Application and Installation Guide EMCP 4 SCADA Data Links
©2013 Caterpillar
All rights reserved. Page 35
6 TROUBLESHOOTING
There are several categories of problems that can occur with the SCADA data links. The first is
intermittent or no communication. If communication is well-established, there may be Modbus error
messages, which are called Exception Responses. And finally, the EMCP 4 may return data which is
outside of the valid data range.
Along with checking the physical connections and EMCP 4 software configuration, the use of a known
software package to test the connection is recommended. A good choice is the EMCP 3 Monitoring
Software, which is available for download from Cat PowerNet.
6.1 INTERMITTENT OR NO COMMUNICATION
There are many factors which can cause poor or no communication. Some may cause a SCADA Data
Link Fault event on the EMCP 4. Others may be observed by the client only.
Certain conditions may activate a SCADA Data Link Fault event. This fault may be viewed on the EMCP 4
display, via ET, or even via Modbus (see section 5.7). The causes and troubleshooting procedure differ
somewhat between data links. For the RS-485 SCADA Data Link, check the following:
Baud rate, parity, and stop bits mismatch
Lack of, or misplaced, termination resistors
Incorrect wiring (MB+ and MB- switched, full-duplex device connections incorrect, etc)
Distance from end to end too great; 1200m (4000ft) is the limit
For the TCP/IP SCADA Data Link, check the following:
Cable disconnected after communication was established
IP address not unique
Subnet mask or gateway mismatch
Incorrect wiring
Poor shielding or no shielding (especially in noisy environments like adjacent to AC power cables
or near the generator housing), or low-spec cable (anything below Cat-5)
Distance from end to end too great; 100m (325ft) is the limit
Other symptoms may be observed at the client only. These may be indicative of poor communication or
no communication:
Wires not connected at all, or incorrectly
Incorrect Slave Address (RS-485) or IP Address (TCP/IP) configured
Baud rate mismatch (RS-485)
Message timeout too short (EMCP 4 RS-485 SCADA has a maximum response time of 200ms,
EMCP 4 TCP/IP SCADA has a maximum response time of 50ms)
Multiple requests being sent from client without waiting for a response (RS-485)
6.2 MODBUS ERRORS: EXCEPTION RESPONSES
The EMCP 4 will send an Exception response if it is able to interpret the request, but there is a problem with
it. The exception response is identified by a function code that is 128 (0x80) or greater. The function code is
related to the function code the EMCP 4 is responding to; it’s value is 128 + the Function Code for Request.
There is additional data, called the exception code, within the exception response message that can help
identify the problem. The different exception responses and exception codes are described below.
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