HEAD OFFICE
: MITSUBISHI DENKI BLDG MARUNOUTI TOKYO 100-8310
TELEX : J24532 CABLE MELCO TOKYO
HIMEJI WORKS : 840, CHIYODA CHO, HIMEJI, JAPAN
6. ALLOCATION OF BUFFER MEMORIES (BFM)
6.1 Buffer memories
The FX
2N
-4AD-TC communicates
with the PLC via buffer memories.
B F M s # 2 1 t o # 2 7 a n d # 3 1 a r e
reserved.
All non reserved BFMs can be read
b y t h e P L C u s i n g t h e F R O M
instruction.
BFMs (buffer memories) marked
with an “*” can be written to, the
special function block using the TO
instruction.
1) Buffer Memory BFM #0: Thermocouple Type K or J selection mode
BFM #0 is used to select Type K or J thermocouples for each channel. Each digit of a 4 digit
hexadecimal number corresponds to one channel, the least significant digit being channel 1.
•
A/D conversion time is 240ms per channel. When “3" (unused) is set for a channel, A/D conversion is
not executed for that channel, therefore, the total conversion time is decreased. In the above example,
the conversion time is as follows:
240ms (conversion time per channel)
×
2channels (number of channels used) = 480ms (total
conversion time)
2) Buffer Memory BFMs #1 to #4: Number of temperature readings to be averaged
When the number of temperature readings to be averaged is specified for BFMs #1 to #4, the
averaged data is stored in BFMs #5 to #8 (
°
C) and #13 to #16 (
°
F). Only the range 1 to 256 is valid for
the number of temperature readings to be averaged. If a value outside of this range is entered, a
default value of 8 is used.
3) Buffer Memory BFMs #9 to #12 and #17 to #20: Present temperature
These BFMs store the present value of the input data. This value is stored in units of 0.1
°
C or 0.1
°
F,
but the resolution is only 0.4
°
C or 0.72
°
F for Type K and 0.3
°
C or 0.54
°
F for Type J.
6.2 Status Information
1) Buffer Memory BFM #28: Digital range error latch
BFM #29 b10(digital range error) is used to judge whether the measured temperature is within the
unit’s range.
BFM #28 latches the error status of each channel and can be used to check for thermocouple
disconnection.
Low : Latches ON when the temperature measurement data drops below the lowest
temperature limit.
High
: Turns ON when the temperature measurement data rises above the highest
temperature limit, or when a thermocouple is disconnected.
When an error occurs the temperature data before the error is latched. If the measured value returns
to within valid limits the temperature data returns to normal operation. (Note: The error remains
latched in (BFM #28))
An error can be cleared by writing K0 to BFM #28 using the TO instruction or turning off the power.
b15 or b8
b7
b6
b5
b4
b3
b2
b1
b0
Not used
High
Low
High
Low
High
Low
High
Low
CH4
CH3
CH2
CH1
BFM
CONTENTS
*#0
Thermocouple Type K or J selection mode.
At shipment: H0000
*# 1 - #4
CH1 to CH4 Averaged temperature reading to be
averaged (1 to 256) Default = 8
#5 - #8
CH1 to CH4 Averaged temperature in 0.1
°
C units
#9 - #12
CH1 to CH4 Present temperature in 0.1
°
C units
#13 - #16
CH1 to CH4 Averaged temperature in 0.1
°
F units
#17 - #20
CH1 to CH4 Present temperature in 0.1
°
F units
#21 - #27
Reserved
*#28
Digital range error latch
#29
Error status
#30
Identification code K2030
#31
Reserved
H
3
3
1
0
CH1
CH2
CH3
CH4
0 = K type
1 = J type
3 = Not used
[ Example ]
2) Buffer Memory BFM #29: Error status
3) Identification Code Buffer Memory BFM #30
The identification code or ID number for this Special Block is read from buffer memory BFM #30 using
the FROM instruction. This number for the FX
2N
-4AD-TC unit is K2030. The PLC can use this facility in
its program to identify the special block before commencing data transfer to and from the special block.
7. SYSTEM BLOCK DIAGRAM
8. EXAMPLE PROGRAM
In the program shown below, the FX
2N
-4AD-TC occupies the position of special block number 2 (that is
the third closest block to the PLC). A Type K thermocouple is used on CH1 and a Type J on CH2. CH3 and
CH4 are not used. The averaging count is four. The averaged values in degrees C of input channels CH1
and CH2 are stored respectively in data registers D0 and D1.
Bit devices of BFM #29
ON
OFF
b0 : Error
When either b2 or b3 is ON
A/D conversion is stopped for
the error channel
No error
b1 : Not used
b2 : Power source
24V DC power supply failure
power supply normal
b3 : Hardware error
A/D converter or other
hardware failure
Hardware Normal
b4 to b9 : Not used
b10 : Digital range error
Digital output/analog input
value is outside the specified
range.
Digital output value is normal.
b11 : Averaging number error
Selected number of averaged
results is outside the available
range -see BFM #1 to #4
Averaging is normal.
