Troubleshooting a Serial Connection

Introduction

Serial connections on a PC are somewhat difficult to troubleshoot. This document is designed to give you some insight into what to look for and some common problems that may occur.
If you are attempting to connect serially over an internal or external modem, see our document on Troubleshooting Modem Connections. If you are working over a modem, but as a PPP or Internet connection, see our document on A Guide to Dial-Up Networking.

The Basics

Serial communications works with an I/O address range and a hardware interrupt. Since PCs are limited on hardware interrupts, conflicts can occur.

With later releases of Windows, the serial settings are all automatic and controlled through the system configuration (see your Control Panel options).

Within Anzio Lite and AnzioWin, you can set the communications method for direct serial or modem. This is done through the Communications Setup screen.

Direct "Serial" lets you select the port, baud rate, data bits and parity. From this you can talk over a serial line to another host, or to a modem (you can also select "Modem" as the communication method, whereby you let Windows control access to the modems through the TAPI protocol).

Setting Up A Loopback Test

A loopback test is the ideal method to determine if a serial port is working correctly. With Anzio running on the PC, you can jumper pins 2 and 3 together (the send and the receive) to see if characters get echoed back to the Anzio screen. Follow these steps:

1. Set Anzio for the proper serial port
2. Turn off Full Duplex.
3. Jumper pins 2 and 3 together. You can do this with a bent paper clip inserted in the proper holes.
4. Type characters. If they appear on the screen, then the serial port is working correctly.

Breakout Box Diagnostics

If you have a breakout box for serial connections available, you can test your serial communications setup with this. A loopback test is easily set up by jumpering the same two pins together as described above. Also you can put the breakout box in the main line, between the PC and the host, and watch each ends response as characters go out and come in.

If you are unfamiliar with serial communications, or for more information on how the results should look, give us a call.

Troubleshooting A Host Direct Connection

Here are some simple steps to troubleshooting a serial connection:

• If this is a new PC replacing a "dumb" terminal, make sure you are plugged into the serial port on the PC, not the parallel port.
   
  
• You will probably have to have a gender changer or change the connecting plug for the serial cable to the 25-pin male connection on the PC, or you would need a 25-to-9-pin adapter). If you did not, you may have plugged the cable into the wrong port on the back of the PC.
   
  
• Place a "dumb" terminal on the same line as the PC and see if it is still responding. If the "dumb" terminal works, but a PC (connected to the right port) still does not, the problem is usually one of two:
   
  
• The PC's serial port has been disabled
  
  
• It is common on today's computers for the vendor to add a modem and hence disable one of the serial ports on the back. Check the Modem settings and the System settings in the Control Panel for conflicts).
   
  
• There is a pin problem in the cable
  

• Many "dumb" terminals do not need all pins connected and sometimes cables are built with a minimum configuration. However PC's must have signals coming in on
  certain pins (check the cable pin-out).
   
  
• Use the loopback test described above to see if serial port is correct.
   
• Type   at   and press enter. If you get a response back of  OK or 0 then you are connecting to the modem. If this is not what you want, try the other serial port(s).
   
• Are you getting an error when Anzio tries to initialize the serial port? Do you get an error when you hit any keyboard character or the enter key? If so, see the list below for more detail.

Serial Errors
Bad Communications Parameters

This usually appears when Anzio is unable to initialize (set baud, parity, etc.) for the serial port. Check to be sure you have the correct serial port selected and that no other software is using that port, i.e. fax auto-receive, voice mail or other modem software.

Unable To Transmit

Similar to the above, except that a communications error of some type has occurred, preventing data from going out over the serial.

Framing

Framing errors occur when the serial chip is not getting the correct number of bits in its characters. If this is intermittent, it may be due to phone noise. If this is a direct-connect serial, it is possible that the BAUD, PARITY, DATA BITS and/or STOP BITS are set wrong for the connection.

Parity

This occurs when the serial chip has detected a parity error. It may mean that the BAUD, PARITY, DATA BITS and /or STOP BITS are not correct. If you are using a modem over a phone line, it may also be due to loss of characters because of phone noise.

Overrun

While most serial programs do not show this particular error, Anzio does. An OVERRUN message can appear on Anzio's status line, when your PC is connected to the host via a direct serial line, or when using a modem through the serial line interface.

The overrun will result in lost data coming to the PC from the host, which may in turn cause a scrambled screen, file transfer problems, screen lockups, and so forth.

An overrun occurs when characters come into the serial communication chip in the PC faster than Windows can service them. It is this chip itself that overruns its buffer.

This problem has shown up lately especially in PCs where the serial communication "chip" is actually part of an integrated chipset motherboard.

There are two approaches to solving this problem:

1) You can adjust the buffer usage on the chip. Note that the following terminology may vary with different versions of Windows.

a. Bring up the Control Panel's "System" item.

b. Click the "Device Manager" tab.

c. Go to "Ports (COM & LPT)".

d. Choose the COM port in question, such as COM1.

e. Click "Properties".

f. Click "Port Settings".

g. Click "Advanced".

h. Make sure the box "Use FIFO buffers..." is checked. Then try a lower setting for "Receive Buffer".

2) If method 1 does not work, your best solution is to add a new serial port to the PC. This can be an internal card, such as a PCI card. Make sure it is "16550 compatible". Or, if your PC and Windows both support USB, you can add a USB-to-serial adapter. This will not require you to open the computer case.