A Practical Guide to Particle Counting for Drinking Water Treatment - Chapter 19 doc

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CHAPTER

19
Serial Communications

All of the manufacturers provide units designed for digital serial communica-
tions. The great majority of applications will be found to require the speed and
efficiency of serial data transfer. For this reason, each of the manufacturers provides
serial data communications as an integral part of its base system. Each of the standard
turnkey systems incorporate RS-485 multidrop communications designed for direct
computer interface. This fact alone should be enough to convince those still wed to
4 to 20 mA interfaces of the inefficiencies of such an approach. If it were cheaper
or easier to follow that path, at least some of the manufacturers would be doing so.

A. INTERFACE

All of the units provide a standard RS-485 interface. This allows for multiple
units to be connected to a single-twisted shielded-pair cable. The number of counters
that can be connected to the system depends on the number of instrument addresses
available, and the limitations of the data collection software. All of the manufacturers
allow for up to 32 units. More units may be added to each system, but will have to
be placed on a separate RS-485 network.
ARTI, Chemtrac, Met One, and IBR provide RS-232 access to the particle
counter. This allows individual parameters to be set without connecting to the RS-
485 network. Only one unit may be connected at a time on an RS-232 network.
Chemtrac provides a separate connector, while Met One and IBR require moving a
couple of jumpers to switch from RS-485 to RS-232.
RS-485 communications can be conducted over two- or four-wire cabling. Two-

wire communications involves transmitting and receiving data over the same pair
of wires, while the four-wire arrangement keeps transmission and receiving lines
separate. There is no difference in speed, as the units do not transmit and receive at
the same time. ARTI, Met One, and Chemtrac are designed to operate on a two-
wire arrangement, although the Chemtrac unit may also be operated in four-wire

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164 A PRACTICAL GUIDE TO PARTICLE COUNTING

mode by removing jumpers on the printed circuit board. IBR requires a four-wire
connection. The only difference in these arrangements is the slightly higher cost of
four-conductor cabling.

B. PROTOCOLS

Each of the available units communicates data by way of a unique protocol.
When a standard turnkey system is installed, the protocol is not important from the
user’s standpoint. Only when the particle counters are to be tied into SCADA or a
data collection system does the protocol come into play.
Only ARTI and Chemtrac use protocols developed to simplify interface with
third-party equipment. ARTI has implemented the Modbus RTU protocol, which is
supported by almost all of the available SCADA software packages. Chemtrac uses
a modified version of the optomux protocol. While the standard optomux protocol
is supported by many SCADA packages, not all of the commands are included. The
standard optomux protocol provides for minimums, maximums, and averages to be
transmitted by the field hardware, whereas the SCADA software programs perform
these tasks themselves. Often these commands are left out of the driver packages
that come standard with the SCADA software. Unfortunately, the particle counters

make use of these commands for other data functions, and so the drivers are not
fully compatible. However, since many of the commands are functional, the existing
drivers can be modified to support these commands, as opposed to being written
from scratch.
Certainly at some point, other units will be designed to communicate via more
widely accepted protocols. Most of these will be built around popular programmable
logic controller (PLC) protocols such as Modbus, since the amount of data commu-
nicated is rather large, and fits the format of the PLC.
The individual protocols are too complex to provide within this book. Met One
includes detailed information on protocols in their standard equipment manuals. The
other manufacturers should be able to provide this information on request.

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© 2001 by CRC Press LLC