Rockwell Automation MHB44IND Pulse Input and Analogue Output Module - Manual (Page 12)

12 MHB44IND October 2005 – Issue 2
Triguard SC300E
Analogue outputs
Five test-sockets are provided for monitoring the output lines: one common terminal via 2.2k
ohm to the +24V field supply, and the other four via 10k ohm to the current-determining
resistors on the termination card. These resistors are 250 ohm, so 20mA of loop-current
corresponds to 5V across them.
The four analogue voltage outputs use 12-bit DACs to produce +0.4V to –6V with respect to
field +24V common, applied across resistors on the termination card. The outputs from the
three channels go through a voting scheme using FETs to select the middle-value output, so
that if one channel fails high or low, one of the other two outputs will be selected (see Figure 2-
3).
The resultant current of 0 to 22mA determined by the voltage and resistor is fed to the field
device, then returns to the supply negative via the termination card.
Figure 2-3 Analogue output section - Schematic block diagram