Reference designators for electrical and electronics parts and equipment


Appendix F: Scenario Questions and Answers.

Answers to frequently asked questions about reference designators.

1. Given that you have a set of three matched band-pass filters used on one printed board assembly (PBA). How do you reference designate the filters on the schematic diagram and what does the parts list (PL) entry look like?

The set of three matched filters would use the reference designator A#. “A” is the class letter for a separable assembly, which this package of three individual filters would be. So the parts list entry for the associated parts list of the assembly would look something like:
A#, Qty 1, EA, Company PN, Set of 3 matched band-pass filters.
The schematic diagram would have A#FL1, A#FL2, and A#FL3 reference designators.

2. Given that you have a set of seven matched diodes and they are to be installed on three different printed board assemblies, four diodes on one PBA, two diodes on another PBA, and one diode on a third PBA.

To start with let’s talk about how the control drawing is set up to procure these seven matched diodes. You would have a drawing number, for example 123, that has the specifications for the matched diodes with each diode allocated a suffix number, -1 through -7, and the set purchased under part number, say 123-10. For each PBA the PN shown for each diode on the associated PL would be a different suffix number like this:

PBA1 with D# PN 123-1, D# PN 123-2, D# PN 123-3, and D# PN 123-4.
PBA2 with D# PN 123-5, and D# PN 123-6.
PBA3 with D# PN 123-7.

Note that it doesn’t matter what the diode reference designations are on the schematic diagrams, on the assembly drawings (parts layout drawings), or the associated PLs. The quantity of each individual diode is 1, with purchasing keyed to the fact that it has to buy PN 123-10 to get the seven diodes, PNs -1 through -7 are not orderable. For the PN 123-10 to be visible to the unit it could be listed on the final/top assembly PL with reference designator A#.

3. How do you reference designate circuitry that has multiple channels of the same parts?

Just use the non-class letter N, as listed in Appendix B, Clause 0.4, as a prefix reference designation to the basic reference designation. For instance, given that there are eight channels on a PBA you would assign the channels as follows:

N1, Channel 1
N2, Channel 2
N3, Channel 3
N4, Channel 4
N5, Channel 5
N6, Channel 6
N7, Channel 7
N8, Channel 8

R1 in Channel 1 would be N1R1, in Channel 2 this would be N2R1, etc. Circuitry on the PBA, not associated with any particular channel, would use basic reference designations.
Note: When I worked for a PBA house I saw a customer with this scenario that was using the Altium Designer program and they have some obtuse way of using underscores and hyphens. The reference designations, if you could call them that, on the schematic diagram and on the parts list were very confusing.

4. Similar to question 3, how would you reference designate circuitry on a single PBA that are segregated into what I would call pods. Maybe the circuitry has metal shielding.

Similar to answer 3, use the non-class letter N, as listed in Appendix B, Clause 0.4, as a prefix reference designation to the basic reference designation.

An example is:

N1, Prescaler
N2, Phase-lock loop (PLL)
N3, Doubler
N4, Amplifier
N5, Filter section

5. What reference designation should be used for a PCB?

IAW Appendix B, Clause 0.2.5, Inseparable subassemblies states:

“...printed circuit boards...shall be assigned the class letter U...”

You have two choices, either use the U numbered one above the last one used on the PL for U numbered parts or use U0 (U zero). I recommend and use U0.

6. How do you reference designate two PCBs that are derived from a single schematic diagram?

Referencing answer 5, I would use the reference designations U0A and U0B. I actually saw this situation once. The RF portion of the circuitry was done on 30 mil thick Teflon-fiberglass material while the control circuitry portion was done on normal 1/16 inch FR4 material.

7. How do you properly reference designate a “resettable fuse” or “polyfuse”?

First of all there is no such thing as a resettable fuse or polyfuse as a fuse is defined to be a one time thermal cutout. These are trade names for a Polymeric Positive Temperature Coefficient (PPTC) device or thermistor. When the current through the device exceeds a set trip point the internal heating makes the device go into a high resistance state, limiting the current to a minuscule amount. Removing the high current condition reduces the internal heating and the device reverts to its normal low resistance condition and will pass its rated current. Thus the device automatically resets. The class letter to use would be RT, for a thermistor, and the graphic symbol to use would be that of a circuit breaker or circuit protector. See Appendix B, Clause 0.2.1.

