GE C70 Instruction Manual page 234

PRODUCT SETUP
5.3.16.2 CRC alarm CH1(2)
SETTINGS  PRODUCT SETUP  DIRECT I/O  CRC ALARM CH1(2)
 CRC ALARM CH1

The C70 checks integrity of the incoming direct input and output messages using a 32-bit CRC. The CRC alarm function is
available for monitoring the communication medium noise by tracking the rate of messages failing the CRC check. The
monitoring function counts all incoming messages, including messages that failed the CRC check. A separate counter
adds up messages that failed the CRC check. When the failed CRC counter reaches the user-defined level specified by the
CRC ALARM CH1 THRESHOLD
FlexLogic operand is set.
When the total message counter reaches the user-defined maximum specified by the
setting, both the counters reset and the monitoring process is restarted.
Configure the operand to drive an output contact, user-programmable LED, or selected communication-based output.
Latching and acknowledging conditions—if required—are programmed accordingly.
The CRC alarm function is available on a per-channel basis. The total number of direct input and output messages that
failed the CRC check is available as the
5
• Message count and length of the monitoring window — To monitor communications integrity, the relay sends 1
message per second (at 64 kbps) or 2 messages per second (128 kbps) even if there is no change in the direct outputs.
For example, setting the
at 64 kbps and 80 minutes at 128 kbps. If the messages are sent faster as a result of direct outputs activity, the
monitoring time interval shortens. Take this into account when determining the
setting. For example, if the requirement is a maximum monitoring time interval of 10 minutes at 64 kbps, then the
ALARM CH1 MESSAGE COUNT
• Correlation of failed CRC and bit error rate (BER) — The CRC check can fail if one or more bits in a packet are
corrupted. Therefore, an exact correlation between the CRC fail rate and the BER is not possible. Under certain
assumptions an approximation can be made as follows. A direct input and output packet containing 20 bytes results
in 160 bits of data being sent and therefore, a transmission of 63 packets is equivalent to 10,000 bits. A BER of 10
implies 1 bit error for every 10000 bits sent or received. Assuming the best case of only 1 bit error in a failed packet,
having 1 failed packet for every 63 received is about equal to a BER of 10
5.3.16.3 Unreturned messages alarm CH1(2)
SETTINGS  PRODUCT SETUP  DIRECT I/O  UNRETURNED MESSAGES ALARM CH1(2)
 UNRETURNED
 MESSAGES ALARM CH1
The C70 checks integrity of the direct input and output communication ring by counting unreturned messages. In the ring
configuration, all messages originating at a given device should return within a pre-defined period of time. The unreturned
messages alarm function is available for monitoring the integrity of the communication ring by tracking the rate of
5-110
CRC ALARM CH1

FUNCTION: Disabled
CRC ALARM CH1

MESSAGE COUNT: 600
CRC ALARM CH1

THRESHOLD: 10
CRC ALARM CH1

EVENTS: Disabled
setting within the user-defined message count
ACTUAL VALUES  STATUS  DIRECT INPUTS  CRC FAIL COUNT CH1
CRC ALARM CH1 MESSAGE COUNT
is set to 10 × 60 × 1 = 600.
UNRET MSGS ALARM CH1

FUNCTION: Disabled
UNRET MSGS ALARM CH1

MESSAGE COUNT: 600
UNRET MSGS ALARM CH1

THRESHOLD: 10
UNRET MSGS ALARM CH1

EVENTS: Disabled
C70 CAPACITOR BANK PROTECTION AND CONTROL SYSTEM – INSTRUCTION MANUAL
Range: Enabled, Disabled
Range: 100 to 10000 in steps of 1
Range: 1 to 1000 in steps of 1
Range: Enabled, Disabled
CRC ALARM 1 CH1 COUNT
CRC ALARM CH1 MESSAGE COUNT
to "10000," corresponds a time window of about 160 minutes
CRC ALARM CH1 MESSAGE COUNT
–4
.
Range: Enabled, Disabled
Range: 100 to 10000 in steps of 1
Range: 1 to 1000 in steps of 1
Range: Enabled, Disabled
CHAPTER 5: SETTINGS
, the
DIR IO CH1 CRC ALARM
actual value.
CRC
–4