Campbell Scientific CDM-VW300 Series Dynamic Vibrating-Wire Analyzer System - Manual (Page 81)

Appendix C. CDM Devices and CPI Bus
C.1.4 CPI Grounding
To keep the common mode of the signals in range, a common reference
(ground) connection is advised for any non-isolated communication link. If a
ground potential difference exists, then current will flow on the common wire
from one end of the cable to the other. This current is restricted only by the dc
resistance of the common wire which is often quite low. To increase this
resistance, a 100-Ω resistor is placed inside each CDM device, SC-CPI
interface, or CDM equipped datalogger between the CPI ground and the local
ground. This is a compromise between directly connecting the two points and
leaving the common connection entirely disconnected. It limits current while
still helping to prevent the two node voltage potentials from drifting too far
apart.
C.1.6 Addressing
The CPI bus uses an addressing scheme that supports up to 120 CDM devices
on the bus connected to a single SC-CPI interface. In the case of the CDM-
VW300, fewer devices must be used because of limitations of bandwidth
caused by high-speed measurement rates. Future CDM devices may be used on
a CPI bus managed by a SC-CPI interface device connected to a datalogger.
C.2 Distributed Architecture
CDM devices can operate with longer cable distances between modules and
the datalogger than might be expected for sub-second data acquisition. A
common long cable network is illustrated in the following FIGURE C-2.
The CDM-VW300 uses the CPI communications protocol to communicate
with Campbell Scientific dataloggers or other CDM devices. Two CPI
connectors are provided on each device. The network kit supplied with each
CDM device provides yellow tape with which to optionally mark RJ45cables
dedicated to CPI communications.
A single CDM device can be connected to a CR800 series, CR1000, or
CR3000 datalogger using an SC-CPI interface. One CPI port on the CDM
device connects to a CPI port on the SC-CPI. The other CPI port is terminated
with a CPI terminator (supplied with pn 29370 network kit). When multiple
CDM devices are used in a daisy-chain topology, one CPI port of the CDM
device nearest the datalogger connects to the SC-CPI interface and the other
CPI port connects to a neighboring CDM device. Any CDM devices in the
middle of the chain will have both CPI ports connected to neighboring CDM
devices. Once CPI port of the CDM device at the end of the CPI bus/chain
connects to a neighboring CPI device, and the other port is fitted with a
termination plug (pn 28558). Contact a Campbell Scientific application
engineer for information about using a optional star topology.
C-3