Parallel ports carry many times more data than serial ports in the same amount of time, which is why they are used for all the connections where speed is an issue, such as for monitors and printers. Serial and parallel devices, cables, ports, and communications are all based on the same basic premises: Serial data is transmitted one bit at a time, parallel data is transmitted eight or more bits at a time.

Serial Devices

     Because a serial transmission moves less data than a parallel transmission through the wire at one time, it has more power and can travel longer distances. A serial cable can be up to 50 feet in length (compared to the 15 feet limit of parallel cable). Most serial devices are external devices that plug onto the PC via a serial port. Serial ports are also called COM ports, or RS-232 ports. RS-232 stands for IEEE standard number 232.

     Serial ports are usually added to the PC via an expansion board, although some of the newer computers now have one or two on the motherboard. Serial ports are easy to recognize becuase they are always either a 9 or 25 pin male connector. In contrast, a parallel port is always a female connection regardless of how many pins it has. Serial connectors are called DB-9 and DB-25. The DB stands for data bus. All serial connectors are DB type "D" (D shaped connectors), but not all DB connectors are serial.

     A PC only uses nine pins in a serial connection, which is why many PCs use the DB-9 connector in place of the DB-25 with its too many and wasted pins. You will find the DB-25 plug on many older PCs, multipurpose adapter cards, and some modems, although it is becoming rare. When serial ports are added to the PC via an expansion card, commonly COM1 uses a DB-9 connector and COM2 uses a DB-25 connector.

• Data bits: Indicates the number of bits used in the character coding scheme, or data word. Some systems use 7 bits and others use 8 bits, with no other choices available.
  
  
• Flow control (handshaking): The embodiment of the protocol used to control the dialog of two serial devices. In general, flow control is sued to manage the data flow by sending a character or signal to stop it.
  
  
• Parity: A group of five choices: even, odd, space, mark, or none. Parity is a way of checking whether the correct number of bits was send and received. Both devices in a serial communication must be set the same.
  
  
• RTS/CTS: request-to-send and clear-to-send. Sends signals to specific pins to stop and start the data flow. The CTS signal indicates that a device is ready to accept data, and the RTS signal indicates when a device is ready to send data.
  
  
• Stop bits: Used in certain serail communications to indicate the begining and end of data words.
  
  
• XON/XOFF: One of the most common forms of flow control, it send control characters to stop the flow of data (XOFF) and restart it again (XON). This is the software method of flow control.
  
  

DTE to DCE

     After a serial device is connected to the PC, each device is designated as either the Data Terminal Equipment (DTE) or the Data Communications Equipment (DCE). When you connect a modem to a PC, it becomes the DCE and the PC is the DTE. These designations are used to determine which device initiates and controls the conversation between the two devices.

Serial Connection Pin Assignments
Pin Number	Designation	Activity
1	Carrier Detect (CD)	Indicates a connection is established.
2	Receive Data (RD)	All incoming data is received on this pin.
3	Transmit Data (TD)	All outgoing data is sent on this pin.
4	Data Terminal Ready (DTR)	The host device (PC) is ready to communicate.
5	Signal Ground	Not used on PC systems.
6	Data Set Ready (DSR)	The connected device (modem) is able to communicate.
7	Request to Send (RTS)	Host device wants to communicate.
8	Clear to Send (CTS)	Connected device is ready to communicate.
9	Ring Indicator (RI)	The telephone is ringing.

     The serial connector's pins are used to send signals to create what amounts to a conversation (the handshaking) between two serial devices. Handshaking accomplishes the hardware flow control between the PC and the modem, as follows:

1. The Data Terminal Equipment (DTE), or PC, turns on the Data Terminal Ready (DTR) signal, indicating it is ready.
   
2. The Data Communications Equipment (DCE) acknowledges this message by turning on the Data Set Ready (DSR) which indicates it is ready.
   
3. The DTE turns on its Request to Send (RTS) signal to let the DCE know it is ready to receive data.
   
4. The DCE acknowledges this request with a Clear to Send (CTS) that means the data is now coming.
   
5. The data flows one bit at a time until one of the devices needs to stop it. This stopping is indicated by either the RTS or CTS being turned off. The flow starts again when the applicable indicator is turned back on.
   

     The cable used to connect a PC to a serial device is called a serial cable, or a straight-through cable. All the pins in this cable are connected one to one without any twists, crosses, etc. On occasion, two PCs are connected in a DTE-to-DTE arrangement. When this happens, the cable's pinout 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.

The UART

     Serial devices are controlled by a Universal Asynchronous Receiver / Transmitter, or UART for short. This specialized integrated circuit is found either on the device adapter card or on the motherboard. The UART chip controls all actions and functions of the serial port, including:

• Controlling all the connector's pins and their associated signals.
  
• Establishing the communication protocol.
  
• Converting the parallel data bits of the data bus into a serial bit stream for transmission.
  
• Converting the received serial bit stream into parallel data for transmission over the PC's internal data bus.
  

Troubleshooting a Serial Port

     If you are having a serial port problem, it is most likely a system resource conflict. System resource conflicts include problems such as serial device that fails intermittently or doesn't work at all, another device that stops working when the serial device is installed, or the PC locking up during the boot sequence. To determine the source of the problem, check the following:

• Inspect the port for bent pins. Certain pins absolutely must be straight in order for the device to work properly.
  
  
• Ensure that the cable is the appropriate cable for the device. Some serial devices can't use a straight-through or null modem cable.
  
  
• Check the Windows Device Manager for system resource conflicts. An IRQ conflict is the most common error in this area.
  
  
• Be sure that the serial cable is not more than 50 feet long. Beyond this distance, you lose data integrity, which shows up any number of ways.
  
  

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Parallel Devices

     Parallel data is transmitted eight bits at a time. The standard covering parallel port technologies is IEEE 1284 and is used primarily for printers. IEEE 1284 incorporates the two parallel port standards that had been used up to the point when it was created, the standards included in IEEE 1284 are as follows:

• Standard Parallel Port (SPP) allows data to travel one-way only - from the computer to the printer.
  
  
• Enhanced Parallel Port (EPP) allows data to flow in both directions, but only in one way at a time. This lets the printer communicate to the processor or adapter to signal it is out of paper, its cover is open, and so on.
  
  
• Enhanced Capabilities Port (ECP) allows bidirectional simultaneous communications over a special cable, one that is IEEE 1284 compliant. Many bidirectional cables exist, but they may be EPP cables, which do not support ECP communications.
  
  

     IEEE 1284 establishes the standard for bidirectional communications on the parallel port, and the ECP protocol allows for full-duplex parallel communications.