# How to determine whether an FPC cable or connector is 1.25mm or 1.27mm pitch?

I'm building an LCD display test harness on a perf board. I'm looking for a solution for connecting an 14 pin FPC/FFC cable to a 2.54mm perf board. Unfortunately, even with a digital caliper, it's very hard to measure the precise pitch of the cable. A quick search lead me to an article titled How to know the Pitch of FPC /FFC Ribbon Cable?, which explains an easier method for finding the pitch.

I measured the width of the cable several times with two slightly different results, 18.8mm and 18.7mm. I divided each by 15 (14 conductors + 1), giving me 1.2466mm (18.7mm width) and 1.25333mm (18.8mm width). Presumably, the pitch is 1.25mm since the article mentions it's a standard FFC/FPC pitch.

However, I'm thoroughly confused when trying to find a suitable connector. Using a pin header as a gauge, the existing connector's PCB footprint is 2.54mm with the second row of holes shifted over by what looks to be half the pitch or 1.27mm, not necessarily 1.25mm I calculated for the cable. I searched for more information and found a DigiKey article. The formula for calculating the pitch doesn't add 1 to the number of conductors which results in around 1.31 which is way off.

Is the pitch of the PCB connector different from the pitch of the cable? Am I dealing with an odd ball connection, or am I measuring incorrectly? How do I determine the pitch of the cable and the connector's footprint? Moreover, is it even possible to adapt a FPC to an IDC connector?

ASIDE: As a software engineer, I'm a bit puzzled by this experience. I always worshipped real engineers for their precision, consistency, and clarity when it comes to specifications and documentation.

• "I divided each by 15 (14 conductors + 1)". Should that not be "I divided each by 13 (14 conductors - 1)"? 14 conductors have 13 gaps, don't they? Commented Jun 7 at 21:23
• I got the formula from the first link, which I suspect to be wrong. N-1 doesn't seem to be correct either as the result isn't anywhere near 1.25mm nor 1.27mm. Commented Jun 8 at 1:45
• Where are the datasheet link for your parts? There should be a dimensioned and toleranced drawing. Why would you want to use a part without specifications? Commented Jun 8 at 3:48
• It's an old STN 14-pin display. The datasheet doesn't mention anything about the connector. Commented Jun 11 at 1:48