Serial Timing Calculator

From baud, parity and stop bits: character time, Modbus RTU 3.5- and 1.5-character times, request/response time, polling cycle and minimum response timeout.

Baud rate (bps)Data bitsParityStop bits

Function codeQuantity (registers/coils)Slave processing time (ms)Extra master gap (ms)Transactions per cycle (devices × requests)

Results
Bits per character
Bit time
Character (byte) time
Max bytes per second
t1.5 — allowed gap inside a frame
t3.5 — minimum silence between frames
Request
Normal response
Exception response (5 bytes)
One transaction (req + processing + resp + t3.5 + gap)
One cycle
Transactions per second
Minimum response timeout (processing + response)
Line utilisation

With a USB-serial adapter, received bytes reach the PC in batches (FTDI default 16 ms). Timeouts and frame splitting can lag this calculation by that much.

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Formulas

Bits per character = start 1 + data (7/8) + parity (0/1) + stop (1/2)
Character time = bits ÷ baud · 9600 bps 8E1: 11 ÷ 9600 = 1.146 ms
t1.5 = 1.5 × character time, t3.5 = 3.5 × character time (up to 19200 bps)
Transmission time = bytes × character time
Transaction = request + slave processing + response + t3.5 + master gap

An RTU receiver treats a gap longer than 1.5 characters inside a frame as an error, and 3.5 characters of silence as the end of the frame. So a sender sends the bytes back to back and waits at least t3.5 between frames.

Timing by baud rate (8E1, 11 bits)

Baud	Char time	t1.5	t3.5	Bytes/s	FC03 ×10 transaction*
1200	9.167 ms	13.75 ms	32.083 ms	109	334.583 ms
2400	4.583 ms	6.875 ms	16.042 ms	218	167.292 ms
4800	2.292 ms	3.438 ms	8.021 ms	436	83.646 ms
9600	1.146 ms	1.719 ms	4.01 ms	872	41.823 ms
19200	572.9 µs	859.4 µs	2.005 ms	1745	20.911 ms
38400	286.5 µs	750 µs	1.75 ms	3490	11.203 ms
57600	191 µs	750 µs	1.75 ms	5236	8.052 ms
115200	95.5 µs	750 µs	1.75 ms	10472	4.901 ms

* Request 8 + response 25 bytes + t3.5 with zero slave processing time — a lower bound. Real devices add processing time. Bold cells are the fixed values recommended by the specification.

RTU frame length by function

Function	Request bytes	Normal response bytes	Quantity limit
01/02 read coils/inputs	8	5 + ⌈N ÷ 8⌉	2000
03/04 read registers	8	5 + 2N	125
05/06 write single	8	8 (echo)	1
0F write multiple coils	9 + ⌈N ÷ 8⌉	8	1968
10 write multiple registers	9 + 2N	8	123

What response timeout should I use?

The specification leaves the response timeout to the application and notes it is typically 1 s to several seconds at 9600 bps, with a turnaround delay after a broadcast (address 0) of typically 100–200 ms. The «minimum response timeout» here is the theoretical floor — slave processing plus response transmission — so set the real value with the device's documented response time and any USB adapter latency added, with room to spare.

FAQ

How many bytes per second at 9600 bps?

With 11-bit characters (8E1 or 8N2) at most 9600 ÷ 11 ≈ 872 bytes; with 8N1 (10 bits) 960 bytes. In practice processing time between request and response and the t3.5 silence between frames make it less.

How long is the Modbus RTU 3.5-character time?

About 4.01 ms at 9600 bps and 2.005 ms at 19200 bps with 11-bit characters. Above 19200 bps the specification recommends a fixed 1.75 ms.

How long does a polling cycle over 10 devices take?

Enter 10 in «Transactions per cycle» and the device's documented response time (or a measured one) as slave processing time. Processing time often exceeds transmission time, so raising the baud rate may help less than expected.

Real request/response timestamps and gaps are visible in the traffic log of PortDock Modbus Master.