8. How do you properly reference designate a hand wound inductor with constituent parts?

For an inductor you need a coil form and a length of wire. For an air wound inductor there may be no coil form but you still need a length of wire. As an example of a homemade hand wound inductor (coil) using a toroid core and a length of wire, the parts list (PL) would look something like this:

L3, Qty 0, EA, Inductor: 2.2 mH
L3E1, Qty 1, EA, Ferrite core: FB-43-2401
L3W1, Qty AR, Wire: 22 AWG enameled, 20 turns

The reference designator of the inductor, in this case L3, would be the only reference designator used on the schematic diagram. The reason the quantity is zero for the inductor in the parts list is because it is made from the quantities of the constituent parts. The reference designator E1 stands for ferrite bead or miscellaneous electrical part (see Appendix B, Clause 0.4). And the abbreviation of AR for quantity means “as required”, for which there is no unit of measure.

To better specify the amount of wire you should state the length used:
L3W1, Qty 4, cm, Wire: 22 AWG enameled, 20 turns

9. Like question 8, how do you properly reference designate a hand wound transformer with constituent parts?

Like answer 8, you would need to list the core and the wire. For a transformer you will have at least two wires, if trifilar wound you would have three wires, and if you have other additional windings or taps you would need the wire for those. Let’s take a look at what some parts lists would look like for various situations.

For a transformer with a primary and secondary winding where the wire is of different size and different lengths.:

T1, Qty 0, EA, XFMR: 115 V ac 60 Hz 200 mA input, 15 V ac 2 A output
T1E1, Qty 1, EA, E-I iron core
T1W1, Qty 50, mm, Wire: 20 AWG, 20 turns
T1W2, Qty 20, mm, Wire: 22 AWG, 10 turns

For an RF transformer with a twisted trifilar winding:

T2, Qty 0, EA, XFMR: trifilar wound
T2E1, Qty 1, EA, Binocular core
T2W1, Qty 0, EA, Wire: twisted trifilar, x twists/cm, 10 turns
T2W1W1, Qty 50, mm, Wire: 22 AWG enameled
T2W1W2, Qty 50, mm, Wire: 22 AWG enameled
T2W1W3, Qty 50, mm, Wire: 22 AWG enameled

Again, T2W1 is a quantity of 0 (zero) because it is made from constituent parts.

10. How do you reference designate two resistors in series or parallel that take the place of a single resistor?

You use suffix letters. If R7 is a 51 Ω resistor and you want to replace it with two 100 Ω resistors in parallel, you would use reference designations R7A and R7B. If you have a resistance value that can only be obtained by connecting two resistors in series then they would use suffix letters. If the original resistor was reference designated R3, then the two resistors in series would be R3A and R3B.

11. For new equipment development where interface connectors are assigned up front in the preliminary specifications, how should this be done?

Don’t put the cart before the horse!

Reference designation of connectors, along with all other circuitry, is done when the schematic diagram is developed and schematic entry is accomplished. If necessary to identify interface connectors, those having to do with the outside world, simply use a bogus class letter such as CN (meaning connector).

An ECO can always be written to change the bogus reference designation to the proper reference designation.

12. How are terminals for parts notated?

The class letter E stands for an individual terminal but can also stand for the terminal (pin or socket) of a part. ASME Y14.44 says that instead of using the class letter E that a hyphen “-” can be used. As an example instead of stating “U3 pin 2”, “U3 terminal 2”, or “U3E2”, use “U3-2”. This is especially helpful with multi-terminal connectors, especially of large ones that have row and column terminals. For instance J7 pin A7 could be reference designated as J7EA7 but that wouldn’t make much sense, use J7-A7.

13. How do you properly reference designate a terminal used as a test point.

The non-reference designation TP is to be used on maintenance diagrams with the class letter E used to reference designate the actual part.

14. How do you designate the collector of a transistor or the pin (terminal) of an IC as a test point?

You use the non-reference designation TP (see Appendix B, Clause 0.4) next to the graphic symbol for a test point, which is a filled in circle (see IEEE 315, Clause 1.5.1), with a leader pointing to the test point location or without a leader.

