| The EXTECH Model 383273
Digital Multi-meter has a built-in RS-232
interface, and can easily communicate with the
FT-100. The meter is relatively inexpensive (less
than $200), and provides several data monitoring
functions that can expand the capabilities of the
FT-100. In addition to the typical DMM features,
there are also several optional probes that allow
the monitoring of several other types of inputs,
such as: humidity, light intensity, wind speed,
etc. Although the meter was originally designed
to interface with a PC, this application note
describes the hardware and software necessary for
interfacing with the FT-100.
The EXTECH 383273
is a 3 1/2 digit digital multi-meter with the
following measurement capabilities:
- Voltage (AC
& DC)
- Current (AC & DC)
- Resistance
- Capacitance
- Frequency
- Diode and Continuity
- Temperature
The meter has its
own RS-232 interconnect cable; however, to
interface with the FT-100, this cable must be
altered. The schematic below shows the necessary
cable interconnections:
The EXTECH meter
always requires a "high" on pin 4 (DTR)
to send data. The above cable connects this line
to pin 9 from the FT-100. Pin 9 is one of the two
lines used for serial port selection in the 1041
Serial Port Expander. Because of this, the meter
must be addressed as either port #2 or #4.
Note: The 1041 does not have a "select"
voltage on pin 9. If you need to interface with
the meter through a 1041, please contact the
factory.
The program shown
can be used to receive all types of readings from
the meter. Since most applications would not
require all the functions of the meter, you may
omit the program lines that are not necessary for
your particular function. In particular, unused
"IF" statements used for function
selection, may be omitted.
HOW THE
PROGRAM WORKS
The meter will
immediately send 5 bytes of data whenever it
receives a "space" character. The first
byte is always 02, and the last byte is always
03. These framing bytes are used to determine
proper communications. The third byte is the
function and range byte, and is used in the
program to select the proper "units"
and "multiplier". The third and fourth
bytes contain the actual reading. This
information is embedded within the bits of these
numbers, so the program extracts the data from
the bytes. The multiplier is used to scale the
meter reading to the basic units for each type of
reading (for example, capacitance is scaled to
farads). The variable "VALUE" is the
final value for the reading, in the basic units.
The program reads
in the 5 bytes and stores them as string
variables. Because these bytes can be any value
from 00 to 0FFH, and therefore cannot always be
represented as ASCII text, we must extract the
actual value from the string variable table. In
order to know exactly where in the table
each string variable is located, it is very
important that the first reference to
string variables folllows the order: A$, B$, C$,
D$, E$. To simplify this requirement, you could
place the following command at the beginning of
your program:
A$="":
B$="": C$="":
D$="": E$=""
This will insure
the placement of the string variables within the
table.
Although somewhat
lengthy, this program is relatively simple, and
you should have very little trouble following the
flow and adapting it to your particular
situation. If you have any trouble, or have a
unique interface situation, please contact the
factory. There is always an application engineer
ready to assist.
EXTECH
'Program Name
'
COM 9600,,,,0,,0
'Set baud to 9600 & disable Direct Command
Mode & Echo
L0: ? @2;" ";:DELAY
10 'Send
"space" character to meter, & wait
10ms for return
GET
@2;A$,B$,C$,D$,E$ 'Read 5 bytes
from meter
IF A$<>CHR$ 2 OR
E$<>CHR$ 3 THEN GOTO L1
'Test
framing bytes
CLS
'
