Serial RS232 Information & FAQ
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Troubleshooting Serial Interfaces
Garbage Data Response
No Response at All
DataPro RS232 Serial & Parallel Products
TROUBLE-SHOOTING SERIAL INTERFACES
Due to the diversity of implementations in the RS-232-C standard,
there are literally hundreds of existing pinouts and wiring configurations
for RS232 serial cables. Fortunately, most cables can be classified as either
"straight/synchronous" or "crossover/null-modem", and if the devices are
compentent in handling signals, they correct type of cable should be suitable
in most situations.
However, even with a known-good cable you may encounter
problems such as garbage data response or no response at all.
Such problems are created by other elements of the installation
which need to be checked, and in such situations the below troubleshooting
guide may be of use.
Problem: GARBAGE DATA RESPONSE
If this occurs, check the switch settings to ensure a proper
match for baud rate, parity, start and stop bits, and data word
length. Also be sure both devices use the same protocol e.g.,
X/ON, X/OFF or READY/BUSY.
NOTE that garbage data response is usually not a cable problem,
although it can be. Usually it is caused by a mis-match, i.e.,
settings on the two devices do not match. You must match up:
Here is the only situation in which an improperly built cable
can cause garbage data: if the devices have hardware flow control
selected, but the flow control line is not connected properly,
data would be clean initially, until the input buffer overflows,
at which time (many) characters will be lost - you will be able
to read the output, but there will be gaps in the data stream.
- Baud rate (speed)
- Start and stop bits
- Word size
- Protocol (also called "flow control" or "handshake")
- XON/XOFF (software)
- RTS/CTS (hardware)
- RDY/BSY (hardware)
- ETX/ACK (software)
Any other type of garbage data will NOT be the result of a cable
Problem: NO RESPONSE AT ALL
If this occurs, the following checklist will help in identifying
- Are all cable plugs connected tightly?
- Are cables going to the right ports?
- Have operating system and software been properly installed?
- Have ports been configured properly?
- Is AC power on for all devices?
- Did you re-power after setting switches and jumpers?
If not, turn device power off, wait 30 seconds, and turn it
- Have switches and jumpers been set properly?
- Are cables wired as shown by the pin-to-pin diagram?
- Are there any broken lines? Any extra pins connected?
NOTE: Many printers will refuse to print under one or more
of the following conditions:
- Is paper properly loaded?
- Is ribbon (and printwheel) installed?
- Is cover closed?
- Is the on-line switch set "on-line"?
- Cover not closed tightly.
- Differences detected between printer settings and
- Out of paper.
- Is phone connection achieved and indicated by the
- Have you pressed several returns to activate remote
- Are brightness and contrast turned up?
- Is full duplex communications mode set?
Written by DataPro International Inc.|
Unauthorized duplication strictly prohibited.