MODBUS, is there a maximum time a device can take to respond?

Question

In talking to a MODBUS device, is there an upper bound on how long a device can take to respond before it's considered a timeout? I'm trying to work out what to set my read timeout to. Answers for both MODBUS RTU and TCP would be great.

Answer 1

In MODBUS over serial line specification and implementation guide V1.0 section 2.5.2.1 MODBUS Message ASCII Framing There are suggestions that inter-character delays of up to 5 seconds are reasonable in slow WAN configurations.

2.6 Error Checking Methods indicates that the timeouts are configured without specifying any values.

The current Modicon Modbus Protocol Reference Guide PI–MBUS–300 Rev. J also provides no quantitative suggestions for these settings.

Your application time-sensitivity, along with the constraints that your network enforces, will largely determine your choices.

If you identify the worst-case delays you can tolerate, take half that time to allow a single retransmission to fail, subtract reasonable transmission times for a message of maximal length, then you should have a good candidate for a timeout. This will allow you to recover from a single error, while not reporting errors unnecessarily often.

Of course, the real problem is, what to do when the error occurs. Is it likely to be a transient problem, or is it the result of a permanent fault that requires attention?

Alexandre Vinçon's comment about the ACKNOWLEDGEMENTs is also relevant. It may be your device does not implement this, and extended delays may be intended.

Answer 2

The specification does not mention a particular value for the timeout, because it is not possible to normalize a timeout value for a wide range of MODBUS slaves.

However, it is a good assumption that you should receive a reply within a few hundreds of milliseconds.

I usually define my timeouts to 1 second with RTU and 500 ms with TCP.

Also, if the device takes a long time to reply, it is supposed to return an ACKNOWLEDGE message to prevent the expiration of the timeout.