SYNCLOOP(8) Maintenance Procedures SYNCLOOP(8)


syncloop - synchronous serial loopback test program


/usr/sbin/syncloop [-cdlstv] device


The syncloop command performs several loopback tests that are useful in
exercising the various components of a serial communications link.

Before running a test, syncloop opens the designated port and configures
it according to command line options and the specified test type. It
announces the names of the devices being used to control the hardware
channel, the channel number (ppa) corresponding to the device argument,
and the parameters it has set for that channel. It then runs the
loopback test in three phases.

The first phase is to listen on the port for any activity. If no
activity is seen for at least four seconds, syncloop proceeds to the next
phase. Otherwise, the user is informed that the line is active and that
the test cannot proceed, and the program exits.

In the second phase, called the "first-packet" phase, syncloop attempts
to send and receive one packet. The program will wait for up to four
seconds for the returned packet. If no packets are seen after five
attempts, the test fails with an excoriating message. If a packet is
returned, the result is compared with the original. If the length and
content do not match exactly, the test fails.

The final phase, known as the "multiple-packet" phase, attempts to send
many packets through the loop. Because the program has verified the
integrity of the link in the first-packet phase, the test will not fail
after a particular number of timeouts. If a packet is not seen after four
seconds, a message is displayed. Otherwise, a count of the number of
packets received is updated on the display once per second. If it becomes
obvious that the test is not receiving packets during this phase, the
user may wish to stop the program manually. The number and size of the
packets sent during this phase is determined by default values, or by
command line options. Each returned packet is compared with its original
for length and content. If a mismatch is detected, the test fails. The
test completes when the required number of packets have been sent,
regardless of errors.

After the multiple-packet phase has completed, the program displays a
summary of the hardware event statistics for the channel that was tested.
The display takes the following form:

CRC errors Aborts Overruns Underruns In<-Drops-> Out
0 0 0 0 0 0

This is followed by an estimated line speed, which is an approximation of
the bit rate of the line, based on the number of bytes sent and the
actual time that it took to send them.


The options for syncloop are described in the following table:

Option Parameter Default Description
-c packet_count 100 Specifies the number of
packets to be sent in the
multiple-packet phase.
-d hex_data_byte random Specifies that each packet
will be filled with bytes
with the value of
-l packet_length 100 Specifies the length of each
packet in bytes.
-s line_speed 9600 Bit rate in bits per second.
-v Sets verbose mode. If data
errors occur, the expected
and received data is
-t test_type none A number, from 1 to 4, that
specifies which test to
perform. The values for
test_type are as follows: 1:
Internal loopback test.
Port loopback is on.
Transmit and receive clock
sources are internal (baud
rate generator). 2: External
loopback test. Port
loopback is off. Transmit
and receive clock sources
are internal. Requires a
loopback plug suitable to
the port under test. 3:
External loopback test.
Port loopback is off.
Transmit and receive clock
sources are external
(modem). Requires that one
of the local modem, the
remote modem, or the remote
system be set in a loopback
configuration. 4: Test using
predefined parameters. User
defines hardware
configuration and may select
port parameters using the
syncinit(8) command.

All numeric options except -d are entered as decimal numbers (for
example, -s 19200). If you do not provide the -t test_type option,
syncloop prompts for it.


Example 1: A sample display of using the syncloop command.

In the following command syncloop uses a packet length of 512 bytes over
the first CPU port:

example# syncloop -l 512 zsh0

In response to the above command, syncloop prompts you for the test
option you want.

The following command performs an internal loopback test on the first CPU
port, using 5000 packets and a bit rate of 56Kbps:

example# syncloop -t 1 -s 56000 -c 5000 zsh0


zsh(4D), attributes(7), syncinit(8), syncstat(8)


device missing minor device number

The name device does not end in a decimal number that can be used as
a minor device number.

invalid packet length: nnn

The packet length was specified to be less than zero or greater than

poll: nothing to read

poll: nothing to read or write.

The poll(2) system call indicates that there is no input pending
and/or that output would be blocked if attempted.

len xxx should be yyy

The packet that was sent had a length of yyy, but was received with a
length of xxx.

nnn packets lost in outbound queueing

nnn packets lost in inbound queueing

A discrepancy has been found between the number of packets sent by
syncloop and the number of packets the driver counted as transmitted,
or between the number counted as received and the number read by the


To allow its tests to run properly, as well as prevent disturbance of
normal operations, syncloop should only be run on a port that is not
being used for any other purpose at that time.

March 9, 1993 SYNCLOOP(8)