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Trouble shooting data transfer problems.

When data transmission comes out garbled, you can either kick the computer or troubleshoot the problem. The technique for the former approach needs no explanation. Troubleshooting is trickier but ultimately more satisfying.

I will outline some key problem-solving methods here. A few ideas will be simplified in the interest of clearer overall comprehension.

First, let's consider what one needs for computer communication. Your microcomputer should have a keyboard, monitor, adequate random access memory, and preferably two disk drives--one for the program that coordinates the communication and the other for storage of the information to be sent or received.

In addition, you need: a modem, the device that sends information over the telephone; a cable to connect the modem to your computer or a different kind of cable to connect your computer with another computer if you are not using a modem; and a software program in each computer that coordinates the sending and receiving of information.

One of the most common problems in communication is use of the wrong cable. There are several ways to learn which one to use.

Of course, if you are buying equipment, it may be with the understanding that the seller will take care of the connecting cable. But quite often, one already has the equipment--an extra printer, say. The trouble is it won't work when it's hooked up, which may or may not mean a cable problem.

Bringing in a consultant to figure out what's wrong may cost more than the printer. Before throwing it away or sending it out for repairs, try to determine if indeed the correct type of cable is in use, if it is properly connected, and if the commands are being sent over the correct wire (a cable has many wires).

Start by checking the type of communication port on the computer and the printer. The port is an electrical outlet. The serial RS232 port is the most common. Serial ports send one bit after another in serial fashion, as opposed to a parallel communication method, which sends many bits concurrently.

RS232, IEEE488, and other kinds of ports each require a unique cable. Furthermore, the wire used to send data in the RS232 cable must be connected to the receive data channel of the receiving device. The problem that usually crops up in communicating with RS232 cables is that the send data wire is connected to the send data channel and the receive data wire to the receive data channel, instead of send to receive and vice versa.

Many mail-order companies sell troubleshooting devices, small boxes through which you can try out reconnections systematically until a hookup works. Prices range from about $100 to $150.

As an alternative, I recommend something called Smartcable (IQ Technologies, 11811 N.E. First St., Bellevue, Wash. 98005), which costs $80. (Incidentally, prices mentioned in this column are subject to change and discounts.) A device in the middle of the cable automatically searches for the optimum allocation of wires and makes the connection that will work. Once you know how to connect the printer, you can replace the smartcable by a lower-cost cable and save the device for another troublesome installation.

There are a variety of modems. The most important difference among them is in the baud rate, the speed at which they send information. At one time, most microcomputer owners used 300 bauds. That's 300 bits per second or approximately 300 characters per second.

With recent price declines, however, 1,200-baud modems have gained in popularity. They still cost several hundred dollars more than a 300-baud modem, but the extra speed and added features can save a user time and money.

Although 2,400-baud modems are now available for under $800, most organizations have only 300- or 1,200-baud modems at present. Bear in mind that each side of the telephone needs a modem of the same speed for communication.

Modems can be purchased as stand-alone equipment connected to the computer RS232 port through a cable or as a card that is plugged inside the computer. The first kind may be used with any computer. The second kind of modem is often cheaper, but it is more difficult to remove and switch to another computer. You need to partially disassemble the computer case to get the card out, and it will only work on a compatible computer.

Other features to look for include how many telephone numbers a modem can remember for automatic dialing and whether it will automatically answer a call. Among 1,200-baud modems, the differences are fewer than in the 300-baud category. For the relatively small spread in prices, it is generally better to choose the fullest array of features. Your decision, however, will also hinge on the communication program. Not all such programs work with all modems.

Whether you transmit over the phone or over a cable, communication software will greatly enhance your ability to communicate. In fact, it is often impossible to send and receive data without the proper software.

Communication programs accomplish several purposes. They connect your computer to another computer; determine when to send data and when to stop if the other computer is busy processing the data received; allow you to store the data received in a disk file or send data from a file; send and detect control characters that instruct the computer to perform a function, such as to run another program; and emulate a terminal. Some communication programs are included with the purchase of a modem.

I will comment on a few that should satisfy most communication requirements. But remember that programs are constantly evolving, and the best one for your specific needs may not be mentioned here. For example, if most of your communication involves use of a special data base, such as the National Library of Medicine's Medline, you may want a program optimized for that purpose.

Here are three programs with a broad range of uses:

* PC Talk III, which may be freely copied, is often available at no cost from many computer user groups. It is considered one of the best, and many extensions have been written for it. These, too, are available from user groups.

* Smarterm 125 emulates the VT50, VT100, VT125, and a few other Digital Equipment Corp. terminals. If most of your communication is done with DEC computers and you need a graphics terminal like the VT125, this is the program for you. I used it to communicate with a DEC VAX and a data base system with graphics, and it worked quite well.

