Four data tables
- Coils: read/write bits, typically output commands.
- Discrete inputs: read-only bits, typically physical input states.
- Input registers: read-only 16-bit measurements.
- Holding registers: read/write 16-bit settings and state.
Modbus Register Map Workbench
Translate Modbus register labels into PDU addresses and choose valid read/write functions.
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Learner-ready Modbus register map workbench with four table types, Modicon-to-PDU address translation, read/write access checks, request/response PDU bytes, bit packing, scenario presets, quick reference, source links, and desktop/mobile visual verification.
Modbus Register Map Workbench
Pick a device table, translate the human label into the address actually carried in the Modbus PDU, and see whether the requested function code is legal for that table. The default example reads two input registers from labels 30002 and 30003.
Selected table
Input registers 3x
Access
Read-only 16-bit
Doc label
30002
PDU address
0x0001
1. Pick table
2. Translate
3. Request
4. Lookup
5. Response
6. Diagnose
Configure lookup
00001
30002
2 registers
25.1 C, 53.6 %RH
Table family
3x input registers
Read-only 16-bit measurement values.
Documentation label
30002
Human-facing labels are one-based and include the table family.
On-wire PDU address
0x0001
Modbus request PDUs carry the zero-based address, not the 3x/4x prefix.
Function check
FC04 is valid
FC04 reads input registers only.
Client request
Unit 17 reads two input registers beginning at PDU address 1.
Server lookup
The server maps address 1 to label 30002 in the input register table.
Response
The response returns a byte count followed by two big-endian register values.
Coils 0x
Read/write output bits. Examples: relays, lamps, motor run commands.
Discrete inputs 1x
Read-only input bits. Examples: limit switches, door contacts, alarms.
Input registers 3x
Read-only 16-bit measurements. Examples: temperature, humidity, pressure.
Holding registers 4x
Read/write 16-bit values. Examples: setpoints, modes, configuration.
Request PDU
Read request: function, starting address, and quantity.
Response PDU
Normal response: function, byte count, and register data.
Valid request: FC04 reads input registers
The PDU address 0x0001 selects documentation label 30002. The response contains two 16-bit values: 251 and 536.
Quick reference
Address translation
- Documentation labels are usually one-based: 30001, 40001, and so on.
- The PDU uses zero-based addresses: label 40001 becomes address 0x0000.
- The 0x/1x/3x/4x prefix is a table hint, not bytes sent in the PDU.
Function match
- FC01/FC02 read bit tables.
- FC03/FC04 read 16-bit register tables.
- FC05/FC15 write coils; FC06/FC16 write holding registers.
Technical accuracy notes
Bit packing
When FC01 or FC02 returns multiple bits, the first addressed bit is packed into the least significant bit of the first data byte. Extra bits in the final byte are padding.
Register values
Register data is carried as 16-bit quantities in big-endian byte order. Device manuals define scaling, units, and signed versus unsigned interpretation.
Real devices vary
A device may implement only a subset of addresses and functions. Valid table family does not guarantee that every address in that range exists.