# What do 3-digit reference designators mean?

While looking at many circuit boards and schematics, I often notice that 3-digit designators are used, for example R101 instead of R1.

What do these digits actually mean? The numbers are not in sequence, for example only a single relay, K101 may be present, still numbered 101.

• Sometimes it's for "channels", as others have noted; sometimes it's just to "make the part numbers line up" (assuming you don't get above R999!). – TripeHound Jul 10 '17 at 7:04

You should be able to figure out it looking at the device as whole. You can find such numbering in the devices with multiple boards, multiple physical or logical blocks in it. First digit may designate block #, other two (usually) designate component # in the block.

• That's what I suspected. Nice to know! – Mark Jul 9 '17 at 19:36

Often, in multi-channel circuits (same circuit repeated several times), the first digit will indicate the channel, while the remaining digits indicate the specific component. For example, R101, R201, R301... will each be the "same" component, but in channels 1, 2, 3...

I personally usually use 4 digits: first two are the sheet number and last two are the unique number for this component. That means I can use up to 99 components of each type on each sheet! Of course sheets 1-9 have 3-digit reference designators.

I suspect this or something similar is the reason you see 3-digit numbers.

I find this method quite convenient especially during layout. When you are in the process of placing components, this is a good way of more or less knowing where does each component belong to.

• In your counting, is 1111 sheet 1 component 111 or sheet 11 component 11? – PlasmaHH Jul 10 '17 at 5:41
• @PlasmaHH It is actually sheet 11, component 11. I have estimated there will not be any sheet with more than 99 components of the same type and this is true until now! – nickagian Jul 10 '17 at 6:08
• Sheet 1, 2, 3, ... should all still use 4 digits (2 digits for the page). That's what 0 is for. Not using 01, 02, 03, etc. for the page results in various sorting issues. It also makes the numbers indeterminate, with potential duplication, when you go over 99 components of a component type on a page (it does happen). – Makyen Jul 10 '17 at 7:43

There is also a common system for reference designators that follow a coordinate zone system for large boards that have thousands of components. (Think PC or server motherboard for example). The board will have zone numbers along the sides that may range from 1 to 9. Along the top/bottom the zone numbers may range from A to K (skipping I) on the primary side and ranging from L to V (skipping O) on the secondary side. Reference designators then become something like R7N16 for the 16th resistor in the "7N" zone region.

This system makes locating the physical resistor on the actual board easier if the reference designators are actually all included in the silkscreen of the board. Particularly useful when testing and qualifying the first or second fabrications of a new board.

In the advent of small components like 0402 and 0201 SMT parts it is getting more common to not even include the reference designators on a board and engineers and techs need to keep their laptop handy right on the lab bench to be able to look up part locations!

Reference designators are arbitrary. Very generally, the first character denotes the type of component (R for resistors, C for capacitors, U or IC for integrated circuits, D for diodes, Q for transistors, Y or X for crystals, J or P for jumpers and/or connectors, F for fuses, K for relays, etc...

The numbers are just as arbitrary. You shouldn't be confused by the number of digits. I typically re-annotate my boards so the designators "flow" according to how they're placed, with 1 being in the upper left and increasing as you move across and down the board. I've tried numbering 1-99 for components on the top and 101-199 for components on the bottom, but over the years my boards sometimes got to be more complex and I opted for just starting at 1 at the top left and letting the numbers just naturally progress as they transitioned to the bottom of the board.

Of course, when you re-annotate based on physical location then the numbers on the schematic are all over the place, but that's fine; trying to find R75 when you see R73 is much, much easier when they're numbered by location. The schematic organization is much easier to observe and doesn't need numeric "locality".

Different people assign reference designators differently. Some use location on the PCB, some location in the schematics. Some the order in which components were added to the design.

One reason to use 3-digit designators even when not strictly needed is to make the length of the designator independent of the value. That means that when you re-annotate the board or copy and paste a subcircuit you don't have to go round re-positioning all the designators on the silkscreen layer.