There are occasional exceptions to its ability to emulate features of DEC terminals, and you should read up on them before you buy. The most significant one, a minor inconvenience really, is that at present it cannot display text and graphics at the same time as a VT125 can. You must switch between screens. Smarterm 125 is available from Persoft, 2740 Ski Lane, Madison, Wis. 53791, for $295.

* Softerm PC emulates a long list of terminals, including the VT100. It has excellent communication capabilities and may be one of your best choices if you want to emulate one of those terminals or for general communication between computers. Its extensive manual explains both the features and their potential uses. Softerm PC may be purchased from Softronics, 3639 New Getwell, Suite 10, Memphis, Tenn. 38118, for $195.

To insure accurate transfer of information, you need a method to detect and correct transmission mistakes. This is particularly important when there's interference on the phone lines or when you are transferring programs or numbers and a single error may throw everything off. In the transfer of text, on the other hand, a single error may cause only one wrong letter to be printed, which is usually of little significance.

Both microcomputers must use the same error correction protocol to transmit and receive information. One method employed in many microcomputer software packages is called Xmodem. It is part of the three programs described above, but they each also have another, more sophisticated protocol.

Let's turn now to troubleshooting of common problems. In order for a transmission to work, a variety of parameters must be set up. For example, most computers must communicate with each other or with printers at the same baud rate. If you try to send data to a printer at the wrong baud rate, you may see something like XX@<<XXXX.

Parity check is a feature on the IBM PC and many other microcomputers. Most computers send characters or data in groups of 8 bits. To these a 9th is added--the parity bit. Either odd or even parity can be used as a check.

Here's one way it can work: With odd parity, the 0s and 1s in the 8 bits plus the parity bit will always have an uneven total in error-free data transmission. The parity bit makes sure the total is odd. It is 0 if the 8 bits add up to an odd number, and 1 if they add up to an even number.

Failure to match parity at both ends may produce a string intermixing correct characters and strange ones, such as: 'C*)NI#?O.

Word length is another parameter. It is a misleading term since it helps the computer determine where a character ends, usually at 7 or 8 bits. If you use the wrong word length, the computer misunderstands most of the characters and may interpret some as control characters (instructions to perform various functions). You may then see random characters widely spaced out over the screen or printout.

Stop bits mark the end of a character. They work in conjunction with parity and word length. The wrong stop bit produces erratic results similar to the outcome of an incorrect word length.

Keeping these rules in mind reduces the combinations you have to try when troubleshooting a computer whose characteristics are not known. As a general rule in troubleshooting a communication system, first try using no parity, a word length of 8, and 1 stop bit. Vary the word length, stop bits, and baud rate until good transmission is achieved.

To check a printer, try no parity--most printers do not check parity--a word length of 7, and 1 or 2 stop bits. The baud rate is 1,200 or less for older printers, 1,200 to 2,400 for newer printers. Printers have switches that determine their baud rate. Look up the switches in a user manual or try these rates in the following order, which goes from the most to the least common: 600, 1,200, 2,400, 900, 1,500, 1,800, and 300. You may also use these settings to check your cable connection.

Once the information is sent in a reasonably trouble-free fashion, you may want to cut the error rate to a minimum by optimizing the communication parameters: baud rate, parity, word length, and stop bits. Many terminals require specific values. All you have to do is be sure that they are the same at both ends of the communication. Changing from no parity to odd or even will help detect errors in transmission.

Handshaking is a function that tells each computer when to wait before sending information because the other computer or printer is not ready. It is implemented by sending a signal over a wire, usually a different wire from the one that transmits data. A widely utilized means of handshaking is called xon/xoff. Again, the sending and receiving units need to use the same method.

When troubleshooting, you may choose not to use handshaking. This will let you send characters and see if they are properly received without having to wonder whether incomplete transmission is due merely to a mismatch of handshaking procedures. Eventually, some data will be lost as you exceed the capacity of the receiving device, but at least you will know whether the other parameters are all right.

After you have selected the other parameters, you must establish the appropirate handshaking. The manual, the manufacturer, or trial and error will tell you which one to use.

In addition, you often need to specify: whether a line feed and carriage return must be set at the end of each line; control codes that fix the start and end of the entire transmission; and when to store the information received in a file. These parameters are chosen after everything else involved in the transmission of information works as well as possible.

To test your transmission, you could send a file with the following information:

* A line of 80 characters containing letters or the numbers 0 to 9 repeated to the end, followed by the same line less one character, followed by a third line less one more character, and so on. Using lines with progressively fewer characters can point up if and where characters are lost.

* As an alternative, you could try a sentence that contains all the letters of the alphabet, such as, "The quick brown fox jumps over the lazy dog." Then run the same sentence entirely in capital letters along with the numbers 0 to 9.

And finally, this: good luck.
COPYRIGHT 1985 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1985 Gale, Cengage Learning. All rights reserved.

Article Details
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Author:Siguel, Edward N.
Publication:Medical Laboratory Observer
Date:Jul 1, 1985
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