Does Modbus RTU require a gap between characters when transmitting?

Question

I'm a little confused about the inter-character gap in Modbus and whether its required when a master sends a message to a slave. The protocol spec says you can't have more than a 3.5 character gap between bytes when transmitting but is there any specific minimum amount of time you must have between bytes?

I've written a Modbus driver (master) that is able to communicate with a variety of devices and most don't seem to care about any gap between characters when receiving messages. However, I've come across a couple of devices where I was unable to communicate reliably without putting in a some kind of delay (measured in microseconds) between bytes, which is determined by the baud rate.

Is the character gap an absolute requirement or does it depend on the manufacture of the device and how the implement the Modbus protocol?

Answer 1

I have encountered uarts which require 2 stop bits on receive. If it set for 2 stop bits, this may explain the reqirement for a gap to extend thes# stop period beyond 2 stop bits.

Usually, only the first stop bit is checked on receive to determine framing error regardless of the stop bit setting.

Answer 2

Does Modbus RTU require a gap between characters when transmitting?

No.
In fact the Modbus spec states in section 2.5.1.1 MODBUS Message RTU Framing that "[t]he entire message frame must be transmitted as a continuous stream of characters."
Requiring intercharacter gaps would be contradictory to specifying "a continuous stream".

The protocol spec says you can't have more than a 3.5 character gap between bytes when transmitting ...

You're misquoting the protocol spec.
Only a 1.5 character gap is tolerated between characters in a RTU message.

From the Modbus spec: "If a silent interval of more than 1.5 character times occurs between two characters, the message frame is declared incomplete and should be discarded by the receiver."

A silent (idle) line with a duration of 3.5 characters must precede a message.
IOW a gap of 2 (i.e. more than 1.5 and less than 3.5) characters would prematurely end the current message, and the following characters (of that malformed message) would not be considered the start of a new message and must be discarded (until the line goes idle for at least 3.5 characters).

... is there any specific minimum amount of time you must have between bytes?

The Modbus spec does not mention any such requirement.
Such a requirement would be impractical.

UARTs (typically) do not have a capability to meter its output by inserting a delay between the transmission of character frames.
Adding such a delay is an additional processor burden as well as the use of a timer.
On the contrary UARTs have evolved to transmit characters as fast as the baudrate allows with the least processor intervention, e.g. hardware FIFO and DMA.

A "minimum amount of time you must have between bytes" is simply a reduction in the effective data rate.
Therefore an appropriate reduction of the baudrate would accomplish the exact same data rate.

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Is the character gap an absolute requirement or does it depend on the manufacture of the device and how the implement the Modbus protocol?

No, you are probably using too fast a baudrate for the device/circumstances in question.

A microprocessor or microcontroller should be able to keep a UART busy and transmit without any intercharacter gaps.
A UART that requires gaps during receiving is IMO an overloaded system and is broken.
For reliable communication (without flow control) use a baudrate low enough so that metering the transmitted characters is not necessary.

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Addendum

Apparently there is at least one UART (from Siemens) that can meter its output by holding the line idle for N bit-times between character frames.

Answer 3

It is at the end of the message that there should be a pause of 3.5 characters or longer.

Usually in the data transmission protocols in the first positions of the byte sequence, the number of bytes that follow is included, but Modbus RTU does not send that length and what determines when a message ends is the pause of 3.5 characters.

If you send the sequence of bytes at once there should never be any pause between characters

If you are writing a Master you should not worry about this, since it is the slave that must wait 3.5 characters to know when the master request is finished. You from the master side simply wait for the slave to reply since you know how many bytes the slave is going to send, in the request you already sent how many bits or 16bit words you want to read.

And if you have communication problems with some devices, it is probably due to the combination of communication speed and poor quality of the line. Try a lower baud rate, but adding wait between characters for me doesn't make much sense.