Rockwell Automation MHB44IND Pulse Input and Analogue Output Module - Manual (Page 11)

MHB44IND October 2005 – Issue 2 11
Triguard SC300E MHB44IND Pulse Input and Analogue Output Module
Pulse Inputs
The overall period of one or more cycles of the input is measured. Frequency is calculated by
dividing the number of cycles by the overall period. The pulse input voltage on each
independent path passes via an isolating transformer to an amplifier, which compensates for
the frequency characteristics of the transformer and performs automatic level control to cater for
the wide range of input amplitude (see Figure 2-2). The amplifier output is converted to a pulse
train feeding a 16-bit frequency counter controlled by the Common Interface (CI). Voting
of the frequency data between paths is performed by the RTTS.
The pulse trains from all three independent paths are combined and trigger circuits which
cause the front-panel LEDs to flash. The four red LEDs, corresponding to the pulse-input
channels, flash when pulse trains are being received. Four pairs of sockets are provided for
monitoring the input lines via 100k ohm resistors.
Figure 2-2 Pulse input section - Schematic block diagram