(between 1 to 256)
b12 to b15 : Not used
Non-
contact
analog
switch
CPU
System
ROM
Buffer
Memory
RAM
A/D
converter
PLC
Command
information
write and
data status
read
24V DC
Power Source
5V Power
Supply
POWER
LED
Photocoupler
±15V
CH1
CH2
CH3
CH4
A/D
LED
Cyclic switching
FX
2N
-4AD-TC Analog Block
DC/DC
converter
Gain and offset
values are
stored in the
EEPROM
Control
signals
Converted
data
24V
LED
FROM
TO
5V
Analog Input
M8002
Initial
Pulse
K2 K30 D2 K1
Y010
FNC78
FROM
K2030 D2 M0
FNC10
CMP
M8000
RUN
monitor
K2 K29 K4M10 K1
FNC78
FROM
M10
M8002
K2 K0 H3310 K1
FNC79
TO
Initial
Pulse
Error found
Specify the type of thermocouples.
H3310 R Block No.2 BFM#0
CH4 and CH3: not used
CH2: Type J(1)
CH1: Type K(0)
Block No.2 BFM #30 R (D2)
Identification code
When (K2030) = (D2), M1 = ON.
i.e. When identification code is K2030, M1 = ON.
This initial step checks that the special function block placed at position 2 is actually
an FX
2N
-4AD-TC, i.e. its unit identification number is 2030 (BFM #30). This step is
optional, but it provides a software check that the system has been configured
correctly.
Block No.2 BFM #29 R (K4M10)
Transfer the error status to (M25 to M10).
When error is found, M10 = ON
Represents b0 BFM #29
This step provides optional monitoring of the FX
2N
-4AD-TC Error Buffer Memory (#29).
If there is an Error on the FX
2N
-4AD-TC, bit b0 of BFM #29 will be set on. This can be
read by this program step, and output as a bit device in the PLC (Y010 in this example).
Additional Error devices can be output in a similar manner, e.g. b10 BFM #29 Digital
range error. (see below)
Y010
M8000
K2 K29 K4M10 K1
FNC78
FROM
M10
Y011
M20
Represents b0 BFM #29
Represents b10 BFM #29
9. DIAGNOSTICS
9.1 Preliminary checks
I.
Check whether the input/output wiring and/or extension cables are properly connected on the FX
2N
-
4AD-TC analog special function block.
II.
Check that the PLC system configuration limits have not been exceeded, i.e. the number of special
function blocks and the total system I/O are within the specified range.
III. Ensure that the correct operating range has been selected for the application.
IV. Check that there is no power overload on either the 5V or 24V power sources, remember the loading
on the main unit or a powered extension unit varies according to the number of extension blocks or
special function blocks connected.
V.
Make sure that the main unit has been switched to RUN.
9.2 Error checking
If the FX
2N
-4AD-TC special function block does not seem to operate normally, check the following items.
•
Check the status of the POWER LED.
Lit
:The extension cable is properly connected.
Otherwise :Check the connection of the extension cable.
•
Check the external wiring.
•
Check the status of the “24V” LED (top right corner of the FX
2N
-4AD-TC).
Lit
:FX
2N
-4AD-TC is ON, 24V DC power source is ON.
Otherwise :Possible 24V DC power failure, if ON possible FX
2N
-4AD-TC failure.
•
Check the status of the “A/D” LED (top right corner of the FX
2N
-4AD-TC).
Lit
:A/D conversion is proceeding normally.
Otherwise :Check buffer memory #29 (error status). If any bits (b0, b2, b3) are ON, then this is why
the A/D LED is OFF.
10.EMC CONSIDERATIONS
Electromagnetic compatibility or EMC must be considered before using the FX
2N
-4AD-TC.
Mitsubishi recommend that the thermocouple sensors used, should be fitted with a form of seild or
screening as protection against EMC noise.
If some form of cable protection is used, the “Shield” must be terminated at the
terminals
as shown in chapter 2.
Because of the delicate nature of all analog signals, failure to take good EMC precautions could lead to
EMC noise induced errors; up to ±10% of actual values. This is an absolute worst case figure, users who
do take good precautions can expect operation within normal tolerances.
EMC considerations should include selection of good quality cables, good routing of those cables away
from potential noise sources.
Additionally it is recommended that signal averaging is used as this will reduce the effects of random
noise “spikes”.
M1
K2 K1 K4 K2
FNC79
TO
K2 K5 D0 K2
FNC78
FROM
P
K2 K29 K4M10 K1
FNC78
FROM
special
block No.2
FX
2N
-4AD-TC
BFM number
result
destination
No. of
words read
(K4)
→
(BFM #1), (K4)
→
(BFM #2)
Number of samples is changed to four on both CH1 and
CH2.
(BFM #5)
→
(D0), (BFM #6)
→
(D1)
Transfer the averaged temperature value in °C to the
data registers.
This step is the actual reading of the FX
2N
-4AD-TC input channels. It is essentially the
only program step which is needed. The "TO" instruction in this example, sets the input
channels, CH1 and CH2, to take the average reading of four samples.
The "FROM" instruction reads the average temperatures (BFM #5 and #6) for input
channels CH1 and CH2 of the FX
2N
-4AD-TC. If direct temperature readings are
required BFM #9 and #10 should be read instead, e.g.
SLD
Manual number : JY992D65501
Manual revision : B
Date
: SEPTEMBER 2002
