Problems with modem communications often are caused by software. If both parties are not using the same settings, the software can be used to change the settings so that they match. Both parties do not have to use the same software to communicate. Some common problems are:
Not being able to connect to the other computer.
Connecting but receiving garbage.
Connecting but getting random characters in addition to the "real" data.
Connecting but seeing dropped or missing characters.
Getting two characters for every one you are keying.
Not being able to see what you are typing.
We will discuss each of these issues in turn.
Inability to Connect
If you are not able to connect to the other computer, check to see that both modems are communicating at the same speed. The communication software may be forcing one of the modems to connect at a given speed with the command:
&Nx
The value x is between 0 and 9. The &Nx command tells the modem what speed to connect with another modem. Setting the modem with &N0 allows the modem to determine what the highest possible connect speed is and uses that speed to connect. Using commands other than &N0, such as &N2, forces the modem to connect only at one speed. See the table below for various forced connect speeds.
Command Connect Rate
&N0 Variable
&N1 300 bps
&N2 1200 bps
&N3 2400 bps
&N4 4800 bps
&N5 7200 bps
&N6 9600 bps
&N7 12 Kbps
&N8 14.4 Kbps
&N9 16.8 Kbps
If the modem is set to a forced speed, try setting the modem to the variable connection rate and reconnecting. If that does not work, try connecting to a different service or modem and see if the problem persists. If you can connect to other computers, you know the trouble is at the other end of the line.
Receiving Garbage
If you can connect, but get garbled characters on the screen, check to see that both modems are set for the same number of data bits. If the data bits are set correctly, consider a line noise problem. Use a different telephone line and see if the problem repeats itself.
Receiving Random Characters
If you can connect, but you are getting random characters on the screen in addition to the "real" data, consider the following
Is there a telephone extension on the line you are using? If so, did someone pick up the extension?
Is the modem in question an external modem? If so, is the modem cable near a fluorescent light or some other source of EMI?
Is the terminal emulation set properly?
Consider a line noise problem.
Dropped or Missing Characters
If you can connect, but are seeing dropped or missing characters, consider the following:
There may be a flow control or handshaking problem between your computer and the modem. Check the cabling for good connections and try another cable of the appropriate type.
There may be a problem with the UART chip on your serial port. Try setting your modem to communicate at a slower speed or replace the current UART with a 16550 model.
There may be an interrupt latency problem. Attempt to establish the communication link with as few open programs as possible.
Double Characters/No Characters
Echo on/off can cause you to not see messages on the screen or to see double characters for everything you type. Local echo causes the modem to repeat back every character it receives. This feature makes the text you type appear on the screen. If the computer you are connected to has "remote echo" enabled, its modem repeats every character it receives. If both local echo and remote echo are enabled, you will see every character that you type twice. This problem is corrected by turning local echo off.
If the software you are using does not support a local echo option, try using "full-duplex" as a substitute for echo off. Conversely, you may use half-duplex as a substitute for echo on. Some software packages have mistakenly used the term duplex in place of local echo options. Duplexing refers to data transmission being limited to transmitting and receiving as single tasks, or simultaneous two-way transmissions.
If you are connected but cannot see what you are typing, the other computer does not have remote echo enabled, and your local echo is not enabled. To correct the problem, turn local echo on.
&N0 Variable
&N1 300 bps
&N2 1200 bps
&N3 2400 bps
&N4 4800 bps
&N5 7200 bps
&N6 9600 bps
&N7 12 Kbps
&N8 14.4 Kbps
&N9 16.8 Kbps
If the modem is set to a forced speed, try setting the modem to the variable connection rate and reconnecting. If that does not work, try connecting to a different service or modem and see if the problem persists. If you can connect to other computers, you know the trouble is at the other end of the line.
Receiving Garbage
If you can connect, but get garbled characters on the screen, check to see that both modems are set for the same number of data bits. If the data bits are set correctly, consider a line noise problem. Use a different telephone line and see if the problem repeats itself.
Receiving Random Characters
If you can connect, but you are getting random characters on the screen in addition to the "real" data, consider the following
Is there a telephone extension on the line you are using? If so, did someone pick up the extension?
Is the modem in question an external modem? If so, is the modem cable near a fluorescent light or some other source of EMI?
Is the terminal emulation set properly?
Consider a line noise problem.
Dropped or Missing Characters
If you can connect, but are seeing dropped or missing characters, consider the following:
There may be a flow control or handshaking problem between your computer and the modem. Check the cabling for good connections and try another cable of the appropriate type.
There may be a problem with the UART chip on your serial port. Try setting your modem to communicate at a slower speed or replace the current UART with a 16550 model.
There may be an interrupt latency problem. Attempt to establish the communication link with as few open programs as possible.
Double Characters/No Characters
Echo on/off can cause you to not see messages on the screen or to see double characters for everything you type. Local echo causes the modem to repeat back every character it receives. This feature makes the text you type appear on the screen. If the computer you are connected to has "remote echo" enabled, its modem repeats every character it receives. If both local echo and remote echo are enabled, you will see every character that you type twice. This problem is corrected by turning local echo off.
If the software you are using does not support a local echo option, try using "full-duplex" as a substitute for echo off. Conversely, you may use half-duplex as a substitute for echo on. Some software packages have mistakenly used the term duplex in place of local echo options. Duplexing refers to data transmission being limited to transmitting and receiving as single tasks, or simultaneous two-way transmissions.
If you are connected but cannot see what you are typing, the other computer does not have remote echo enabled, and your local echo is not enabled. To correct the problem, turn local echo on.
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