Allen-Bradley 1794-VHSC Counter Module 2 Channels 1MHz
Introduction:
The Allen-Bradley 1794-VHSC is a Flex I/O very high speed counter module. It offers 2 input channels with 2 counters per channel. This module supports encoder inputs up to 1MHz.
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Product Overview
The Allen-Bradley 1794-VHSC belongs to the Flex I/O 1794 series. This intelligent I/O module performs high speed counting for industrial applications. It provides signals to any compatible programmable controller with Ethernet capability. The 1794-VHSC supports encoder modes with 5V DC or 15-24V DC terminations. You can use this module for motion control, position monitoring, and flow measurement.
Input Specifications
The 1794-VHSC has 2 input channels with 2 groups of 3 inputs per counter. Each channel supports two independent counters. The module accepts 5V DC input voltage with appropriate termination.
Input voltage and current ratings:
Input off-state voltage: <1.8V DC
Input off-state current: <0.25 mA
Input on-state current: >5 mA
Input frequency maximum: 1 MHz
Current draw with 5V DC termination:
19.1 mA at 5V DC
25.7 mA at 6V DC
Current draw with 15-24V DC termination:
6.1 mA at 15V DC
10.2 mA at 24V DC
Off-state voltage varies by termination type. For 5V DC termination it is <1.25V DC. For 15-24V DC termination it is <1.8V DC.
Output Specifications
The 1794-VHSC provides 2 isolated groups of 2 outputs each. Total of four isolated output points.
Output electrical ratings:
Output supply voltage range: 10-31V DC
Output current at 5V DC: 0.5A
Output current at 12-24V DC: 1.0A
Off-state leakage current: <300 μA
Voltage loss at 0.5A (5V DC): 0.9V DC
Voltage loss at 1.0A (12-24V DC): 0.9V DC
Power Requirements
The 1794-VHSC requires an external power supply. Nominal voltage is 24V DC with a range of 19.2-31.2V DC. External supply current draw is 100 mA at 24V DC. The module also draws 75 mA at 5V DC from the FlexBus.
Power and thermal data:
Maximum power dissipation: 5.0W at 31.2V DC
Maximum thermal dissipation: 17.1 BTU/hr at 31.2V DC
The 1794-VHSC dissipates 5W of heat at maximum load
Isolation Specifications
The 1794-VHSC provides 50V isolation using basic insulation type. Isolation exists between 6 separate areas. Each area is tested at 850V DC for 1 second. This design protects the control system from field side noise and transients.
Compatible Terminal Bases
The 1794-VHSC installs on specific terminal base units. Compatible bases are:
1794-TB3G terminal base
1794-TB3GS terminal base (screw type with shielding)
The module uses keyswitch position 1. Always verify the keyswitch setting matches the terminal base before installation. Proper keying prevents incorrect module insertion.
Environmental Specifications
Temperature limits:
Operating temperature: 0 to 55 °C (32 to 131 °F)
Full operation capacity across entire range
Humidity and physical ratings:
Relative humidity: 5 to 95% non-condensing
Dimensions installed: 3.7 x 3.7 x 2.7 inches (94 x 94 x 69 mm)
The 1794-VHSC weighs approximately 0.30 kg
Installation Guidelines
Mount the terminal base onto a grounded DIN rail first. Set the keyswitch to position 1 on the terminal base. Push the 1794-VHSC straight into the terminal base until it locks. Connect the external 24V DC power supply to the terminal base terminals. Wire encoder inputs according to the termination voltage you select. Use shielded cables for high speed signals to reduce noise.
Do not exceed 1 MHz input frequency on any channel. Keep input off-state current below 0.25 mA to prevent false counts. The 1794-VHSC supports hot insertion under power.
Package Contents
One Allen-Bradley 1794-VHSC Flex I/O Very High Speed Counter Module
One keyswitch label (position 1)
Product identification label
Frequently Asked Questions (FAQ)
1. What is the maximum input frequency of the 1794-VHSC?
The 1794-VHSC accepts input frequencies up to 1 MHz. This supports very high speed encoder and pulse train applications.
2. How many input channels does the 1794-VHSC have?
The 1794-VHSC provides 2 input channels. Each channel has 2 groups of 3 inputs per counter.
3. What terminal bases work with the 1794-VHSC?
The 1794-VHSC works only with 1794-TB3G or 1794-TB3GS terminal bases. Standard Flex I/O bases are not compatible.
4. What is the output current rating per channel?
The 1794-VHSC outputs provide 0.5A at 5V DC or 1.0A at 12-24V DC. Outputs are isolated in two groups of two.
5. Does the 1794-VHSC support 24V DC encoder inputs?
Yes, the 1794-VHSC supports 15-24V DC terminations. You must use the compatible terminal base and proper wiring.