15. How do you reference designate a multi-element part that is a potentiometer with an on-off switch? This would be a volume control with a power switch.

The graphic symbol would show an envelope and within the envelope would be shown a potentiometer, with reference designation R1, and a switch with reference designation S1, connected by a mechanical linkage (the shaft that turns). The mechanical linkage could be shown with IEEE 315A, Clause 4.6.3 Indication of operating method, namely IEEE 315A, Clause Manually operated switch; general symbol or IEEE 315A, Clause Turn-switch (locking). The overall reference designation of the envelope (the part) would use class letter U (for an inseparable assembly). On a schematic diagram you might see the potentiometer and switch in different places, without mechanical linkage. These would then be reference designated as U#R1 and U#S1 respectively.

18. How would you reference designate a part that needs a knob? This would be for parts like a potentiometer or a rotary switch.

Similar to answer 1 you would treat this as a separable subassembly. On the schematic diagram you would use A#R1 for a potentiometer and on the parts list you would have A#R1 for the potentiometer and A#MP1 (MP meaning mechanical part) for the knob. For a rotary switch you would do the same thing. On the schematic diagram you would have A#S1 for the switch and on the parts list you would have listed A#S1 and A#MP1.

Following Appendix B, Clause 0.1 that states some class letter designations are for parts or for an assembly, for the potentiometer you could use reference designators R#R1 and R#MP1. Likewise for the rotary switch you could use S#S1 and S#MP1.

19. How do you reference designate ferrite beads on the leads of a THT transistor or on the leads of a THT connector?

The symbol for a ferrite bead is shown in IEEE 315A, Clause 6.2.11, Ferrite bead, shown on a conductor. The graphic symbol is similar to the IEC symbol for a fuse, a rectangular box, with one long side missing. The class letter to use is E (see Appendix B, Clause 0.4). The ferrite bead symbol would be shown on the leads of the part in question, along with its reference designation, but would not have a land pattern associated with it (for PCB mounting) as the ferrite bead does not mount to the PCB.

20. How do you reference designate a multi-element DIP type switch?

As an example let’s take a look at a part that has eight switches. One way to reference designate the switch package is S#, with the individual switches with suffix letters (in this case you would have S#A through S#H). However, I have seen the datasheet for these parts to have the switches labeled with numbers, in this case 1 through 8. You could use reference designation U# for the package and the switches would be U#S1 through U#S8. But Appendix B, Clause 0.1 says, “Certain item names and designating letters may apply to either a part or an assembly”. Thus, you could reference designate the package (an inseparable assembly) with S#, which I would highly recommend. The individual switches within the package would be identified as S#S1 through S#S8.

21. Why do people/organizations keep using old graphic symbols for RF connectors?

Because people/organizations are unaware of newer standard symbols. For the newer symbols, since 1986 for IEEE Std 315A and before that for the IEC (for over 30 years now), see IEEE 315A, Clause 5.6.1A Coaxial plug and socket symbols. I don’t know the exact call out for IEC 60617 as I do not have access to that standard.

22. What are the standard graphic symbols for ground?

You will find them under IEEE 315, Clause 3.9 Circuit Return. See ASME Y14.44, Fig. 3 Reference Designation in a Typical Schematic Diagram, that shows usage of these symbols.

A1. The earth ground symbol (notated as being an IEC symbol), which is three horizontal lines one below another with the 2nd line being shorter than the 1st and the 3rd line being shorter than the 2nd is found in IEEE 315, Clause 3.9.1 Ground, general symbol. The symbol has the following two statements:
1) A direct conducting connection to the earth or body of water that is a part thereof.
2) A conducting connection to a structure that serves a function similar to that of an earth ground (that is, a structure such as a frame of an air, space, or land vehicle that is not conductively connected to earth).
I have always found that the maintenance manuals for vehicles that I have worked on follow this standard.

A2. What I call the pitchfork symbol (notated as being an IEC symbol) is delineated in IEEE 315, Clause 3.9.2 Chassis or frame connection; equivalent chassis connection (of printed-wiring boards). This is the main symbol to use, when the circuitry is for a printed circuit board (PCB), and NOT the earth ground symbol. Additional explanation text is: “A conducting connection to a chassis or frame, or equivalent chassis connection of a printed-wiring board. The chassis or frame (or equivalent chassis connection of a printed-wiring board) may be at substantial potential with respect to the earth or structure in which this chassis or frame (or printed-wiring board) is mounted”.

