You just installed a Modbus TCP device. The manual says “set Unit ID to 1.” Your PLC configuration asks for a Unit ID. Another device manual says “Unit ID is ignored.” A third says “Unit ID must be 0xFF.” What is going on? Which one is right? All of them are right. The Unit Identifier — usually called… Read More »
You just downloaded a new device manual and it says “Modbus client” and “Modbus server.” Your RS-485 documentation says “master” and “slave.” Your PLC code has variables named slaveId. Which is right? Do you need to change your code? Are these different protocols? Short answer: no. The protocol is identical. The terminology depends mostly on the transport you… Read More »
Does anyone actually use Modbus broadcast? Some do. Most engineers go their whole careers without touching it. Broadcast is one of those Modbus features that lives in the corner of the spec, gets one or two paragraphs in most books, and shows up unexpectedly when you have to commission a large multi-drop bus at 3 AM. This article… Read More »
Every Modbus TCP packet starts with 7 bytes called the MBAP header (Modbus Application Protocol header). Those 7 bytes are what turn Modbus from a serial protocol into a network protocol. Get them right and everything works. Get them wrong, and the server may ignore the request, close the connection, or return an exception. This article decodes a… Read More »
Standard Modbus TCP has zero security. No authentication. No encryption. No integrity checks. Anyone who can reach TCP port 502 can issue Modbus requests. Depending on the device configuration, this may allow reading registers, writing coils or registers, changing operating parameters, or disrupting industrial processes. For decades, the answer was “put it behind a firewall and hope for… Read More »
You read a float from a Modbus device. The value should be 25.3 degrees Celsius. You get 1.3 × 10³⁸. Or maybe -8.5 × 10⁻⁴¹. Or something that swings wildly between reasonable and absurd every time you re-read. Welcome to Modbus byte and word order. It is the single most common headache when you connect a new device.… Read More »
On an Allen-Bradley controller, “CIP” and “EtherNet/IP” are the same conversation. CIP is the protocol — the object model, the services, the way data is named. EtherNet/IP is CIP running on top of standard Ethernet. When you configure a Rockwell PLC to talk to a drive, a remote I/O block, a camera, or another controller, you’re configuring EtherNet/IP,… Read More »
Every FTP client on this list is free. Not free trial. Not free with locked features. Free. So the question is not “which one costs nothing.” It’s “which one fits the work you do.” A tech pulling event records from relays needs a different tool than someone running nightly backup scripts. This guide sorts the clients by job,… Read More »
FTP is the oldest protocol you will still find running in a substation. It moves files between two machines over TCP. That is the whole job. But the way it does the job — two separate connections, plain-text commands, a server that calls you back — trips up more engineers than any other legacy protocol. This article walks… Read More »
MQTT was born as a publish/subscribe protocol — messages flow from many publishers to many subscribers with no expectation of a response. This model fits telemetry perfectly: sensors publish, dashboards subscribe, nobody waits for anyone. But when you need one client to ask another client a specific question and get one specific answer back, pub/sub alone falls short.… Read More »