'------ Extract readings from STRING VARIABLE
table ------
PTR=PEEK 59+(PEEK
60*256)+((PEEK 61+(PEEK 62*256))*16)-4
FUNCT=(PEEK PTR AND
1)*0FFH AND (PEEK (PTR-1))
'Extract Function byte
PTR=PTR-2:IF
FUNCT<>0 THEN DEC PTR
VAL1=(PEEK PTR AND
1)*0FFH AND (PEEK (PTR-1))
'Extract #1 Data
byte
PTR=PTR-2:IF
VAL1<>0 THEN DEC PTR
VAL2=(PEEK PTR AND
1)*0FFH AND (PEEK (PTR-1))
'Extract #2 Data
byte
'
'------ Test for invalid or overload readings
------
IF (VAL1 AND 1FH)=3FH
THEN CLS:GOTO L0
IF (VAL1 AND 1FH)=0FH
THEN CLS:CURSOR 2,5:DISP "+
OVERLOAD":GOTO L1
IF (VAL1 AND 1FH)=0EH
THEN CLS:CURSOR 2,5:DISP "-
OVERLOAD":GOTO L1
'
'------ Determine Function and Value Multiplier
of Meter Setting------
IF FUNCT=0 THEN
F$="V DC":M=0.0001
'200mv DC scale
IF FUNCT=1 THEN
F$="V DC":M=0.001
'2v DC scale
IF FUNCT=2 THEN
F$="V DC":M=0.01
'20v DC scale
IF FUNCT=3 THEN
F$="V DC":M=0.1
'200v DC scale
IF FUNCT=4 THEN
F$="V DC":M=1
'1000v DC scale
IF FUNCT=5 THEN
F$="Hz":M=1
'Frequency
IF FUNCT=6 THEN
F$="V DC <CONT/DIODE>":M=0.001
'Diode/Continuity scale
IF FUNCT=8 THEN
F$="Ohms":M=0.1
'200 W Resistance scale
IF FUNCT=9 THEN
F$="Ohms":M=1
'2K W Resistance scale
IF FUNCT=10 THEN
F$="Ohms":M=10
'20K W Resistance
IF FUNCT=12 THEN
F$="Ohms":M=100
'200K W Resistance
IF FUNCT=16 THEN
F$="Ohms":M=1000
'2M W Resistance scale
IF FUNCT=17 THEN
F$="Ohms":M=10000
'20M W Resistance scale
IF FUNCT=18 THEN
F$="CAP (F)":M=1E-8
'20 F Capacitance scale
IF FUNCT=20 THEN
F$="CAP (F)":M=1E-9
'2 F Capacitance scale
IF FUNCT=24 THEN
F$="CAP (F)":M=1E-10
'200nF Capacitance scale
IF FUNCT=32 THEN
F$="CAP (F)":M=1E-12
'2000pF Capacitance scale
IF FUNCT=33 THEN
F$="A DC":M=0.01
'20 Amp DC scale
IF FUNCT=34 THEN
F$="A DC":M=0.0001
'200 mA DC scale
IF FUNCT=36 THEN
F$="A DC":M=1E-5
'20 mA DC scale
IF FUNCT=40 THEN
F$="A DC":M=1E-6
'2 mA DC scale
IF FUNCT=48 THEN
F$="A DC":M=1E-7
'200 A DC scale
IF FUNCT=64 THEN
F$="DEG F":M=0.1
'Temperature 200E F
IF FUNCT=65 THEN
F$="DEG F":M=1
'Temperature 2000E F
IF FUNCT=66 THEN
F$="DEG C":M=0.1
'Temperature 200E C
IF FUNCT=68 THEN
F$="DEG C":M=1
'Temperature 2000E C
IF FUNCT=128 THEN
F$="V AC":M=0.0001
'200mv AC scale
IF FUNCT=129 THEN
F$="V AC":M=0.001
'2v AC scale
IF FUNCT=130 THEN
F$="V AC":M=0.01
'20v AC scale
IF FUNCT=131 THEN
F$="V AC":M=0.1
'200v AC scale
IF FUNCT=132 THEN
F$="V AC":M=1
'750v AC scale
IF FUNCT=161 THEN
F$="A AC":M=0.01
'20 Amp AC scale
IF FUNCT=162 THEN
F$="A AC":M=0.0001
'200 mA AC scale
IF FUNCT=164 THEN
F$="A AC":M=1E-5
'20 mA AC scale
IF FUNCT=168 THEN
F$="A AC":M=1E-6
'2 mA AC scale
IF FUNCT=176 THEN
F$="A AC":M=1E-7
'200 A AC scale
IF FUNCT=255 THEN
F$="*** HOLD ***"
'HOLD
reading (HOLD button is pressed)
'
POL=1-(1-(VAL1 AND 1))*2
'Polarity
IF FUNCT=5 THEN POL=1:IF
(VAL1 AND 1)=1 THEN M=1E4
'Frequency Range
'
'--- Extract meter reading from data bytes ---
NUM=(VAL2 AND
3CH)/4:CALL XPOSE:VALUE=Q
'LSD of result
NUM=(VAL2 AND 3)*4+(VAL1
AND 0C0H)/64:CALL XPOSE
VALUE=Q*10+VALUE
NUM=(VAL1 AND
3CH)/4:CALL XPOSE:VALUE=Q*100+VALUE
VALUE=(VAL1 AND
2)*500+VALUE
'MSD of result
'
IF (VAL2 AND 40H)=1 THEN
VALUE=VALUE*100
'Decimal position
IF (VAL2 AND 80H)=1 THEN
VALUE=VALUE*10
VALUE=VALUE*M*POL
'Scale value to basic units & add polarity
'
CURSOR 2,1:DISP
#D;VALUE;" ";F$
'Display
meter reading and units on LCD
'
L1: DELAY 500:IF FUNCT=5 THEN DELAY
500 'Delay 2 sec. (If frequency,
delay 1 sec.)
GOTO L0
'
'
'******* SUBROUTINE TO TRANSPOSE BITS *******
' Input: NUM = number to transpose
' Output: Q = result
'
XPOSE: Q=0:FOR I=1 TO 4
SHL
Q:Q=Q+(NUM AND 1):SHR NUM
NEXT:RETURN
/
NOTICE:
Every effort has been made to insure the accuracy
of the information contained in this document,
however
Y-tek is not responsible for any consequences
resulting from erroneous or inaccurate
information.
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