The International System of units (SI) defined many ways to express measurements in print. Yet, hardware developers should be ready to encounter some variations on the SI system from time to time. Below are 3 common examples that electronics developers and programmers should be aware of.
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RKM Code
RKM code notation replaces decimal points with a unit of measure. That’s to say: everything normally located left of the decimal place is printed left of the unit, and the rest is placed on the right. The method is frequently associated with resistors and capacitors (per IEC 60062:2016 “Marking codes for resistors and capacitors”) . Yet, engineering documentation (especially schematics) may apply it to other measurable quantities such as voltage.
| SI UNITS | RKM EQUIVALENT |
| 0.1 Ω | 0R1 |
| 100 Ω | 100R |
| 3.3 kΩ | 3k3 |
| 33 kΩ | 33k |
| 33 MΩ | 33M |
| 3.3 V | +3V3 |
| 2.7 V | +2V7 |
| 1.8 V | +1V8 |
The notation was created to address at least two problems in schematic capture. The first deals with typography/fidelity problems caused by decimal point characters. A decimal point’s small size makes it prone to loss during photocopying, compression, and other lossy image-transfer processes. Secondly, not every text encoding standard has a code point for the omega symbol (Ω) of resistance; so RKM code represents electrical resistance an uppercase R. RKM code is not mandatory by any means.
Decimal Prefix vs. Binary Prefix
The metric prefix system is based on exponential increases in the number 1000…
| ENGLISH | NOTATION | EXPONENT | MULTIPLICAND |
| kilo | 1 k | 10001 | 1 000 |
| Mega | 1 M | 10002 | 1 000 000 |
| Giga | 1 G | 10003 | 1 000 000 000 |
| Tera | 1 T | 10004 | 1 000 000 000 000 |
Digital data storage is usually structured in exponents of two. The SI system’s letter prefixes were not made for this purpose, and can lead to confusion when applied to digital storage. For example: a sales brochure may inaccurately express the amount of storage as “64 kB” which would mean 64,000 bytes under the SI system. A trained student or professional may tacitly understand the storage will actually be 65,536 (which is 216). But ultimately, this is open to interpretation.
The binary prefix system was created to address this discrepancy.
| ENGLISH | NOTATION | EXPONENT | MULTIPLICAND |
| kibi | 1 ki | 10241 | 1 024 |
| Megi | 1 Mi | 10242 | 1 048 576 |
| Gibi | 1 Gi | 10243 | 1 073 741 824 |
| Tebi | 1 Ti | 10244 | 1 099 511 627 776 |
The two systems are still applied somewhat inconsistently between industries. Even departments within the same organization may use different prefixes, so when in doubt: consider going straight to the source to ask for clarification.
Ad Hoc Prefixes in SPICE
SPICE (meaning: Simulation Program with Integrated Circuit Emphasis) was developed in the 20th century, and occasionally it diverges from the modern SI system. Consequentially, the SPICE engine accepts (and expects) a few non-standard prefixes.
| PLAIN ENGLISH | QUANTITY | SI SYSTEM | SPICE System |
| Mega | 106 | M | MEG |
| milli | 10-3 | m | M |
| micro | 10-6 | µ | u |
Additionally, many modern SPICE simulators are case-insensitive. Not only are “m” and “M” considered the same prefix; but typing “F” for “farad” will be read as “femto” (meaning 10-15). So be mindful of SPICE prefixes when using the application.
Interestingly, LTspice XVII (version 17.0.35.0) appears to recognize RKM notation.
References
| [1] | National Institute of Standards and Technology, “NIST Special Publication 330: The International System of Units (SI),” 2019. [Online]. Available: https://doi.org/10.6028/NIST.SP.330-2019. [Accessed 20 Nov. 2019]. |
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