How do I log Modbus data to CSV, and trend a register over time?
Updated 18 September 2026
You do not need a SCADA system or a laptop to trend a Modbus value. Read the registers you care about, name them once so the numbers mean something, and export the result as a CSV you can open in Excel or email to whoever needs it. Easy Modbus does this from an Android phone: it saves the register map you build, so every export already has real names, scaling and units attached rather than a wall of raw numbers.
Why log to CSV at all
Two jobs come up constantly in the field and both are a CSV underneath. The first is proof — capturing what a meter or drive was doing at a moment, to send to a vendor or file with a commissioning record. The second is trend — watching a value move over minutes or hours to catch something intermittent. A phone that can read Modbus and write a CSV covers both without dragging out a laptop or standing up a logging server.
Make the numbers mean something first
A raw Modbus dump is a column of integers, and a column of integers is close to useless a week later. Before you export, give each register the four things the protocol leaves out:
- A name — “Supply air temp”, not “40007”.
- A data type and word order — so a 32-bit float reads as 72.5, not as two random halves. See why your value looks wrong.
- A multiplier — so a raw 725 becomes 72.5.
- Units — °F, kW, ppm.
Do this once and it is saved. Every later reading and every export carries it, which is the whole point of building a register map rather than re-deciphering the device each visit. See what a Modbus register map is.
Capturing a trend
For a snapshot, read the device and export — the CSV holds each named register with its value, type and units. For a trend, re-read on an interval and export the set; each export is timestamped, so a sequence of them lines up into a time series in Excel. Keep the interval sane: on a serial chain each read costs real wire time, and hammering a device that the building controls also rely on is a way to make enemies. See why is my Modbus reading slow or unreliable?
What the CSV is good for
- Open it in Excel and chart a column to see a value drift or spike.
- Send it to a vendor as evidence of what the equipment reported, with names they can read.
- Leave it as documentation — a CSV of a device's real, named registers is worth more to the next person than the undocumented device they would otherwise inherit.
The CSV lives on your phone and in the email you choose to send. Easy Modbus has no account and no server, and a register map names a customer's equipment — so nothing leaves the phone unless you send it.