|
I've learned a lot about the electronic hardware "inside the serial port", and I take that knowledge for granted, expecting that many of you also have some similar knowledge base. I finally realized that many of you may NOT have this knowledge base, at least in very basic terms. This page is intended to help fill that knowledge gap. |
There are only a couple of basic parts that make up the
serial interface, whether you call it TTYS0, COM 1, the Modem Port, or
whatever label your device has.
|
|
|
| The data moving around in your computer is, generally
speaking, moving on a parallel bus. That is, there are 8 data leads in
parallel, and the computer moves data around a byte at a time. (We're
going to ignore things like USB, Firewire, and SATA drive interfaces in
this paper.) When a hardware maker wants to add a serial port, they use
some form of Async Receiver/Transmitter (ART) chip, which
will connect to the computers parallel bus and hardware interrupt
architecture, and the chip is responsible for being the gatekeeper
between the computer and the serial world. The first chips were called the Universal Async Receiver/Transmitter (UART), and you can still find these today on some add-in serial cards for PCs. Later, the Dual UART (DUART) was developed, putting a pair of UARTs on a single chip, to save space. As terminal servers became popular (early 1990's), we saw QuadART and even OctART chips, though they are not common for hobbyist use. The common abbreviation for this class of hardware is the *ART, where the quantity is defined in the nearby text. |
|
When you set the serial port speed, parity, data bits (the data word),
and stop bit settings, you are talking to the *ART chip associated with
the physical port you are trying to use. The ART is responsible for forming the serial characters, including the start and stop bits, and for calculating the parity bits for sending data. It is also responsible for receiving the incomin data, calculating the parity, for buffering some small number of bytes as needed while it signals the computer bus that the ART has some received data that needs to be examined. |
| Note: the serial output and input signals to the *ART chips are at the same levels as that of the computer. This may be TTL (+5V and ground), or even less in portable equipment and laptops (+3,3V and ground). The ART only makes the data words, and it is up to the I/O interfaces to convert the computer levels to the desired I/O levels. |
|
In the early days, serial communications between devices used the computer bus signalling levels. But, due to noise on the lines, the distances were limited. As a result, various "line driver" interfaces were tried. These interfaces took data (the "ones and zeros") from one physical layer, and translated them to another physical layer, with no interpretation or changes to the data, in efforts to extend the useful length of the data transmission. RS-232 is one of many serial communication protocols. The *ART chip makes the chain of zeros and ones, and the interface chips translate the voltages between the computer bus and the RS-232 serial port. These chips have no intelligence, they are basically voltage level converters. |
| Serial communication meant needing fewer wires than extending a parallel bus, and copper was expensive in those days, so if you could save on the copper wires, you could afford to spend some of the money on electronics.) |
| The computer chips used various Digital Logic designs, with TTL (transistor-transistor logic) becoming the most common. This meant the data wa expressed as +5v for a "high" (or "1"), and 0v (equal to signal ground) for the "low" (or "0") signal state. |
| Serial interface chips needed bridge two voltage realms
while isolating them. In the case of RS-232, the "high" state was
expressed as being between -3v to -12v, while the "low" state was
between +3v to +12v. The early driver chips were seperate from the receiver chips, and each of them had four connections for power: ground, +5v, +12v, -12v. (The early memory also required negative voltages, and the disk drive motors required +12v, so most power supplies already had these voltages available, which is why they were tried.) When an interface chip failed (or a technicians screwdriver shorted the wrong pins), an ART chip could be introduced to the ugly side of a power supply, and the smoke in the ART chip usually escaped, making the ART useless. In later years, the ART chips were often in sockets, rather than being soldered to the computer board, beacuse this was a common point of failure. |
| The DALLAS Semiconductor company developed a family of interface chips (starting with the MAX-232) that used only a single supply voltage to the driver/receiver set on a single chip, and there was a "charge pump" built into the chip. This was a marriage of TTL and Linear technologies on the same piece of silicon, and was a revolution at that time. Using a few external capacitors, these chips created the +/- 12v drive voltages needed using just the +5v and ground of the computer. |
| The Serial BREAK signal is used to reset modems, or clear a transmission channel. It is defined as asserting the serial signal for longer than two complete serial data characters, including the time for the start and stop bits. (Early implementations of break sent the signal for 500 milliseconds (ms), or just 250 ms. However, as serial speeds have become faster, you can see that the length of time needed for a Serial BREAK signal becomes shorter. |
| Some devices send a Serial BREAK when they are powered
off. This is usually because they use the older serial driver chips,
using many independant voltages. When the power is turned off, the
computer (+5v) voltage usually falls faster than the drive motor (+12v)
voltage. This results in the serial driver chips still having I/O power
when the logic voltage fails, so the driver sees a "logic low" signal
on the input, and so the driver aserts it's output accordingly. Since
there are no start or stop bits, the attached host believes it has
received a valid Serial BREAK. Using the "single supply" interface driver/receiver chips eliminates this trouble, because the I/O voltage goes away as soon as the logic voltage goes away. |
| I hope this page has been a useful foundation from some of my readers. |