Some slaves truncate response on CRC mismatch. Modbus Poll may still show partial data.
Diagnose:
Use case: Logging 40 holding registers from a solar inverter over RS485 (9600 baud, 8N1)
Old behavior (v8.x): Frequent “Bytes missing” errors, especially during polls every 200 ms.
New behavior (v9.2.1 build 1824): Rock-solid for 72 hours of continuous polling.
RS485 requires a twisted-pair cable with proper termination. A loose ground or a missing termination resistor creates reflections, corrupting the frame and dropping bytes.
If the problem persists, post the Modbus Poll communication log (Display → Communication → Save) and the exact expected vs. received byte count — that usually reveals the exact cause.
This error occurs when the Modbus Poll master expects a specific number of bytes based on the requested function code (e.g., Read Holding Registers) but receives a truncated or incomplete message from the slave device. Common Causes & Solutions Response Timeout (Too Short):
Fix: Increase the "Response Timeout" in the Connection > Setup menu. If the slave device is slow to process requests, the master may close the listening window before all bytes arrive. Incorrect Inter-character Timing (RTU Mode):
Fix: Modbus RTU requires a silence of at least 3.5 characters to signal the end of a frame. If your serial-to-USB converter or the slave device has "jitter," the frame might be cut off prematurely. Ensure the Baud Rate and Parity settings match exactly on both ends. Electrical Noise & Interference:
Fix: Use shielded twisted-pair cabling and ensure proper grounding. High electrical noise can corrupt the CRC (Cyclic Redundancy Check), causing the software to discard part of the message as "missing" or invalid. Buffer Overflows: modbus poll bytes missing error fixed
Fix: If using a Serial-to-Ethernet gateway, ensure the packetization timeout is set correctly. If the gateway sends data in fragments, Modbus Poll may interpret the first fragment as an incomplete (missing byte) message. Troubleshooting Checklist
Check the Communication Traffic: Use the Display > Communication window in Modbus Poll to see the raw Hex strings. Look for the RX (receive) line to see if the slave is actually sending fewer bytes than requested.
Verify Slave ID: Ensure you aren't receiving a "noisy" response from a different slave on the same bus.
Update Drivers: If using a USB-RS485 adapter, ensure you are using the latest FTDI or Prolific drivers.
The "Bytes Missing" error in Modbus Poll is a classic ghost in the machine. It’s the digital equivalent of someone hanging up the phone halfway through a sentence. Fixing it isn't just about a checkbox; it’s a masterclass in the fragility of industrial communication. The Anatomy of the Error
In the Modbus RTU world, timing is everything. Unlike a modern web page that waits for a full packet, Modbus relies on a strict silent interval (the 3.5-character rule) to know when a message ends. When you see "Bytes Missing," it means the master (Modbus Poll) expected a specific response length based on your request, but the line went quiet too soon. The Usual Suspects (and Their Fixes)
1. The "Cheap Hardware" TaxMost "Bytes Missing" errors are born in low-quality USB-to-RS485 converters. These devices often struggle with "direction control"—the split-second switch from sending to receiving.
The Fix: Use a converter with an FTDI chip or an "auto-direction" circuit. If you’re stuck with what you have, try decreasing the Baud Rate. Lower speeds are more forgiving of poor hardware timing. Some slaves truncate response on CRC mismatch
2. The Response TimeoutIf the Slave device is a bit slow (perhaps it’s an old PLC or a complex sensor), it might start sending data, pause to process, and then finish. Modbus Poll might see that pause as the end of the message.
The Fix: Increase the Response Timeout in your Connection Setup. Giving the device an extra 500ms can often bridge the gap.
3. Inter-character DelayThis is the "stutter" of the industrial world. If there is a delay between individual bytes within a single packet, Modbus Poll gets confused.
The Fix: In Modbus Poll’s setup, look for "Force Inter-character Delay" or similar advanced timing settings. Sometimes, adding a tiny buffer (1-2ms) helps the software "wait" for the trailing bytes of a slow packet.
4. Electrical Noise (The Invisible Wall)If your cable is running next to a Variable Frequency Drive (VFD) or a heavy motor, EMI can "chop" the end of your data packet.
The Fix: Ensure your twisted-pair cable is shielded and grounded at one end. More importantly, check for Termination Resistors (120 ohms) at both ends of the bus to prevent signal reflection. The Philosopher's Fix
If the hardware is perfect and the settings are right, look at the Request Length. Sometimes we get greedy and ask for 100 registers at once. If the Slave's internal buffer can only handle 32, it will simply stop talking halfway through. The Fix: Break your read requests into smaller chunks.
Fixing "Bytes Missing" is rarely about the software being "broken"—it’s about harmonizing the speed of light with the limitations of copper and silicon. Once you find that sweet spot of timing and shielding, the error vanishes like smoke. If you are polling every 20 ms, some slaves cannot keep up
Are you seeing this error on a USB converter, or is it happening over a long-distance serial run?
It is structured to identify the problem, explain the root causes, and provide a step-by-step solution.
If you are polling every 20 ms, some slaves cannot keep up.
Situation: A client had a Schneider Electric Altivar 32 VFD connected via RS485 to a PC running Modbus Poll. Every 22 seconds, "Bytes Missing" appeared.
Attempted old fixes:
The final fix applied:
Lesson: The bytes missing error was caused by the USB converter holding the last two bytes of the CRC in its buffer. The 1 ms latency timer forced an immediate flush.
Isolate the problem: