PC Hardware A Beginner’s Guide

Chapter 19: Ports and Connectors^489

3. The DTE signals over its Request to Send (RTS) line requesting the DCE to send
   any data it has.


4. The DCE replies with a signal over the Clear To Send (CTS) wire to alert the
   DTE that it ready to send data.


5. The data flows from the DCE over its Transmit line, which is the DTE’s Receive
   line, one bit at a time and is placed in a receiving buffer in the main memory of the
   PC (DTE).


6. If the data comes in faster than the PC can process it or moves it to another
   location in memory, the receiver turns off the RTS, which the sender detects,
   and stops transmitting data until the receiver has been able to process some of
   the data in the buffer. When it has room in the buffer again, the receiver turns
   on the RTS and the sender resumes transmitting data. If the sender needs to
   halt the transmission for any reason, the CTS signal is turned off and back on
   when it wishes to resume the transmission.

Cabling the Connection

The cable used to connect a PC to a modem is called a serial cable,a modem cable, or a

straight-through cable.In this cable, all the pins are connected one-to-one without any

twists, crosses, or other fancy arrangements (that is, unless you need to use a 9- to 25-pin

converter should the modem cable come with a 25-pin connector and the PC have a 9-pin

serial port).

Although few serial port questions are on the A+ Core Hardware exam, there are some.

Expect at least one with “null modem cable” as its answer. On occasion, two PCs are con-

nected in a DTE-to-DTE arrangement. When this happens, the cable’s pinouts is changed to

simulate the action of the modem by cross-connecting a number of the pins and creating

what is called a null modem, or modem eliminator,cable. Both the modem cable and the

null modem cable are generic, and you can purchase them at any electronics store.

Configuring a Serial Port

Nearly all PCs include at least one serial port, which is designated as COM1. Additional

ports are designated by the BIOS as COM2, COM3, and COM4. Multiple serial ports can

be added to the PC individually or in sets of two or four with multiport expansion cards.

Individualserialportsrequireindividualsystemresourceassignments,whichmaycause

conflicts with already installed devices. A multiport serial card typically shares a single

IRQ (interrupt request) among the ports with an onboard processor handling the traffic

management duties. So, if a PC requires multiple serial ports, it may be more efficient for

thesystemtoinstallamultiportcard(orconsiderUSB—moreonthislaterinthechapter).

Configuring the serial port on a PC involves setting its system resource assignments.

Luckily, most PCs use the default assignments for the COM ports. Table 19-2 lists the default