Difference Between Hard Wired Signal and Modbus

The Modbus protocol is used extensively to 'read' data from 'slave' devices for reporting and historian purposes. Modbus can also 'write' data to slave devices, giving it control capability.

Modbus is a master/slave protocol where a slave is silent unless spoken to. The implication is that an alarm or an 'event' on a slave device is not broadcast via Modbus because there's no provision for a slave to talk unless first spoken to. Hence, the master has to poll the slave and get the slave's response in order to find out about the alarm or event. Depending on the application, whatever time that takes might or might not be a serious consideration.

The Modbus standard is not be considered deterministic, with fixed and known timing intervals.

An issue in Modbus is the absence of slave's response which ties up the Master until the time-out interval expires.

The transmission medium, the physical transport layer, is typically RS-232 for short distance (<10m) point-to-point, RS-485 for longer distances (<1000m) or ethernet, any of which have some susceptability to ground faults, lightning strikes and electrical noise. The robustness of the physical layer is solely up to the implementator, because there are no Modbus wiring components, like there are for Profibus, for example.

Modbus is an open protocol so its implementation is up to whomever implenments it. It has only recently in the last couple years had a compliance process, yet the protocol has been around for 30+ years.

I'm not familiar with the operational or safety issues of control of switchgear, but the selection of a control communications has to take into consideration the full range of comm's performance. There might well be conditions where hard-wired controls (I'll define that as on-off or analog signals to dedicated I/O points over copper wire) is necessary because either the physical transport or protocol limitations of Modbus are not appropriate for the task.

None of this detracts from the hundreds of thousands of Modbus installations that successfully do what they're supposed to, within the limitations of what Modbus does and how it does it. Many of these control implementations have hard-wired ESD circuits required by electrical code.