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A Modem Communications Primer - Serial Ports: Communicating One Bit at a Time

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3/8/2011 10:45:48 PM
The link between your computer and your modem is the serial port (also known as a COM port or RS-232 port). For an external modem, this link usually comes in the form of a serial cable that runs from the port to an interface in the back of the modem. The exception is the pocket modem, which usually plugs into the serial port directly. For internal and PC Card modems, the serial port is built right into the modem’s circuitry.

They’re called serial ports because they transmit and receive data one bit at a time, in a series. (This is opposed to working with data in parallel, in which multiple bits are transmitted simultaneously.) As such, serial ports can be used by many kinds of devices that require two-way communication, such as mice, infrared adapters, bar code scanners, and, of course, modems.

Serial Port Pin Configurations

Like most computer interfaces, serial ports send and receive data and signals via wires that correspond to single bits. For a serial port, these wires are metal pins that come in two configurations: 9-pin and 25-pin, as illustrated in Figure 1. From the context of modem communications, there is no essential difference between the 9-pin and 25-pin connectors, other than their layout. Table 1 shows the pin assignments for the 9-pin connector, and Table 2 shows the partial pin assignments for the 25-pin connector (the other pins can be safely ignored).

Figure 1. The 9-pin and 25-pin serial port connectors.

Table 1. Pin Assignments for a 9-Pin Serial Port Connector
Pin NumberSignal
1Carrier Detect (CD)
2Receive Data (RD)
3Transmit Data (TD)
4Data Terminal Ready (DTR)
5Signal Ground
6Data Set Ready (DSR)
7Request To Send (RTS)
8Clear To Send (CTS)
9Ring Indicator

Table 2. Partial Pin Assignments for a 25-Pin Serial Port Connector
Pin NumberSignal
2Transmit Data (TD)
3Receive Data (RD)
4Request To Send (RTS)
5Clear To Send (CTS)
6Data Set Ready (DSR)
7Signal Ground
8Carrier Detect (CD)
20Data Terminal Ready (DTR)
22Ring Indicator

The key pins in both layouts are Transmit Data (TD) and Receive Data (RD). The computer uses the TD pin to send the individual bits of outgoing serial data to the modem. For incoming data, the modem uses the RD pin to get the bits into the computer. 

The UART: The Heart of the Serial Port

You might be wondering how the computer’s processor and the serial port can possibly get along with each other. After all, the CPU deals with data in parallel: the eight bits (one byte) that are required to represent a single character of information. I’ve just told you, however, that serial ports are one-bit wonders. How do you reconcile these seemingly incompatible ways of looking at data?

The answer is a special chip that resides inside every serial port (or sometimes on the computer’s motherboard): the Universal Asynchronous Receiver/TransmitterUART). (For an internal or PC Card modem, the UART chip sits on the card itself.) It’s the UART’s job, among other things, to take the computer’s native parallel data and convert it into a series of bits that can be spit out of the serial port’s TD line. On the other end, individual bits streaming into the destination serial port’s RD line are reassembled by the UART into the parallel format that the processor prefers. (

It’s clear, then, that the role of the UART in data communications is crucial. In fact, the UART is often the source of transmission bottlenecks that can hold up the entire process. To see why, consider what happens when serial data arrives via the modem. The UART assembles the incoming bits until it has a full byte, and it stores this byte in a special memory buffer. It then notifies the CPU—by generating an interrupt request—that data is waiting. Under ideal conditions, the CPU grabs the data from the buffer immediately, the UART processes the next byte, and the cycle repeats. However, what if the CPU is busy with some other task when it receives the interrupt request? The UART continues processing the incoming bits, and if the processor can’t get to the buffered data in time, the UART simply overwrites the existing buffer with the new data. This means, at best, that the lost byte must be retransmitted, and the overall performance of the download suffers as a result. (At worst, you might lose the character altogether!)

This isn’t usually a problem at relatively slow data transfer rates (for example, up to 9,600bps), but it can cause all kinds of problems with modern modems running at up to 56,600bps. To prevent these overruns, you need a UART that can keep up with the deluge. Here’s a summary of the various UART types and their suitability for fast data transfer rates:

8250This was the chip used in the original IBM PC XT. Its design calls for a one-byte data buffer, so it isn’t suitable for high-speed transfers.
16450This was the chip used in the IBM PC AT and compatible machines. Although it sported some improvements over 8250 (essentially the capability of working with computers that have higher internal clock speeds), it still used the one-byte data buffer, so it too is limited to 9,600bps.
16550This chip represented a huge improvement over its predecessors. The major innovation was a 16-byte FIFO (first in, first out) buffer that enabled the UART to handle high-speed data transfers, and reduced retransmissions and dropped characters. Also, the 16550 had a variable interrupt trigger that the user could configure to send an interrupt to the CPU when the buffer reached a certain number of bytes. (You’ll see later that Windows XP lets you configure this trigger.) The 16550, however, had a defective FIFO buffer that often caused data loss.
16550AThis is a replacement chip that fixes the bugs in the 16550 but is otherwise identical. The 16550A (or the updated 16550AF, 16550AN, or 16550AFN) is the chip of choice for modems that support data transfer rates of 14,400bps and up. Almost all modern systems have this type of UART.
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