Convert F-CH_AI Raw Input Values to Engineering Units

F_CH_AI processes an analog F-I/O raw value and presents a REAL process value at its V output; a displayed hexadecimal WORD is a bit representation of the raw channel value, not a floating-point engineering-unit value. Interpret the raw code using the selected analog module's range and channel configuration, then apply the block's configured engineering limits. When SIM_ON is active, SIM_V supplies the simulated value in the F-channel block's expected input-value range; it does not mean that a WORD should be reinterpreted as an IEEE-754 REAL.

Raw channel value and engineering value are different

The input WORD represents the channel's raw digital code. Siemens analog documentation uses nominal bipolar values such as −27648 to +27648 for −100% to +100% in applicable ranges, but raw encoding depends on module type, selected range, and diagnostics. A code displayed in hexadecimal should first be interpreted according to the block/module data definition, including signedness; do not convert its displayed hex digits directly into a REAL bit pattern.

Understand the F_CH_AI output and scaling

The source reply describes F_CH_AI scaling the raw input to configured high and low engineering limits (VHRANGE and VLRANGE). The exact parameter names, data types, and block version must be confirmed in the installed F-library help. Conceptually, for an applicable linear range, raw input is normalized according to the configured module range and mapped to the engineering span. Bipolar and unipolar/live-zero ranges have different raw-to-percent relationships, so one generic raw formula cannot cover every channel.

Interpret simulated input

Siemens Safety Matrix documentation describes SIM_ON switching the F-channel driver to simulation and routes simulation values through SIM_V; where preprocessing is configured, the simulation value may first be transformed and inverse-transformed so it matches the F-channel driver's value range. Therefore, ensure the source value feeding SIM_V is expressed in the expected range for that specific block and signal path. Do not assume it is the final temperature or pressure unit unless the block documentation/configuration says so.

Practical conversion workflow

Identify the exact F-module and analog channel, including configured measurement range and wiring mode.
Check the module manual's raw-value representation and diagnostic/overflow codes for that range.
Inspect the F_CH_AI version and its parameter help for raw input, SIM_V, SIM_ON, and output V behavior.
Read the configured engineering high/low limits and any upstream preprocessing.
For live data, compare the raw input and V against a known input or approved simulator. For simulation, enable it only in a controlled test and verify that changing the intended simulated value produces the expected V without affecting production operation.

Verification

Use the block online view and the module's diagnostic/status information to inspect raw input and converted V together. Confirm at least two safe, known points spanning the configured range and check the output unit. Test SIM_V separately from the live channel and verify simulation status. A plausible display at one point does not confirm correct range, polarity, wiring, or scaling.

Limitations and Operational Precautions

The source omits the F-module model, measurement range, F-library version, and exact hexadecimal example, so an exact conversion cannot be calculated. Siemens' ±27648 convention applies only to the relevant ranges; unipolar and live-zero channels differ. F-channel simulation and safety-related signals must be tested under the plant's approved safety and operating procedures. Do not force simulation or change range parameters on a live safety system without authorization and risk review.

Engineer FAQ

Is the WORD hexadecimal value already a REAL? No. It is a raw code representation; the engineering value is separately scaled and output as a REAL by the configured channel processing.

Can I always treat 0x9400 as −27648? No. Decode according to the defined signed representation and module/channel format, then confirm the configured range.

Does SIM_V take the final engineering-unit value? Only if the installed block and signal path specify that range; the F-channel block's input range may be different from a downstream preprocessed range.

What determines the conversion? The module's measurement range and raw encoding, the F_CH_AI version/configuration, its engineering limits, and any preprocessing.

Technical References

Siemens, SIMATIC S7 analog value processing application: raw value representation and nominal ±27648 scaling for the applicable range. Confirm module-specific details in the actual module manual: https://support.industry.siemens.com/cs/attachments/109748759/109748759_Application_Oversampling_V1_1_de.pdf
Siemens, PCS 7 V10.0 / SIMIT V11.2 Getting Started: describes conversion from raw channel values to process values; later release than the unspecified source configuration: https://support.industry.siemens.com/cs/attachments/109746485/109746485_PCS7_V100_SIMIT_V112_GS_DOC_V3_1_en.pdf
Siemens, Safety Matrix Programming and Operating Manual, 01/2019, simulation with SIM_ON and SIM_V: https://support.industry.siemens.com/cs/attachments/109766685/S7FSMUGB_en-US.pdf

Convert F-CH_AI Raw Input Values to Engineering Units