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Understanding Serial Devices and Other Ports



-Have you ever gone to the computer store and wanted to by a component but was unsure which type connection was right for you? This is when we need to look at serial and parallel devices and decide which one is in your future. There is more to these than you think and it's a good idea to get familiar with the workings and limitations of both ports. Of course as I always do, I will add in what seems to be important and what doesn't along with some goofy facts about these little creatures.

Serial Ports

-Serial data is transmitted one bit at a time but this doesn't mean it's a bad thing. The serial data can be shot across a cable with a little power behind it unlike the parallel device. A good way to think about this is by using a single ASCII* character. In order to transmit one character over a serial port you will need 8 separate transmissions through the cable. In a parallel you only need one transmission to achieve this. Another way of thinking is the Serial Port is a one-lane road while the Parallel is a major interstate. We can pass on a larger volume with the parallel.

-Now when it comes to cables and connections I have received several questions about length. A Serial cable can send information up to 50 feet before we get into some serious errors. The reason behind this is simply the blast of the information behind it. In the same respect our Parallel cable can only send information but 15 feet before it becomes error prone.

-Most serial devices that we come into contact with are external such as modems, camera's, and many other toys or necessities we men need. Another name for the serial port is also COM port which when dealing with modems you get more than your fair share of the COM port. To confuse the situation again they can also be called RS-232 ports. This stands for IEEE (Institute Of Electrical and Electronic Engineers Inc/ reference standard number two hundred and thirty two) try swallowing all of that one.

-On the back of your computer you will commonly find two serial ports or more. This can be added in via an expansion card or already integrated into the motherboard. Most new systems will find it integrated into the motherboard (ATX) or attached with a cable with the (AT) motherboard. Of course its almost better to have it separate should something go bad but there is commonly more and more integration on our systems. The serial port on your computer is either a 9 or 25 pin connector and is male. When shopping for connectors you will commonly see this as a DB-25 or DB-9 which means the same as above. The DB means Data Bus and the numbers after it are simply the number of pins in the connector. When you get into Parallel ports you will also find them with the DB notation on it which mean's the same thing. Now to get back to the the DB-9 and 25 you will commonly see just the DB-9 in your system if its newer and the old style DB-25 in older PC's. The older DB-25 is really just wasted pins.

-Now lets turn the serial port over and figure out what the individual pins do and their assignments. As with any device we hook into the connector itself will have a #1 pin and so does the serial device. Look below for a chart on assignments.

Pin Number Designation Action
1 Carrier Detection (CD) Indicates that there is a connection established with port.
2 Receive Data (RC) This pin receive all incoming data
3 Transmits Data (TD) All outgoing data is sent through this pin
4 Data Terminal Ready (DTR) Host is ready to communicate this can be your PC telling the modem it is ready to talk
5 Signal Ground Not used on the PC system
6 Data Set Ready (DSR) The connected device is ready to communicate
7 Request to Send (RTS) Host device wants to communicate
8 Clear Ro Send (CTS) Connected Device is ready for communication
9 Ring Indicator (RI) Notification Of another device calling

-Now you know more than you wanted to know about the serial port and its pins. How about getting more into its typical operation with say a serial device such as a modem? This is actually kind of neat and keys you into what's happening when you switch on that external modem.

1) Your PC will turn on the DTE (Data Terminal Equipment) and basically indicates its ready for action.

2) Now the DCE (Data Communications Equipment) acknowledges the message from step 1 and turns on the DSR (Data Set Ready).

3) Now the DTE turns on its RTS (Request To Send) signal and lets the DCE know its ready to except data.

4) Now the DCE (Data Communications Equipment) comes back into the pictures again and accepts the request with a CTS (Clear To Send) reply.

5) Now data is flowing one bit at a time.

-This is to say the least a little confusing but a basic idea of how the serial device works. Now how is all this controlled? Get ready for more acronyms.

-In serial devices we need a way to control data and this is done with UART (Universal Asynchronous Receiver/Transmitter). This is a chip that is found on the motherboard and its port or on the expansion card on the motherboard. You will find that the controller controls the pins and there signals, establishes the communication protocols, converts bits from other types of data streams, and converts the data into parallel data for the internal bus on the motherboard.


-Now how about those pesky Parallel ports? We messed around enough with the Serial so we should touch on the Parallel a little since we all like to print out pictures and use our web cams from time to time.

-As I was going over in the beginning the Parallel port can shoot out 8 bits of data in a single transmission which is pretty darn fast. This may let you in on why the internal bus of our motherboards sends data in this format. Of course it's a bigger highway than 8 bits. The main idea behind the parallel port was to use it for printers rather than all the devices that use it today. I myself have about four different components using the parallel port such as a printer, camera, scanner, and a few other oddball items. The main reason that the parallel port is so popular is the data width and that it is bi-directional. This can come in real handy when you want to move data in a quick fashion. The port itself isn't near as complicated as the Serial Port and only has a few acronyms to back it up unlike the serial port. When looking at a parallel device make sure and think about the length of the cable itself. You do not want to go beyond 15' when hooking up devices. Anything longer will cause data corruption in your system and we don't want that to happen.

-In 1984 like everything else in the world we needed a standard for the parallel port and its workings. This is were the IEEE came in just like with the serial port and decided on IEEE 1284. This is the standard for all bi-directional devices, parallel, and ECP. Now lets get familiar with some of the stuff that comes along with your port that makes up the IEEE 1284 standard.

-Standard Parallel Port (SPP), this standard allows data to flow only from the computer to the device such as the printer.

-Enhanced Parallel Port (EPP), this allows data to flow in both directions but only one at a time.

-Enhanced Capabilities Port (ECP), allows bi-directional simultaneous communications over specific cables especially one that is IEEE 1284 compliant. This is the better one to say the least. Make sure when shopping for cables that they support all of the above and not just an EPP cable. Look for ECP when you can.




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