Fluke 2180A - Manual (Page 55)

21X0A-002
602-11. OPERATION
NOTE
For RS232Cor20 mA current loop operation,
the -002 address switches may be ignored.
The Data Set Ready (DSR, pin 33) or Clear
To Send (CTS, pin 32)must be at +5 Vto +75 V
in order to output data on the RS232C bus.
These lines can be tied to Request To Send
(RTS), pin 31) which provides the required
+ J S V .
602-12. Once the Output Unit Option is installed, the
only operator functions deal with the connection of
external equipment to the analog or digital output
connectors.
602-13. The positive and negative analog terminals have
available a dc millivolt output with the same polarity, and
proportional to the temperature displayed. For example,
if the thermometer displayed +105.7° F the analog output
would be +105.7 mV dc and for -53.1°C the output
would be -53.1 mV dc. The full resolution of the
temperature display (tenths or hundredths of degrees) is
reflected on the output.
602-14. The digital output can be connected to a printer
or any device accepting parallel ASCII data, or to a device
accepting RS232C or Current Loop signals. Connections
for all three types of signals are available simultaneously
on the 36-pin output connector previously described.
602-15. THEORY OF OPERATION
602-16. The Output Unit Option converts the
temperature displayed by the output into a format usable
by the customer’s equipment. The output may be
available as a scaled analog voltage or as formatted
parallel and serial ASCII digital output. The following
paragraphs describe operation of the Accessory Bus that
handles communication between the instrument and the
options, the analog output circuitry and the digital output
circuitry. Refer to the schematic in Section 8 during the
following discussion.
602-17. Accessory Bus Communication
602-18. The option communicates with the
thermometer on the clocked bit-serial accessory bus.
Transmitted on the bus are channel number, range,
conversion type, scale, and digits of the temperature
reading. When the WRTADR line is held low, DCLK
clocks the address of the Output Unit (4), followed by the
thermometer data to the microprocessor on the DATA
line. Once into the microprocessor, the data is converted,
formatted, and output to the analog and digital output
circuitry.
602-19. Analog Circuitry
602-20. The temperature reading received by the
microcomputer is used to generate an integrate control
signal, the length of which is proportional to the
magnitude of the temperature reading. This signal is used
to turn on (close) the FET switches Q6 and U12-2, open
the switches Ull-2, U11-3 and U12-3, and set the output
polarity with switches at U12-9 and U12-10. (U12-9 is
closed when a negative reading is being processed and
U12-10 when a positive signal is being processed.)
602-21. With Q6 on, the capacitor C1 is charged linearly
to a voltage proportional to the length of the control
signal at Q6. When Q6 has been on for the time required,
it is opened, and switch U11-13 is closed, so the output of
U13-8 can be sampled and held on C2. After 10 ms switch
Ull-3 opens and switch Ull-11 closes to zero the
integrate capacitor Cl, until the next conversion cycle.
602-22. The voltage held on C2 is buffered by U13-7.
U13-14 either passes the voltage directly to the output
stage, or amplifies it as controlled by switches U12-9 and
U12-10, which alternate states to set the polarity. The
output stage at U13-1 has a constant gain of -1.
602-23. Digital Output Circuitry
602-24. The thermometer reading transmitted on the
accessory bus every 333 ms is formatted by the
microcomputer and, if requested, made available on the
PTI Bus, RS232C, and current loop outputs. Refer to the
schematics in Section 8 during the following description.
602-25. Eight data, four address, a data valid, an address
valid, and an acknowledge line are used by the PTI Bus
Interface. The external device requests data from the Out­
put Unit by applying the preselected address to the address
lines. The address is preselected by setting switch S3 to the
desired number (0-9). When the proper address is decoded
the tri-state output buffers (U8 and U9) are enabled, and
the microcomputer and external device are notified that the
Output Unit has a valid address. When the conversion pro­
cess is complete, the microcomputer applies the first char­
acter of the formatted data to the output lines and pulls the
DATVAL line low. The external device reads the data and
pulls the ACK line low, causing the Output Unit to reply
with a new character. The process is repeated until data
transfer is complete. The message formats are shown in
Figure 602-1, and a timing diagram is shown in Figure
602-2.
602-3