Eight transmitters share one twisted pair (RS-485, wires A and B). They cannot all talk at once, so the SCADA master asks them one at a time: it sends a request, only the addressed transmitter sends a response, and the SCADA screen updates. Watch the packets travel, read the bytes, then change the process values and cause faults.
Frame inspector: the actual bytes on the wire
REQUEST from master
RESPONSE from transmitter
blue address, orange function code, purple register start and quantity, green data, grey CRC error check, red damaged byte.
SCADA screen (click a tag to trend it)
Trend
blue = what the SCADA shows (only changes when a poll succeeds), grey dashed = the real process value. Last 120 s.
1Set the process values
Untick “on” to unplug a transmitter from the bus (cable break or power loss).
2Choose how values are sent
3Communication settings
The animation is slowed down. The real times (in ms) are shown below and depend on the baud rate.
What is happening now
Bus statistics
One poll (request + answer), real time
All 8 transmitters (one scan), real time
Each SCADA value refreshes every
Good polls
CRC errors (damaged replies)
Timeouts (no reply)
Faster baud rate means shorter frames and a faster scan. A disconnected transmitter costs a full timeout (200 ms) in every scan.
How Modbus works in short
One master, many slaves. Only the master starts a conversation. Each transmitter has its own address (1 to 8 here) and only answers when its address is in the request.
Registers hold the numbers. A transmitter keeps its measurement in a holding register (40001 is register 0). Function code 03 means read holding registers.
The master converts back. The SCADA knows the transmitter range (for example 0 to 200 C) and scales the raw number into engineering units.
CRC catches damage. Two CRC bytes are added to every frame. If noise flips even one bit, the CRC no longer matches and the frame is thrown away.
Quality matters. If a transmitter does not answer, the SCADA keeps the last value but shows it as STALE, then BAD after 3 failed polls.
Modbus RTU Simulator
The Modbus RTU Simulator is an interactive learning tool that demonstrates how multiple field transmitters communicate with a SCADA or PLC system using RS-485 communication.
Explore how a master device sends requests to eight transmitters, receives responses, and updates process values on the SCADA screen. Observe Modbus data frames, register values, function codes, and CRC error checking.
Change baud rates, introduce electrical noise, disconnect transmitters, and experiment with different data formats to understand communication faults and troubleshooting.
This simulator is ideal for instrumentation engineers, technicians, and students learning Modbus RTU, RS-485, and industrial communication systems.
Try the simulator below to see how Modbus communication works in real time.
