FMR43 ISDU Reading: Decode IFM AL1920 SINT Data Types

The SINT[32] response is a raw transport buffer, not a typed process value. The read operation can succeed while Studio 5000 still displays unusable numbers because the controller does not know whether a given response represents UINT8, INT, DINT, or text. Resolve the transaction first, then decode only the returned payload according to the FMR43 parameter definition.

Stop applying the usual quick fixes

Do not change the destination tag to a larger integer and expect the value to become readable. A numeric move from SINT to INT or DINT converts one signed number; it does not assemble multiple bytes into the device value. Likewise, displaying all 32 elements only exposes the wire representation.

Quick fix	Why it fails	Correct next check
Use a numeric move on the first element	It converts one byte and may sign-extend it.	Read the parameter's declared type and byte length.
Read the cyclic input assembly	Indexed service parameters are acyclic and may not exist in cyclic process data.	Check the master service request transaction.
Copy all 32 bytes into one value	The buffer can contain unused bytes or protocol fields, and the destination has a different size.	Identify the payload offset and returned length.
Assume a fixed byte order	A reversed byte sequence can produce a valid-looking but wrong number.	Compare the bytes with a known parameter value.
Search for one universal conversion AOI	The byte array does not carry enough Logix type information to select every conversion automatically.	Build a type-specific decoder driven by parameter metadata.

Check whether the service transaction completes

Start with the IFM request status, not the payload. Record the selected AL1920 port, completion indication, error indication, returned byte count, and the first returned bytes. A changing buffer without a valid completion state is not a trustworthy result.

1. Trigger one read request. Do not continuously retrigger it every scan.
2. Wait for the master interface or AOI to report completion before consuming the buffer.
3. If the request reports an error or returns no bytes, stop decoding. Check the physical port, device connection, request path, and service-channel configuration.
4. If the request completes and returns bytes, retain that response and continue with the parameter checks.

The AL1920 provides a separate service request channel for each device channel. The interface may describe this as an SDO request channel even though the device operation is an IO-Link indexed service read. The request supplies the Index, Subindex, and requested length, then invokes the read operation.

If the IFM AOI already performs that transaction, keep it as the sole transaction owner. If it only maps data or does not expose the required service request, use a Studio 5000 explicit message to the documented AL1920 request address for the selected port. Obtain the exact message path, address, and control layout from the AL1920 documentation; do not guess those fields.

Match the Index, Subindex, and length

Open the FMR43 ISDU parameter table beginning on page 45 and record four fields for the target parameter: Index, Subindex, declared data type, and data length. Also check whether the entry is readable and whether the length is fixed or variable.

Reading	Outcome	Next action
Request fails for every Index on one port	The problem is the port-specific service path or device communication.	Verify the port and master request interface.
Known Index works, target Index fails	The target Index, Subindex, access, or requested length is wrong.	Recheck the FMR43 table entry.
Request succeeds but byte count differs	The length or payload location has been interpreted incorrectly.	Read the master's response layout and use its reported length.
Request succeeds with the expected byte count	The transport layer is working.	Determine byte order and decode the declared type.

Do not treat the 32-element destination size as the parameter length. It is only the capacity of the AOI response buffer. Decode the number of payload bytes reported by the transaction or specified for that parameter. Also locate the first payload byte from the IFM interface definition; do not assume element zero if the response includes service information.

Determine byte order before calculating values

A Logix SINT is signed, but each element still contains one intact eight-bit pattern. A displayed value of -1 therefore represents the byte pattern 255. Normalize each buffer element before assembling a multi-byte value:

byte value = SINT value, when SINT value is 0 or greater
byte value = SINT value + 256, when SINT value is negative

Use a parameter whose value is independently visible or measurable. Compare its expected representation with the returned bytes. If a two-byte value is low byte first, calculate:

unsigned value = first byte + 256 * second byte

If it is high byte first, calculate:

unsigned value = 256 * first byte + second byte

For a signed value containing n bits, first assemble the unsigned value U. When U is at least 2^n-1, the signed result is U minus 2ⁿ. Confirm byte order from the device definition or a known-value test before using either formula in production.

For UINT8, normalize one SINT element to 0 through 255. For INT, assemble two payload bytes and apply signed interpretation. For DINT, assemble four bytes in the verified order and apply 32-bit signed interpretation. For a string, follow the table's stated length and encoding; copy only character bytes into a controller string buffer and set its length from validated data. Do not copy the complete 32-byte response over a Logix string structure.

Build one controlled decode path

1. Select one FMR43 parameter and record its Index, Subindex, type, and length from the ISDU table.
2. Select the AL1920 port carrying that transmitter.
3. Configure either the IFM acyclic-read AOI or the documented explicit-message request channel. Do not let two routines issue requests to the same port concurrently.
4. Load the Index, Subindex, and requested length, then issue a one-shot read.
5. Wait for completion. On an error, preserve the error state and reject the old buffer contents.
6. Copy only the returned payload bytes into a stable working buffer.
7. Dispatch the payload to a decoder for the declared type. Pass the payload length and the verified byte order explicitly.
8. Publish the decoded value together with valid, busy, and error states so downstream logic cannot use stale data as a fresh reading.

A reusable AOI can wrap this sequence, but it needs configuration metadata for every parameter. At minimum, give it the Index, Subindex, expected length, type selection, and byte-order selection. The incoming SINT[32] alone cannot reliably identify the intended type.

Prove the decoded value before returning service

Test the complete path with a stable parameter whose value can be read from the transmitter interface or another trusted display. Capture the raw bytes, decoded value, transaction status, and returned length together. Change the source value enough to affect more than its lowest byte; this exposes reversed byte order that a small value can hide.

For signed parameters, test a negative value if the process permits it. For strings, check character order, actual length, unused bytes, and termination behavior. Repeat several reads and verify that a failed transaction clears validity rather than retaining a previous good value.

After the first parameter passes, add other parameters one type at a time. Keep the transport transaction separate from decoding so a message-path fault cannot masquerade as a conversion fault.

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