Work out Modbus RTU character time, the 1.5 and 3.5 character gaps, and how long it takes to poll every device on a serial line.
Time to poll every device once
4,605ms
Check the values to see a result.
tchar = (1 + 8 + parity + stop) ÷ baudt3.5 = 3.5 × tchar, or 1.75 ms above 19,200 bit/scycle = devices × (request + delay + response + t3.5)
Polling 30 devices at 9,600 bit/s takes 4,605 ms per round, 153.5 ms for each request and reply, or 13.03 complete rounds a minute.
Tip: Most of a polling cycle is often the devices thinking, not bytes on the wire. Measure the response delay rather than guess it.
A Modbus RTU line carries one exchange at a time: the master sends a request, the device thinks, the device replies, and the line stays quiet for at least 3.5 character times before the next frame. Add those up for every device and you have the time to poll the whole line once. At 9,600 bit/s, ten devices each returning ten registers take about half a second.
tchar = (1 + 8 + parity + stop) ÷ baud
Each byte travels as 11 bits: a start bit, eight data bits, a parity bit and one stop bit, or two stop bits with no parity. At 9,600 bit/s that is 1.146 ms per byte.
t3.5 = 3.5 × tchar; recommended 1.75 ms above 19,200 bit/s
A silence of 3.5 character times marks the end of a frame, and a gap of more than 1.5 inside a frame makes it invalid. Above 19,200 bit/s the serial line guide recommends fixed values of 1.75 ms and 0.75 ms.
exchange = (request + response) × tchar + delay + t3.5
Request and response lengths include the address and CRC: 8 bytes to ask for registers, and 5 plus 2 per register back. The device delay is how long it takes to start answering, and it is never shorter than t3.5.
cycle = devices × exchange
Polling is sequential, so the round time grows with every device added. Retries and timeouts add more.
Reading ten registers from each of ten meters, with 5 ms for each to respond, takes 46.82 ms per exchange and 468.2 ms for the round. Every meter can be read about twice a second.
Time to poll every device once 468.2 ms Open in the calculator
Fifty registers from each of twenty devices: each reply is 105 bytes, and the round takes 1,435 ms even at twice the speed. Longer reads of fewer, contiguous blocks are usually faster than many short ones.
Time to poll every device once 1,435 ms Open in the calculator
Thirty devices that each take 20 ms to answer and return fifty registers: 153.5 ms per exchange and 4,605 ms per round. At this size, splitting the devices across two lines halves the round.
Time to poll every device once 4,605 ms Open in the calculator
For 11-bit characters (even parity, one stop bit), with the time for one 8-byte request and a 25-byte reply of ten registers, allowing 5 ms for the device to respond.
| Baud rate (bit/s) | Character (ms) | t1.5 (ms) | t3.5 (ms) | One exchange (ms) |
|---|---|---|---|---|
| 1,200 | 9.167 | 13.750 | 32.083 | 366.7 |
| 2,400 | 4.583 | 6.875 | 16.042 | 183.3 |
| 4,800 | 2.292 | 3.438 | 8.021 | 91.7 |
| 9,600 | 1.146 | 1.719 | 4.010 | 46.8 |
| 19,200 | 0.573 | 0.859 | 2.005 | 25.9 |
| 38,400 | 0.286 | 0.750 | 1.750 | 16.2 |
| 57,600 | 0.191 | 0.750 | 1.750 | 13.1 |
| 115,200 | 0.095 | 0.750 | 1.750 | 9.9 |
The silent interval of 3.5 character times that separates frames. A receiver treats a gap that long as the end of a frame. At 9,600 bit/s with 11-bit characters it is 4.01 ms; above 19,200 bit/s the serial guide recommends a fixed 1.75 ms interval.
Add up the exchanges: one per device per round. More devices, longer replies, slow devices and retries all lengthen the round. For more readings per second, raise the baud rate, read contiguous blocks, or split the devices over more lines.
The Modbus serial line guide specifies 11: start, eight data bits, parity and one stop bit, or no parity with two stop bits. Many devices also accept no parity with one stop bit, but that is outside the specification.
Only up to the point where device response time dominates. At 115,200 bit/s the bytes take a fraction of a millisecond, and a device that needs 20 ms to answer sets the pace. Long cables and poor termination also limit the rate that works reliably.
A ZMB polls its own RS-485 line next to the equipment, so each Modbus device or small group gets a short, fast line instead of sharing one long bus.
