Fluke 2180A - Manual (Page 76)

21XOA-006
NOTE 606-18. THEORY OF OPERATION
Once installed, there are no provisions for
disabling the Limits Option. If a temperature
display is desired without the relay or LED
indications, select one of the Limits functions
f> or and the maximum setting on the
numeric thumbwheel switches.
606-10. OPERATION
606-11. The position and general description of the
Limits Option front panel controls is given in Section 2. A
more detailed description of the three functions is given in
the following paragraphs. The term "thermocouple" =
"RTD" for the 2180A.
606-12. Limits Function
606-13. The Limits function is enabled when the
function portion of the thumbwheel is set in either the ^
or > position. The front panel LED illuminates and the
rear panel relay contacts close when either of the preset
conditions are met. In the greater than (>) function the
indications (LED and relay contacts) result from any
temperature reading that exceeds the value set on the
limits thumbwheels. The less than (^) function gives its
indication when the temperature read by the thermometer
is equal to, or less than, the value set on the limits
thumbwheels. The thumbwheel LSD is a whole number,
fractional entries cannot be made.
606-14. Delta Function
606-15. When the Delta (A) function is selected on the
thumbwheel the thermometer display reads the difference
between the temperature at the thermocouple and the
whole number setting of the thumbwheels. The formula
used for the computation is:
T displayed = T at thermocouple - T thumbwheel setting
(in degrees)
606-16. MINIMUM/MAXIMUM Display
Function
606-17. The microcomputer acculmulates and stores
the highest and lowest temperatures recorded since the
last reset. When the Maximum (T) function is selected
the highest temperature recorded and stored in the
microcomputer is displayed. Selection of the Minimum
function (i) displays the lowest recorded temperature
since the last reset. To record the current temperature for
either function, select the applicable function and depress
the front panel reset switch.
606-19. The -006 Option supplies the thermometer with
the function and numeric data selected on the front panel
mechanical thumbwheel switches. When a Limits
function is selected, the data is stored for comparison on
the option pcb. All communication between the option
and the thermometer is done on the clocked serial
accessory bus. This bus transmits and receives addresses,
thumbwheel data, reset data, and limit status. Refer to the
schematic in Section 8 during the Theory of Operation
discussion.
606-20. Addressing
606-21. Each of the options on the bus is addressed with
a different code. The thermometer uses the address "6" to
talk to the Limits Option. To talk to any option the
WRTADR (P4-3) line must be brought low, with WRT
(P4-4) high, followed by the applicable four address bits
applied to the DATA (P4-5) line in succession, toggling
DCLK (P4-6) for each bit. This clocks the address into the
shift register (U2-15) where it is compared to the Limits
Option address and, if valid, (U1-1 low) enables the gates
required to shift data to the thermometer (U5-11) and into
the Limits Option (U5-2). Details of the data transfer are
in subsequent paragraphs.
606-22. Limits Option Outputs
606-23. Before the thermometer can input data from the
Limits Option, the Limits circuitry must be addressed as
described previously (Ul-1 low). Once addressed the
WRTADR and WRT lines go high. The low to high
transition of WRTADR (U3-6) loads the shift registers
from the thumbwheels and RESET switches. The two
lines enable the output data line (Ul-13) and as the
thermometer accepts the data it clocks the serial output
shift registers with DCLK, transferring the data from the
shift register to the thermometer.
606-24. In the thermometer the data is processed by the
microcomputer to perform the proper action. For the
Limits function the output is compared to the state of the
limits and, if exceeded, the indicator illuminated and the
relay energized. For the Delta function the temperature is
compared against the transmitted value and the
difference displayed. If either the Maximum or Minimum
function is selected the stored value is displayed, but, in
addition, the status of the RESET switch is checked.
606-25. Limits Option Inputs
606-26. The thermometer transmits to the Limits
Option only the Limits Exceeded status. After the option
has been addressed (Ul-1) WRTADR goes high to
606-2