Baud Rate / Cycle Time Calculator

This calculator helps determine communication timings for serial field buses like Modbus RTU, Profibus DP, etc. It calculates bit time, character time, message transmission time, and estimates polling cycle time for multiple devices, incorporating real-world factors like slave delays and network overhead.

Communication Parameters

Protocol Type

Baud Rate

Data Bits

Stop Bits

Parity

Message & Network Parameters

Request Message Length (Bytes)

Response Message Length (Bytes)

Number of Devices (Slaves)

Slave Device Response Delay (ms)

Network Propagation Delay (ms)

Inter-Message Overhead Time (Value)

Overhead Unit

Communication Timing Results

Timing Status: N/A

Parameter	Value

Applicable Standards and Guidelines

Understanding communication timings is crucial for designing reliable and responsive industrial control systems.

Baud Rate: The number of signal changes, or symbols, per second. For simple binary signaling, it's equivalent to bits per second (bps).
Character Frame: In serial communication (like RS-485), each character is transmitted within a "frame" that includes: 1 Start Bit, Data Bits (e.g., 8), an optional Parity Bit, and Stop Bits (e.g., 1).
Inter-Message Overhead: Time delays between messages. For Modbus RTU, this is typically 3.5 character times (T3.5) to signify the end of a message.
Slave Device Response Delay: The internal processing time of the slave device before it begins transmitting its response.
Cycle Time: For polling-based networks (like Modbus RTU master-slave), the cycle time is the total time it takes for the master to poll all slave devices and receive their responses.
Protocol Specifications: Always refer to the specific fieldbus protocol's documentation (e.g., Modbus Application Protocol) for exact timing requirements.

This tool provides a refined estimation of communication timings. Actual performance can be influenced by network topology, cable quality, and device processing delays.

Engineer's Guide to Serial Comms & Network Timing

In industrial automation, serial communication protocols like Modbus RTU (over RS-485) remain the backbone for connecting field devices (sensors, VFDs, actuators) to controllers (PLCs, DCS). While newer Ethernet-based protocols are faster, serial networks are simple, robust, and cost-effective. However, their performance is entirely dependent on timing. Understanding the relationship between baud rate, message size, and protocol overhead is the key to designing a stable and responsive control system.

Baud Rate vs. Bit Rate vs. Cycle Time

These terms are often confused, but they mean very different things.

Baud Rate: This is the "speed" of the wire, measured in symbols per second. A symbol is a change in the electrical state (e.g., a voltage change). In simple binary systems like RS-485, one symbol represents one bit, so Baud Rate = Bits Per Second (bps). 9600 baud means the system can transmit 9,600 bits every second.
Bit Time: This is the duration of a single bit: $Time_{bit} = 1 / \text{Baud Rate}$. At 9600 baud, one bit takes $1/9600 = 0.000104s$, or 104 microseconds (μs).
Cycle Time: This is the *total time* it takes for a master (PLC) to complete one full "round" of polling all the slave devices on the network. This is the value that matters for your control loop. A 10-second cycle time is fine for monitoring a tank farm, but disastrous for controlling a high-speed packaging machine.

Key takeaway: A high baud rate (e.g., 115,200 bps) does *not* guarantee a fast cycle time. A poorly designed network with large messages and long delays can be slow, even at high speeds.

The Anatomy of a Serial "Character" (Byte)

In serial communication, you don't just send 8 bits of data. You send a "frame" that includes overhead bits for synchronization and error checking. This is the single most important concept for timing calculations.

A typical asynchronous frame (like in Modbus RTU) consists of:

1 Start Bit: Always 1 bit. Signals the start of a new character.
Data Bits: Typically 8 bits (to send one full byte).
Parity Bit: Optional (0 or 1 bit). Used for basic error checking.
Stop Bits: Typically 1 or 2 bits. Signals the end of the character.

Total Bits per Character = 1 (Start) + Data Bits + Parity Bits + Stop Bits

A "standard" 8-N-1 (8 data bits, No parity, 1 stop bit) configuration actually takes 10 bits to send on the wire. This calculator computes this for you. At 9600 baud (104.17 μs/bit), one character (byte) takes $10 \text{ bits} \times 104.17 \mu s = 1.0417 \text{ ms}$.

The "Gaps": Where Does All the Time Go?

A network's cycle time isn't just the sum of its message transmission times. The real-world performance is dominated by the "silent time" or "gaps" between messages. This calculator accounts for all of them.

A single master-slave poll consists of 5 distinct phases:

Request TX Time: The time to physically send the master's request (e.g., 8 bytes).
$$T_{req} = (\text{Request Bytes}) \times (\text{Character Time})$$
Propagation Delay (x2): The time for the electrical signal to travel from the master to the *farthest* slave. This happens twice (once for the request, once for the response). It's very small for short RS-485 runs but can be significant (milliseconds) in long-distance networks.
Slave Response Delay: This is the time the slave device takes to *process* the request and formulate its response *before* it starts transmitting. This is a critical value found in the device's datasheet and can range from <1 ms to >100 ms.
Response TX Time: The time to physically send the slave's response (e.g., 20 bytes).
$$T_{resp} = (\text{Response Bytes}) \times (\text{Character Time})$$
Protocol Overhead (Inter-Message Gap): The "silent time" the master must wait before sending its *next* poll. For Modbus RTU, this is famously the T3.5, or the time it takes to send 3.5 characters. This gap is a "bus-free" condition that signals the end of one transaction and the start of the next.

Putting It All Together: The Full Cycle Time

The total time for one poll is the sum of all these parts:

$$T_{poll} = T_{req} + T_{prop} + T_{slave\_delay} + T_{prop} + T_{resp} + T_{overhead}$$

The Total Polling Cycle Time is this value, multiplied by the number of devices you are polling in the loop.

$$T_{cycle} = T_{poll} \times (\text{Number of Devices})$$

This is the number that your PLC's control loop must be able to tolerate. If you have 20 devices, each with a 50ms slave delay, your *minimum* cycle time is already $20 \times 50\text{ms} = 1 \text{ second}$, and that's *before* any data is even transmitted!

Network Utilization & Best Practices

Network Utilization is a measure of how "busy" the bus is. It's the ratio of "time spent sending useful data" to the "total cycle time." A high utilization (> 70-80%) is dangerous. It means your network has no "breathing room" to handle a retry if a message is corrupted. This leads to dropped packets and intermittent failures.

How to Improve a Slow Network:

Increase the Baud Rate: This is the single biggest fix. Moving from 9600 to 115200 bps makes every bit *and* every T3.5 gap 12 times shorter. Ensure all devices and your cable (e.g., shielded, twisted-pair) can support the higher speed.
Optimize Polling: Does every device need to be updated every cycle? Poll critical control devices (like a VFD) every cycle, but poll non-critical monitoring devices (like a tank temperature) every 10 cycles.
Reduce Message Size: Don't poll for 100 registers if you only need 2. Smaller messages transmit faster.
Segment the Network: If you have 100 devices, don't put them all on one RS-485 bus. Use 4 separate bus gateways, each handling 25 devices. This divides your cycle time by 4.
Check Slave Delays: If one old, slow device has a 200ms response delay, it is holding up the *entire network*. Replace it or move it to its own, separate, slow-speed bus.