PIN NAME EIA CCITT DTE DCE FUNCTION 1 CG AA 101 --- Chassis Ground 2 TD BA 103 --> Transmit Data 3 RD BB 104 <-- <-- Pos. Test Voltage 10* <-- Neg. Test Voltage 11 (usually not used) 12+ SCDC SCF 122 <-- Sec. Data Car. Detect 13+ SCTS SCB 121 <-- Sec. Clear To Send 14+ STD SBA 118 --> Sec. Transmit Data 15# TC DB 114 <-- Transmit Clock 16+ SRD SBB 119 <-- Sec. Receive Data 17# RC DD 115 <-- Receive Clock 18 (not usally used) 19+ SRTS SCA 120 --> Sec. Request To Send 20 DTR CD 108.2 --> Data Terminal Ready 21* SQ CG 110 <-- Signal Quality 22 RI CE 125 <-- Ring Indicator 23* CH 111 --> Data Rate Selector CI 112 <-- Data Rate Selector 24* XTC DA 113 --> Ext. Transmit Clock 25* --> Busy In the above, the character following the pin number means: * rarely used + used only if secondary channel implemented # used only on synchronous interfaces also, the direction of the arrow indicates which end (DTE or DCE) originates each signal, except for the ground lines (---). For example, circuit 2 (TD) is originated by the DTE, and received by the DCE. Certain of the above circuits (11, 14, 16, and 18) are used only by (or in a different way by) Bell 208A modems. DEFINITION OF THE MOST COMMON CIRCUITS 1 CG Chassis Ground This circuit (also called Frame Ground) is a mechanism to insure that the chassis of the two devices are at the same potential, to prevent electrical shock to the operator. Note that this circuit is not used as the reference for any of the other voltages. This circuit is optional. If it is used, care should be taken to not set up ground loops. 2 TD Transmit Data This circuit is the path whereby serial data is sent from the DTE to the DCE. This circuit must be present if data is to travel in that direction at any time. 3 RD Receive Data This circuit is the path whereby serial data is sent from the DCE to the DTE. This circuit must be present if data is to travel in that direction at any time. 4 RTS Request To Send This circuit is the signal that indicates that the DTE wishes to send data to the DCE (note that no such line is available for the opposite direction, hence the DTE must always be ready to accept data). In normal operation, the RTS line will be OFF (logic 1 / MARK). Once the DTE has data to send, and has determined that the channel is not busy, it will set RTS to ON (logic 0 / SPACE), and await an ON condition on CTS from the DCE, at which time it may then begin sending. Once the DTE is through sending, it will reset RTS to OFF (logic 1 / MARK). On a full-duplex or simplex channel, this signal may be set to ON once at initialization and left in that state. Note that some DCEs must have an incoming RTS in order to transmit (although this is not strictly according to the standard). In this case, this signal must either be brought across from the DTE, or provided by a wraparound (e.g. from DSR) locally at the DCE end of the cable. 5 CTS Clear To Send This circuit is the signal that indicates that the DCE is ready to accept data from the DTE. In normal operation, the CTS line will be in the OFF state. When the DTE asserts RTS, the DCE will do whatever is necessary to allow data to be sent (e.g. a modem would raise carrier, and wait until it stabilized). At this time, the DCE would set CTS to the ON state, which would then allow the DTE to send data. When the RTS from the DTE returns to the OFF state, the DCE releases the channel (e.g. a modem would drop carrier), and then set CTS back to the OFF state. Note that a typical DTE must have an incoming CTS before it can transmit. This signal must either be brought over from the DCE, or provided by a wraparound (e.g. from DTR) locally at the DTE end of the cable. 6 DSR Data Set Ready This circuit is the signal that informs the DTE that the DCE is alive and well. It is normally set to the ON state by the DCE upon power-up and left there. Note that a typical DTE must have an incoming DSR in order to function normally. This line must either be brought over from the DCE, or provided by a wraparound (e.g. from DTR) locally at the DTE end of the cable. On the DCE end of the interface, this signal is almost always present, and may be wrapped back around (to DTR and/or RTS) to satisfy required signals whose normal function is not required. 7 SG Signal Ground This circuit is the ground to which all other voltages are relative. It must be present in any RS-232 interface. 8 DCD Data Carrier Detect This circuit is the signal whereby the DCE informs the DTE that it has an incoming carrier. It may be used by the DTE to determine if the channel is idle, so that the DTE can request it with RTS. Note that some DTEs must have an incoming DCD before they will operate. In this case, this signal must either be brought over from the DCE, or provided locally by a wraparound (e.g. from DTR) locally at the DTE end of the cable. 15 TC Transmit Clock This circuit provides the clock for the transmitter section of a synchronous DTE. It may or may not be running at the same rate as the receiver clock. This circuit must be present on synchronous interfaces. 17 RC Receiver Clock This circuit provides the clock for the receiver section of a synchronous DTE. It may of may not be running at the same rate as the transmitter clock. Note that both TC and RC are sourced by the DCE. This circuit must be present on synchronous interfaces. 20 DTR Data Terminal Ready This circuit provides the signal that informs the DCE that the DTE is alive and well. It is normally set to the ON state by the DTE at power-up and left there. Note that a typical DCE must have an incoming DTR before it will function normally. This signal must either be brought over from the DTE, or provided by a wraparound (e.g. from DSR) locally at the DCE end of the cable. On the DTE side of the interface, this signal is almost always present, and may be wrapped back around to other circuits (e.g. DSR, CTS and/or DCD) to satisfy required hand-shaking signals if their normal function is not required. All of the above applies to interfacing a DTE device to a DCE device. In order to interface two DTE devices, it is usually sufficient to provide a 'flipped' cable, in which the pairs (TD, RD), (RTS,CTS) and (DTR,DSR) have been flipped. Hence, the TD of one DTE is connected to the RD of the other DTE, and vica versa. It may be necessary to wrap various of the hand-shaking lines back around from the DTR on each end in order to have both ends work. In a similar manner, two DCE devices can be interfaced to each other. pin 01 - pin 01 pin 02 - pin 03 pin 03 - pin 02 pin 04 - pin 05 pin 05 - pin 04 pin 06 - pin 20 pin 20 - pin 06 ok... the above 180 lines is mostly informative.. only the last little section where i typed pin xx - pin xx is needed.. you only need to connect the give pins, and just make sure they go where they belong, and the two computers should talk to each other... y be wrapped back around (to DTR and/or RTS) to satisfy required signals whose normal function is not required. 7 SG Signal Ground