Installation guide

Testing and Maintaining an RIO Network
890 USE 101 00 October 2006 139
Broken Cable
Detection and
Remedies
Unlike coaxial cable, fiber cable contains physically separate transmit and receive
lines. It is possible to lose communications through the Rx line while the Tx line
remains intact. A break in the Rx line will deprive the PLC of input data. Under
ordinary circumstances, the PLC continues to drive outputs via the intact transmit
line. This could lead to outputs turning ON or OFF due to invalid (INPUT STATE: 0)
input data. A method to prevent this from happening uses STAT and SENS
instructions in ladder logic to detect the loss of input communication and inhibit
improper output state changes:
STAT and SENS monitor the I/O status of Drop #2 and inhibit output 00001 if
communications are lost. STAT provides access to the system’s status, including
the status of S908 communications. The status information is stored in a table
starting at register 40101 and has a length of 187 words (as shown in the top and
bottom nodes of the STAT instruction).
SENS senses the first (communications health) bit (SENS top node value = 1)of the
185th word in the status table (SENS middle node value = 40285). This bit is the
communications health for Drop #2 of the S908.
Coil 00001 has been configured as an output in the I/O Map. If the PLC’s Rx line is
broken, the sensed bit becomes 0 (OFF). The middle node output to coil 00097 is
set to 0 (OFF). Coil 00097 controls a normally open relay which, when power is
removed, opens the circuits to coil 00001, thus inhibiting this output.The coils can
now be used in ladder logic to inhibit specific output writes.
As an alternative, the coil can be used to control a SKP instruction to prevent
execution of that portion of the network which would ordinarily output data.
40101
STAT
#0187
#0001
SENS
#0001
40285
00097
00097 00001
This document provided by Barr-Thorp Electric Co., Inc. 800-473-9123 www.barr-thorp.com