCE. When the circuit is implemented, an ON voltage from DTE will cause the data call to be established in the high speed mode (e.g. 1200 baud for a 212A compatible modem). An OFF condition will cause the connection to be established in the low speed mode.