CAT EMCP 4 - BIAS RESISTORS; 2.4 ERROR CHECKING

Application and Installation Guide EMCP 4 SCADA Data Links
©2013 Caterpillar
All rights reserved. Page 9
BIAS RESISTORS
Bias resistors are used to keep the network voltages at well-defined levels during silent times, to prevent
potential false diagnostics and communication failure. They are almost always software-configured,
including on the EMCP 4. This is unique from Termination resistors, which must be user-installed outside
of the EMCP 4. There should only be one device on the network with bias resistors enabled.
NOTE: Some devices, such as PLCs or converters, may already have termination built-in.
Consult your device documentation.
This is configured on the EMCP 4 via the setpoint “RS-485 BIAS RESISTOR ENABLE STATUS”.
2.4 ERROR CHECKING
Error checking in RS-485 SCADA is done via a Cyclic Redundancy Check (CRC). The CRC algorithm
checks the contents of the entire message. The CRC field consists of a 16–bit value, and is appended to
the end of the message. When this is done, the low–order byte of the field is appended first, followed by
the high–order byte. The CRC high–order byte is the last byte to be sent in the message.
The CRC value is calculated by the sending device, which appends the CRC to the message. The
receiving device recalculates a CRC using the same algorithm during receipt of the message, and as it
receives the CRC field, compares the calculated value to the received value. If the two values are not
equal, the message is discarded.
A detailed description as well as sample code and a calculation example are given in the Modbus over
Serial Line Specification and Implementation Guide.