You read the address straight out of the manual, and the value comes back wrong — or one register late. This is the single most common PowerFlex Modbus problem, and it is not a fault in the drive. Here is what actually happens, which function codes the drive answers, and how to settle it on a live drive in a few minutes.
Windows 10/11 · RS-485 adapter · No PLC needed to test
Modbus has never agreed with itself about where counting starts. Some masters number holding registers from 0, others from 1. The wire protocol always sends the zero-based number, so the same physical register can legitimately be called 8192 or 8193 depending on which tool you are holding.
Rockwell says so plainly in the 520-series user manual: the addresses in the tables may need to be offset by +1 depending on the master, and it gives the example that Logic Command is register 8192 on a ProSoft scanner but 8193 on a PanelView.
So when a colleague insists the command word is at 8193 and your gateway needs 8192, you are both right. What you need is not another opinion — it is one confirmed reading from the actual drive.
Read a register whose value you can predict — the status word of a stopped, healthy drive. If the answer looks like a status word, your offset is right. If it is garbage or an exception, shift by one and read again.
| Hex, as documented | 2000H |
| Zero-based decimal | 8192 |
| One-based decimal | 8193 |
| 5-digit convention | 48193 |
Before hunting for parameter addresses, get these four facts straight. They account for most of the failed first attempts.
| Fact | Detail | What goes wrong without it |
|---|---|---|
| Read with FC03 | Read Holding Registers | FC04 (Input Registers) returns an illegal data address exception — drive parameters live in the holding register space |
| Write with FC06 or FC16 | Single or multiple registers (16 decimal = 10 hex) | Other write codes are not supported by the drive's serial interface |
| Logic Command | Register 2000H | Writes are accepted but ignored unless the start source is set to Serial/DSI |
| Logic Status | Register 2100H (2101H one-based) | Reading the wrong word gives plausible-looking nonsense instead of an obvious error |
A PowerFlex register has one physical location and at least six written forms. Nothing below is a different register — they are all the same word, written the way a particular tool expects it. Find the column that matches your master and read across.
| Register | Hex as documented |
Decimal 0-based |
Decimal 1-based |
5-digit 4xxxx |
6-digit 4xxxxx |
|---|---|---|---|---|---|
| Logic Command write, FC06/FC16 |
2000H | 8192 | 8193 | 48193 or 48192 | 408193 or 408192 |
| Logic Status read, FC03 |
2100H | 8448 | 8449 | 48449 or 48448 | 408449 or 408448 |
The two registers above are the ones worth memorising. For everything else, the arithmetic is the same every time — this is the recipe, with 2000H worked through as the example.
| Form | How to get there | Example: 2000H |
|---|---|---|
| Hex, as printed in the manual | starting point | 2000H |
| Decimal, zero-based | convert hex to decimal | 8192 |
| Decimal, one-based | add 1 | 8193 |
| 5-digit holding register | 40000 + one-based | 48193 |
| 6-digit holding register | 400000 + one-based | 408193 |
| 3xxxx or 3xxxxx | do not use | input registers — the drive answers with an exception |
The status and command words are bit-packed, so there is no separate address per bit. Read the whole 16-bit register and mask in software: bit_n = (value >> n) & 1. Some masters also accept dotted notation such as 408449.7 to pull one bit out directly — convenient, but it is the same single read underneath. Viewing the register in binary is usually faster than either, because every bit is visible at once while you operate the drive.
This is the fastest way to pin down addressing and scaling on the drive in front of you. No PLC program, no download to the controller — a laptop, an RS-485 adapter and a Modbus master.
8448 — then at 8449. One of the two returns something that behaves like a status word: stable while the drive is idle, changing bits when you start or stop from the keypad. That single observation fixes your offset for every other address on the drive.You will not find a full parameter-to-register table here, and that is on purpose. Parameter numbering differs across the 520-series and between firmware revisions, and the secondary sources that publish these tables disagree with each other — including on the speed reference address and its scaling.
The authoritative answer is in Rockwell's own documentation for your exact drive and firmware: the 520-series user manual (520-UM001) and the accompanying reference and programming manuals. Anything else, including this page, is a second opinion.
What a tool adds is not a better table. It is the ability to confirm the table you have against the drive on the bench, in minutes, before the number ends up in a PLC program that someone else will inherit.
Take addresses from the manual. Take the offset and the scaling from the drive. Never take either from a forum post — including this one.
Read the status word, watch the bits move, and write the command word — from a laptop, before the PLC program exists. Free for 14 days, no credit card.
Windows 10/11 · Modbus RTU, ASCII and TCP · One-time license
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