Campbell CR10 PROM Instruction Manual download pdf (Page 29)

D-1

APPENDIX D. SDM COMMUNICATIONS AND

LONG SIGNAL CABLES

D.1 BRIEF DESCRIPTION OF SDM

CLOCK RATES

The fastest rate that a datalogger and SDM

sensors can communicate via the SDM bus is

determined by the number of SDM sensors

connected to the SDM bus and properties of the

cable used to connect the sensors. This rate,

called the SDM clock rate, has been optimized

for the standard 7.62 m (25 ft.) SDM signal and

power cable supplied with the CSAT3.

Instruction SDM-CSAT3 (P107) and SDM

Group Trigger (P110) on the CR23X, 21X, and

CR10(X), and CSAT3() and SDMTrigger() on

the CR9000 use this optimum SDM clock rate

by default. For cases when longer lead lengths

are required, the SDM clock rate may be slowed

down using Instruction Set SDM Clock (P108

for a 21X and P115 for a CR23X and CR10(X))

or SDMSpeed() (CR9000). The SDM Clock

rate is changed by entering a parameter

between 1 and 255, where 1 is the fastest SDM

clock rate and 255 is the slowest.

The appropriate SDM clock rate for a given

system configuration, can be found with the

example datalogger programs listed at the end

of this section. The example programs start

with the default SDM clock rate, clock

parameter equal to 1, and systematically slow

the rate down until there are no SDM

communication errors. They then begin to

speed up the rate until SDM communications

fails. The processes of slowing down and

speeding up the SDM clock rate around the fail

threshold continues for approximately three

minutes. The datalogger outputs three arrays.

The first (array ID = 150) is the SDM parameter,

the second (array ID = 151) is the number of

failures at an SDM parameter as a percent of

attempts at that SDM parameter, and the third

(array ID = 152) is the number of attempts at an

SDM parameter. The fastest SDM clock rate,

that can be used with a particular system setup,

can be found from the failure array by selecting

the first clock parameter with zero failures. To

provide a safety margin select the next slowest

clock parameter. Test the new SDM clock rate

in the datalogger program that will be used in

the experiment.

D.2 CR23X EXAMPLE

Copies of the SDM search programs for the

CR23X, 21X, CR10(X), and CR9000 are

available on the Campbell Scientific ftp site at

ftp://ftp/campbellsci.com/pub/outgoig/files/

sdm_srch.exe. The file sdm_srch.exe is a self

extracting DOS file.

Connect all the CSAT3s to the datalogger with

the cable in question. Download the example

CR23X SDM search program to the datalogger,

after the program compiles, set Flag 3 high to

begin the experiment. When Flag 2 is set low

the experiment is complete. Collect the data

from the datalogger and run the example SPLIT

parameters files, SDM_TRAN.PAR and

SDM.PAR, to sort the data. Identify the correct

SDM clock parameter to use for your system

configuration from the report generated by the

SPLIT parameter file SDM.PAR.

Take the following case, a 575 foot SDM signal

and power cable is used to connect a CR23X

datalogger and a single CSAT3 anemometer.

Communication between the CR23X and

CSAT3 fails at the default SDM clock rate.

Thus, a slower SDM clock rate must be used.

The example CR3X SDM clock rate search

program generated the data in Table D1.

TABLE D-1. Data from Example CR23X SDM

Clock Rate Search Program

150,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16, ...

,253,254,255

151,100,99999,99999,100,99999,99999,100,99

999,100,96,0,0,0,99999,99999,99999, ...

,99999,99999,99999

152,1,0,0,1,0,0,1,0,17,450,449,449,432,0,0,0,

... ,0,0,0

Run the SPLIT parameter file SDM_TRAN.PAR

on the above data set to transpose the data into

columns.

Now run the SPLIT parameter file SDM.PAR, to

filter out the SDM clock parameters that were

not tested, 99999 in array 151. This parameter

file also creates a report of the SDM clock

1 2 ... 24 25 26 27 28 29 30

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