Industrial Engineer · phase 5 of 16 · noun
Holding / Input Registers
Sixteen-bit read-write and read-only data areas in the Modbus data model.
The Explain card
- Plain English
- Holding registers are sixteen bit values you can read and write, and input registers are sixteen bit values you can only read. Setpoints and parameters usually live in holding registers, measurements in input registers.
- Example
- A drive exposes its speed reference as a holding register the PLC writes, and its output current as an input register the PLC reads.
- Why it matters
- A thirty two bit value spans two registers, and vendors disagree about which half comes first, so a float that reads as garbage usually needs its words swapped. The offset by one confusion applies here too.
- Hook
- The numbers, in two tables, one of them read only.
Word knowledge
How the word is built, where it came from, and what it sits beside in memory.
In a sentence
The PLC writes the speed setpoint to Holding register 40001 before commanding the variable frequency drive to run.
How it is built
- hold root keep or retain
- -ing suffix forms a noun for an action, result, or function
Where it came from
- Origin
- Old English healdan, meaning to keep, possess, or retain
- Entered the language
- 15th century
- What changed
- The verbal noun first referred to possession or retention, then developed specialized uses for places or structures that keep something available.
How it is spelled
- Pattern
- The base word hold keeps its spelling when -ing is added.
- Pattern
- The final consonant is not doubled after the consonant cluster ld.
Spelled like
- folding
- scolding
- upholding
Broken into chunks
-
hold
- holder
- withhold
- uphold
-
-ing
- reading
- writing
- scaling
What it sits beside
Same subject
- Input Registers
- Modbus
- register address
- function code 03
- PLC memory
Same shape
- folding
- molding
- scolding
Modbus data areas
- coils
- discrete inputs
- Holding Registers
- Input Registers
Where it sits in the deck
Phase 5: PLC Industrial Communications
The protocols that let controllers, drives and operator panels talk, plus the addressing and description files that make them agree on what was said.
