1. DESCRIPTION
The BA578C is a 4/20mA loop powered transmitter
incorporating a large easy to read display. The
instrument may be programmed on-site to accept
most common thermocouples and resistance
thermometers, and will provide a linear 4/20mA
output proportional to temperature, plus a display in
o
C or
o
F. Voltage inputs may be scaled allowing
the BA578C to display variables other than
temperature.
Optional alarms provide two galvanically isolated
solid state outputs which may be independently
programmed for high or low operation. For
installations in poorly illuminated areas an optional
display backlight is available.
The BA578C transmitter is housed in a 144 x 72mm
panel mounting DIN enclosure with an IP65 front
panel.
Fig 1 Simplified block diagram
2. OPERATION
Fig 1 shows a simplified block diagram of the
transmitter. The input signal, which may be from a
thermocouple, resistance thermometer or a dc
voltage, is digitised and transferred to the
instrument processor via an optical isolator. The
processor scales the signal and for temperature
measurement applies linearisation using look-up
tables stored in permanent memory. The resulting
temperature is shown on the internal display and
controls the 4/20mA output current. The
transmitter and the optional alarms are powered
from the 4/20mA current loop, but the optional
display backlight requires a separate supply.
Each time power is applied to the instrument,
initialisation is performed. After a short delay the
following display sequence occurs:
-1.8.8.8.8
Display test in which all
segments of the display
are activated for 2.5
seconds.
. . . .
For 1 second
0
For less than 0.5
seconds.
Input signal
Using calibration
displayed
information stored in
instrument memory.
2.1 Controls
All functions of the transmitter can be programmed
on-site via four front panel push-buttons - see Figs
5A and 5B. In the display mode i.e. when the
transmitter is displaying the input signal, these
push-buttons have the following functions. The
4/20mA output current is not affected when these
push-buttons are operated, and when released, the
input signal will again be displayed.
Button
Function
Down
While this button is pushed the
transmitter will show the display
corresponding to a 4mA output.
Up
While this button is pushed the
transmitter will show the display
corresponding to a 20mA output.
For transmitters fitted with alarms
E+Up
While these buttons are pushed the
transmitter will display setpoint 1.
E+Down
While these buttons are pushed the
transmitter will display setpoint 2.
It is also possible to adjust the two alarm setpoints
from the display mode - see section 10.4
4
