Field Engineer’s Notes (From the Trenches)
The “Gotcha” is 1-5V Scaling vs 4-20mA and Shunt Resistors.
1-5V “Current” Mode: The ALG320 has an internal 250Ω shunt for 4-20mA. If you have a voltagetransmitter (0-10V) but want to use the “4-20mA” range for convenience (scaling 4mA=0%, 20mA=100%), you can’t just land 0-10V on the terminals set to “4-20mA”—you’ll fry the input (rated for 30V max, but 10V across 250Ω = 40mA, which is ok, but the scaling is wrong: 10V -> 40mA reading, which the card sees as Overrange if set to 20mA max).
- Correct Method for 1-5V (representing 4-20mA): Set the channel to “Voltage – 1-5V” (if available in your firmware/config) or use “0-10V” and apply a linear scale in the PLC (1V=4mA, 5V=20mA). Do notuse “4-20mA” range with a voltage source unless you add an external shunt and understand the math.
Wiring 2-Wire Transmitters: For 4-20mA loop-powered (2-wire) transmitters: Land Transmitter (+) to Input (+), Transmitter (-) to Input (-). The ALG320 sources the loop voltage (via its 24V DC supply from the BIU) through the 250Ω shunt. Ensure the BIU has enough 24V DC capacity for all loop-powered transmitters on the station (typical draw: 20-30mA per transmitter + module draw).
Filter Setting: Default is often “No Filter.” In a VFD-heavy area, set it to 8ms or 16ms. A 0ms filter will make a 4-20mA flow signal look like a sawtooth on trend due to 60Hz ripple. Don’t over-filter fast signals (position feedback); 4ms is usually safe for process vars.
Real-World Applications
- Ammonia Refrigeration Skid (Food Plant): An ALG320 on a CHS001 base reads 4 x 4-20mA transmitters: Suction Pressure, Discharge Pressure, Liquid Level (Float), and Saturated Temp. The 16-bit resolution catches a 0.2 PSI drift in the suction pressure, allowing the lead/lag pump staging logic to react before the setpoint is violated. Open Wire detection caught a chewed wire on the level transmitter before the vessel overfilled.
- Power Plant Precipitator Rapper Control: Using the ALG320 to read 4-20mA from the High Voltage Rectifier Current Transformers. The 5ms update rate is fast enough to catch the half-wave ripple for diagnostics, while the 16ms filter rejects the EMI from the 60kV precipitator pulses.
High-Frequency Troubleshooting FAQ
Q: Module LED is Blinking, BIU shows “Config Mismatch” or “Fault” for this block.
A: Config Not Downloaded or Station Number Issue.
- Ensure the Station Number set via the BIU/HHP matches the I/O definition in the BIU’s config for this block.
- The stores its config in the BIU’s memory, not on the module. If you replaced a block, the BIU should auto-download the config when it powers up (if “Auto Config” enabled).
- If not, use the Hand-Held Programmer (HHP) to “Send Configuration” from the BIU to the block, or power cycle the BIU station to force a re-download. Verify the Input Range (e.g., 4-20mA vs 0-10V) is set correctly in the config.
Q: Reading “0” or “32767” (Full Scale) on a channel, wiring checks out, transmitter is alive.
A: Range Mismatch or Wiring Mode.
- If set to “4-20mA” but you wired a “Self-Powered” (4-wire) transmitter that outputs 4-20mA (has its own 24V supply): You must land the transmitter’s 24V supply common to the ‘s Input (-) terminal AND to the system 0V. The ‘s internal shunt still measures the current. If the transmitter’s 24V is floating relative to the ‘s 0V, you get a ground loop or offset.
- If set to “0-10V” but measuring a 4-20mA loop (using external 250Ω shunt to make 1-5V): Ensure the shunt is landed correctly (Signal+ to Shunt+, Shunt- to Input-, Input+ to Shunt+). Set range to “1-5V” or “0-10V” and scale in PLC.
- Check Input Range Config: Accidentally setting “0-5V” for a 1-5V signal (4-20mA equivalent) will read 50% low (1V in reads as 20% of 0-5V scale instead of 0% of 4-20mA scale).
Q: “Open Wire” alarm on a channel, but DMM shows continuity through the transmitter (low ohms).
A: Open Detect Current vs Live Transmitter.
- The sources a small test current to check for open circuit when the loop is de-energized or the transmitter is off.
- If the transmitter is powered (loop alive) and you see “Open Wire,” check the loop polarity (swapped + and – can sometimes confuse the detector on certain firmware versions, though usually it just reads negative) or check for a very high source impedance in the transmitter’s output stage (rare).
- More commonly: The “Open Wire” is real, but your DMM on “Ohms” is powering the loop momentarily, showing continuity. Disconnect the transmitter leads from the , measure resistance between + and – terminals on the module(with power off). It should read ~250Ω (the shunt). If open, the module’s shunt is damaged (rare) or the terminal block pin is bent.
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