In telecommunications, RS232, Recommended Standard 232 is a standard introduced in 1960 for serial communication transmission of data. It formally defines the. Serial Port and Modem Cables. The serial port on your computer can be used in one of two ways. To connect a computer, terminal, or other Data Terminal Equipment DTE. Data Communications Equipment DCE. To connect one DTE directly to another, for example a PC to another. Each way requires a different kind of cable. Connections between a PC and a. This chapter covers the terms, technology, and protocols used in serial connections, introducing the HighLevel Data Link Control HDLC and PointtoPoint Protocol. To complicate matters. While the DCE. port is almost always a female DB2. PC port is always supposed to. PC port can take many shapes DB2. DB9, Din. 8, RJ4. CABL230_U01-2.jpg' alt='Standard Serial Cable Connections' title='Standard Serial Cable Connections' />USB, etc thus many combinations are possible. Connecting a serial port to an external modem is usually straightforward. DB2. 5 and. for every kind of computer that has a serial port, no matter what the. Connecting two computers together, however, can be more challenging. The Electronic Industries Association EIA RS 2. The standard describes a number of electrical signals, each carried. DTE and DCE. Although RS 2. DB2. 5. a 2. 5 pin connector, in which usually no more than 1. DB 9 a 9 pin connector, in which usually all 9 wires are used. The table. shows the circuits used in computermodem connections. Frame Ground. Electrical safety. Transmitted Data. Sims 2 Hood. Data from computer. Data to computer. Request To Send 2To DCE. Hardware Flow Control. Hardware Flow Control. DCE on and in data mode. Voltage measurement reference. Carrier Detect 3To DTE. Modems are communicating. Data Terminal Ready. DTE on and in data mode. Phone is ringing. V. 2. 4 is the European standard equivalent RS 2. RTS is also known as Ready to Receive RTR. CD is also known as Data Carrier Detect DCD or Received Line Signal. Indicator RLSI. The TD and RD wires carry the data, the Signal Ground wire allows the. Figure II 5 from Using C Kermit shows. DB2. 5, DB9, Mini Din. As you can see, the two genders are mirror. Click diagram to enlarge. The serial port of a computer or terminal is designed and intended to be. The device driver andor controller for the port. Transmitted Data TD. Carries data from the computer to the modem. Note that the modem. Transmit Data pin. Received Data RD. Carries data from the modem to the computer. Note that the modem. Received Data pin. Request to Send RTS. Today this signal is generally used to indicate that the computer is. Clear to Send CTS. Indicates that the modem is ready to receive data from the computer. RTS and CTS acting together provide an instantaneous, effective, and. RTSCTS or hardware flow control. Without this, data is likely to be lost or corrupted due to differing speeds. Data Set Ready DSR. Indicates that the modem is turned on and in data mode. Carrier Detect CD. Indicates that the modem has a telephone connection to another modem. When the CD signal goes from. Off to On, it means the other modem has just answered the phone. When it. goes from On to Off, it means the phone connection was just hung up. Data Terminal Ready DTR. Indicates that the computer is on, in data mode, and is paying. When the DTR signal. On to Off, and the. Ring Indicator RI. Indicates that the phone to which modem is connected is ringing and a. A modem cable is simple and the various signals do their jobs in a. The cable must convey all the signals listed in the. Signal Ground and Frame Ground for electrical. Most computers have a male serial port connector, either DB2. DB9. If it. is a DB2. Wires for pins 9 1. If the computer has a DB 9 connector, then the wires connect the. Computer and terminal serial ports are supposed to be male, so the computer. In case the serial port has a. Gender Changer. When two computers or other DTEs are to be connected directly, each one. DCE. The trickery is. The result is a. null modem cable a cable that presents itself to each computer. There are several classes. Minimum 3 or 4 Wire Cable. In this cable, RD and TD are cross connected and Signal Ground goes. Frame Ground. Such a cable provides. It might. present problems for some software or device drivers that require a serial. CD, DSR, andor CTS signals before allowing. This cable Model A in Figure II 6 also has only three functional wires. DTR to CD, RTS to CTS, and so on. This gets past. difficulties with software and device drivers, but in fact provides no. True Null Modem Cable. A fully functional Null Modem cable Model B in the figure allows for. In this cable, complementary circuits are crossed over. RD and TD, RTS and. CTS, DTR and CD, etc, so for example the software on one end of the. DTR signal and the. CD signal go down, even though there. The figure shows the connections for DB2. DB9. By the way, although it was common in days gone by for us to build our own. This is a double ended connector that already has all of the Model B. They come in male male, male female, and. Attach a regular modem cable to each computer and. The modem eliminator that you want has a connector of the appropriate shape. DB9 or DB2. 5 and gender male or female on each end, and is. NOT synchronous. A synchronous null modem is something. For greater detail, see the Kermit books. Serial Port and Modem Cables. The Kermit Project. Columbia University.