Topic
Modbus
16 guides across the knowledge base.
Communication Failures
What to read when a controller stops talking to a device, another controller, or SCADA: port and connection status, message error codes, timeouts and retries, addressing, limits, and the order of checks that separates controller, cable, device, and network.
8 min readUpdated Sep 5, 2026
Serial Communications
RS-232, RS-422, and RS-485 in practice: which is which, cable and distance, termination, biasing, the common reference wire everyone leaves off, and the settings that have to match at both ends.
9 min readUpdated Sep 5, 2026
Protocol Gateways
Protocol converters between a controller and devices that do not speak its language: what a gateway does, the common pairings in water and wastewater, the mapping table that is the whole configuration, the failure modes a gateway adds, and when to use a native interface instead.
9 min readUpdated Sep 5, 2026
SCADA Communications
How SCADA servers talk to controllers and the field: polling and report by exception, the protocols and what each is for, media from plant Ethernet to radio and cellular, poll rate arithmetic, timeouts and quality, time sync, store and forward, and security.
11 min readUpdated Sep 5, 2026
Deep Packet Inspection
What a firewall can do when it understands the control protocols: allowing reads and blocking writes, permitting programming only from one workstation in a window, rejecting malformed messages, and alerting on function codes that should never appear.
8 min readUpdated Sep 5, 2026
How to Configure a Modbus Connection
Set up a Modbus RTU or TCP link between a controller and a device: get the register map, match the physical and protocol settings, build the reads and writes, handle data types and the addressing offset, and verify with a protocol tool before trusting a value.
10 min readUpdated Sep 5, 2026
How to Diagnose Read and Write Communications
Diagnose a controller that cannot read from or write to a device: capture the exact request and response, classify the failure as no link, no answer, or a rejected exchange, read the error or exception code, compare with a test tool, and fix the port.
8 min readUpdated Sep 5, 2026
Tag Shows Bad Quality
A tag flagged bad or uncertain by the driver. What the quality codes mean, the difference between a device that is not answering and an address that does not exist, and how to clear each.
7 min readUpdated Sep 5, 2026
Device Times Out
A polled device that answers late or not at all: how to read the driver counters, tell a dead device from a slow one, and find the timeout setting, the bus loading, the radio latency, the duplicate address, or the device that is simply too busy to answer.
9 min readUpdated Sep 5, 2026
High Retry Count
Polls that succeed on the second or third try: a weak or noisy path, interference, collisions from a second master, a timeout shorter than the radio turnaround, a data rate the path cannot carry, or a slow controller. Reading retry statistics per site.
8 min readUpdated Sep 5, 2026
Read Works but Write Fails
Polling a device returns good data and writing a setpoint does nothing or returns an exception: a write-protected device, a read-only register, the wrong function code, a wrong address or type, logic that overwrites the value, a security rule, or a local mode.
8 min readUpdated Sep 5, 2026
Wrong Register Data
The poll succeeds and the numbers are nonsense: an offset by one, the wrong table, swapped words in a 32-bit value, a float read as an integer, signed read as unsigned, a scale factor not applied, a stale gateway value, or the wrong unit answering.
8 min readUpdated Sep 5, 2026
Protocol Gateway Stops Passing Data
The gateway is powered and pingable and the data behind it is frozen or gone: the device side failed while the master side answers, a stalled gateway, configuration lost on a power cycle, a stale mapping, or a master polling too fast. Testing each side.
8 min readUpdated Sep 5, 2026
Modbus Device Intermittently Offline
A device that answers most of the time. Termination, bias, timing, and the physical layer details that produce a bus which almost works.
9 min readUpdated Aug 21, 2026
Modbus TCP
Modbus over Ethernet: what the MBAP header changes, why the CRC disappears, how the unit identifier works, and what does not get easier just because it is on Ethernet.
8 min readUpdated Jul 28, 2026
Modbus RTU
Serial Modbus over RS-485: function codes, register addressing, the off-by-one that catches everyone, and the physical layer details that decide whether a bus is stable.
10 min readUpdated Jul 21, 2026
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