Follow the request from the PLC 110 master through itsRS-485-25. Error 81 identifies the failed transaction at the PLC, but its precise meaning must be read from the PLC 110 diagnostic table.
Where Does the Request Stop?
It does not prove that the PLC cable, polarity, port configuration, Modbus mode, or request is correct.
| Path element | Configured role or value | Proof required |
|---|---|---|
| PLC 110 | Modbus master | A request is transmitted from the selected port |
| PLC port | RS-485-2 |
Transmit activity appears on this physical port |
| Network | Two-wire RS-485 | Correct polarity, common reference, and stable idle state |
Run one request at a time and observe the PLC transmit and receive indicators or a serial capture. No transmit activity points to PLC task, port, or master configuration. Transmit without any returned bytes moves the investigation to wiring, framing, mode, address, or response timing. Returned bytes followed by error 81 point to parsing, function, register range, or response validation. The check passes when the PLC transmits on RS-485-2 and receives a reply frame.
Is the RS-485 Physical Layer Connected Correctly?
Layer one first. Disconnect other nodes while commissioning this point-to-point link. If no reply appears, reverse only the differential pair and retest; do not change protocol settings at the same time.
- Confirm that the PLC is wired to
RS-485-2, the same port selected in the project. - Connect the signal reference where required by the equipment wiring diagrams.
- Check for duplicate termination, missing bias, shorts, shield faults, and excessive common-mode voltage.
- Remove the PC interface before connecting the PLC so that two masters cannot drive the pair.
A PC test is most useful when it uses the same device connector and field cable. If the PC works only through a different cable or converter, the untested PLC segment remains suspect.
Do the Serial Frame Settings Match?
RS-485 carries bytes; it does not define their baud rate or framing. A mismatch can make a valid reply look like silence or corrupted data.38400baud, no parity, and two stop bits.
| Baud rate | 38400 |
Set 38400
|
| Parity | NONE |
Select no parity |
| Stop bits | 2 |
Select two stop bits |
| Data bits | Ambiguous between the attempted 7 and 8
|
Read the applicable passport or duplicate the working PC framing |
Change only the data-bit selection while holding 38400, no parity, and two stop bits fixed. A serial analyzer or oscilloscope can distinguish a framing mismatch from a missing response. The check passes when received characters decode without framing or parity errors.
The installation information conflicts: one passport is reported to specify Modbus ASCII, while another document is reported to describe RTU. ASCII and RTU are different frame encodings and cannot communicate merely because baud rate, parity, and address match. An ASCII master sends printable hexadecimal characters in an ASCII frame; an RTU master sends binary fields with RTU frame checking.
| Observation | Decision |
|---|---|
| Applicable passport explicitly states RTU | Configure the PLC master for RTU |
| Documents conflict | Capture one successful PC request and identify its frame encoding |
| No reply in either mode | Return to polarity, framing, slave address, and timing checks |
Do not alternate data bits, protocol mode, and timing together. First reproduce the working PC mode, then change one variable per test. The check passes when the PLC receives a syntactically valid frame in the selected Modbus mode.
The configured request targets slave 5, starts at register 0, and uses Read Input Registers function 0x04.0x04, so substituting a holding-register function changes the request rather than testing the same data.
| Request field | Configured value | Commissioning check |
|---|---|---|
| Device type | Universal Modbus device | Master driver builds a standard Modbus request |
| Function | 0x04 |
Read Input Registers is selected |
| Starting register | 0 |
Resolve zero-based versus one-based notation |
| Destination | Real input module | Allocated data width matches the requested register count |
Register documentation may print the first register as one while the PLC driver expects an offset of zero. Inspect the outgoing frame to learn the actual transmitted address; the configuration-screen label alone is insufficient. Request only a documented contiguous range and confirm that the destination has enough space. The check passes when slave 5 returns a normal 0x04 response rather than silence or a Modbus exception.
How Should Timing and End-to-End Verification Be Set?
A slow instrument can be electrically and logically correct yet fail when the master polls again before the transaction or internal measurement cycle is ready. A two-second polling interval was identified as a commissioning trial. Values 1500 and 10000 also appear in the PLC setup, but their field names are not identified; verify their meanings in the project instead of treating them as timeout or interval values.
- Set the serial port to
38400, no parity, two stop bits, and the data-bit value confirmed from the working PC path. - Select the confirmed ASCII or RTU mode.
- Address slave
5and issue function0x04from starting register0, subject to the driver’s address convention. - Use a two-second poll interval as an isolation test and allow one request to finish before starting another.
- Record the raw transmitted request, raw response, response time, PLC status, and mapped value.
- After stable operation, adjust timing only within the limits documented for the instrument and PLC master.
The check passes only when repeated polls produce one valid reply per request, error 81 remains cleared, and the mapped real input changes plausibly with the measured process.
FAQ
No. Configure the PLC for the mode stated by the applicable passport, or identify the mode from a successful PC capture when documents conflict.
Does error 81 prove that the RS-485 wiring is wrong?
No. Decode 81 with the PLC 110 diagnostic table, then separate no-transmit, no-reply, framing, and Modbus-response failures by observing the bytes on RS-485-2.
The stated supported request is Read Input Registers 0x04. Use 0x04 and a documented register range.
Does register 0 always mean the first documented register?
No. Confirm whether the PLC driver uses zero-based offsets or one-based register notation by inspecting the address transmitted in the request.
Can I verify the repair without a protocol analyzer?
Yes. Poll slave 5 with 0x04 at a two-second test interval, confirm one reply per request, verify that error 81 clears, and check that the mapped real input follows the process value.