Identify decimal-coded nibbles before converting a raw byte into a reported number.
Keep the encoding definition
Beckhoff’s BCD conversion reference checks the admissibility of the high and low nibbles before returning a decimal result. Primary reference: BCD conversion.
In the stated packed two-digit BCD model, each four-bit nibble represents one decimal digit. That rule differs from interpreting the complete byte as an unsigned binary integer. Retain digit order and any additional sign or special-code definition from the actual interface.
An invented byte comparison
Imagine the unchanged byte 0x45. As an unsigned binary integer, it is 4×16+5 =69. Under the stated two-digit BCD rule, it represents decimal 45. The storage bits did not change; the interpretation did.
Now consider 0x4A. Its low nibble has numeric value 10, which is not a decimal digit under this model. A conversion should retain that invalid condition rather than silently manufacture a value. These original examples identify encoding behavior, not actual PLC register contents or an approved conversion implementation.
Make the format test explicit
Create fields for raw bytes, declared encoding, digit order, admissibility result and converted value. Preserve invalid data separately from a genuine zero result.
Request the actual register or device definition before decoding a recorded value. The worksheet can explain why two applications display different numbers from one byte while keeping representation errors separate from the process condition the value is intended to describe.
Customer Questions
Does 0x45 always mean 69?
That follows an unsigned binary interpretation, not the stated BCD rule.
Is nibble A a decimal digit?
Its value 10 is invalid in ordinary decimal-digit BCD.
Should invalid encoding become zero?
Retain the invalid state under the actual application rule.
Primary References
These references support the technical principles discussed in this guide. The worked examples and review questions are educational.
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