A3. The third type of ground/circuit return symbol is the triangle pointing down. This is covered by IEEE 315, Clause Potential level not specified by a numerical value. This symbol is not notated to be an IEC symbol. Explanation text states: “To be used when identically annotated common-return connections are at the same potential level”. Also there is Note 3.9.3A: “The asterisk is not part of the symbol. Identifying values, letters, numbers, or marks shall replace the asterisk. For the triangular symbol, this identification shall be placed within the triangle or, if essential for legibility, adjacent to the triangle”.

23. What is the standard symbol for showing a voltage connected to a part?

This is covered by IEEE 315, Clause Specific potential difference. This is simply a short straight line with the voltage value indicated. Two comments:

1) I recommend and use IEEE 315A, Clause 1.7 Direction of Flow of Power, Signal, or Information; specifically IEEE 315A, Clause 1.7.1 One-way, the added IEC symbol of the open arrow “>”. Do not use the unfilled circle “o” as this is the graphic symbol for a terminal and there is NO terminal. You might have seen something like “o—“, instead use “−−>−−” [the dashed line should be a solid line running through the arrow]. And DO NOT put an arrow pointing back to the source.

2) When stating voltage SI usage says it should be the value followed by a space and then the SI letter symbol, like this, “3.3 V”. However, software programs don’t like spaces so nomenclature like, “3.3V”, “3V3” or “3.3_V” are used. You should put a note on the schematic diagram that says, “3V3 indicates the 3.3 V DC line”.

24. How do you reference designate the gold plated fingers on a PCB?

Do not use the class letter “P” as the fingers are not a separate item, they are part of the board and any reference designation used would not be entered on a parts list. The socket that the printed board assembly (PBA) plugs into generally has the terminals lettered and numbered, letters on one side and numbers on the other side. The class letter to use for the socket would be “X”, of course. My recommendation is to use the sudo class letter “A#”, and thus the fingers would be reference designated “A#-1”, “A#-2”, “A#-3”, “A#-A”, “A#-B”, “A#-C”, etc. The terminal letter or number would match to the mating socket terminal letter or number. The reference designation would be made invisible with an added graphic letter or number indicated on the PCB.

25. How do you reference designate large connectors, like ARINC 600 type, that have several areas     of pins, sockets, and coaxial connectors?

These types of connectors have the areas or sections lettered with the individual terminals or coaxial connectors numbered, and for large numbers of terminals these are in a matrix layout with rows numbered and columns lettered. Considering that these connectors are the main connector of the equipment that would slide into a tray with a mating connector in the tray, the class letter to use would be “P” because it will be the most movable of a mating pair. The reference designation would be P# and the terminals would be reference designated P#-B3 or P#-E7K for the large number of pins in a matrix layout. The letter after the hyphen corresponding to the lettered area with the number being the terminal number or if the letter is followed with a number and another letter you know this is the terminal of a matrix.

26. The question has come up about how to make a single schematic diagram for multiple printed     board assemblies (PBAs) where the individual PCBs are from the same board layout. The single board layout would use mouse bites or the V-groove method to separate out the individual boards.

Under the Unit Numbering Method of ASME Y14.44 use reference designation prefixes added to the basic reference designations. Assign reference designation prefixes on a system subdivision diagram (see Appendix D) as follows:

Ref des prefix A1 for PBA1 (PCB1)
Ref des prefix A2 for PBA2 (PCB2)
Ref des prefix A3 for PBA3 (PCB3)


For PBA1 you would have an A1R1, an A1C1, an A1U1, etc.
For PBA2 you would have an A2R1, an A2C1, an A2U1, etc.
For PBA3 you would have an A3R1, an A3C2, an A3U1, etc.

If the schematic diagram is too large for a single sheet of drawing, use separate sheets as needed. No need to use the hierarchical method.

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About the author

The articles in this series are written by Lawrence W. Joy (Larry)
[email protected]
Michigan USA.

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