Campbell Scientific CDM-VW300 Series Dynamic Vibrating-Wire Analyzer System - Manual (Page 79)

Appendix C. CDM Devices and CPI Bus
Campbell Scientific is introducing the CDM (Campbell Distributed Module)
line of peripherals, starting with the CDM-VW300 series of vibrating-wire
analyzers. As multiple CDM devices become available, they will be able to be
networked to communicate measurement information to a central datalogger.
The communications protocol used is the CAN (Controller Area Network)
Peripheral Interface (CPI) protocol.
CPI is a proprietary interface for communications between Campbell Scientific
dataloggers and Campbell Scientific CDM peripheral devices. It consists of a
physical layer definition and a data protocol. CDM devices are similar to
Campbell Scientific SDM devices in concept, but the use of the CPI bus
enables higher data-throughput rates and use of longer cables. CDM devices
require more power to operate in general than do SDM devices.
C.1 CDM Interconnection and Datalogger Connection
C.1.1 Power
The transmission of dc power to CDM modules is not governed by the CPI
specification but is left to the system integrator to implement. Most CDMs
require modest power from a regulated dc voltage between 11 and 32 Vdc.
This power may come from the datalogger power source or through the
datalogger wiring panel.
C.1.2 Interconnect Cable
Current Campbell Scientific dataloggers require the SC-CPI Interface to
connect with one or more CDM devices. Cat5e cable with RJ45 terminations
connects a CDM devices with the SC-CPI and other CDM devices. Cabling
and connections must be managed carefully to achieve the highest data rates
and longest transmission distances. Ideally, four twisted-pair (data,
synchronization, clock, and ground) are contained in a single shielded cable.
The data pair carries data between the SC-CPI and CDM using the CAN
protocol with ISO 11898 as the electrical standard. The CAN protocol covers
device addressing, bus arbitration, prioritization, bit-error checking, and failed-
message retransmission. The synchronization pair uses the RS-485 (TIA/EIA-
485A) electrical standard. This signal is a pulse that marks the beginning of
each scan. CDMs use this precision timing to remain synchronized, to latch,
and to buffer and transmit data to the datalogger unprompted over the data pair.
Use a pn 28558 termination resistor plug (included with the pn 29370 CPI
network kit) on the final device of a CPI chain to ensure cable communications
are working at high speeds and over very long distances. CPI pin assignments
are provided in the following figure as implemented with Cat5e cabling.
C-1