An Adventure in Identifying ConnectorsFollow article
Finding the correct mil-spec connectors for an ex-military laptop.
In the office, we have a rather interesting ex-military laptop — an MBM Termite LT450, featuring a whopping 64MB of RAM, and a 166MHz Pentium processor. We thought this would make quite a nice rugged terminal machine with dual serial ports and USB, and a Linux variant suitable for running on resource-constrained systems.
This being designed as a military device means that it is able to withstand some abuse, such as being dropped (we’ve not tested this for fear of damaging the floor), and having cables yanked upon and no doubt tripped over. This calls for the requirements of rugged connectors that can survive things no ordinary DB9 and USB-A connectors would be able to survive.
As seen in the image above, all the connectors are different from one another, but in the same product series - they all share the same features such as the bayonet locking and keying pins, contact arrangement and labelling, and the internal keyways.
The first clue as to what series of connector these might be is the marking on top of the connector caps; in this case, we looked at the USB connector which has “LMA1054-12-00-00”. A quick search returned the Weald website with an equivalent line of connector caps and the military standard the caps conformed to.
The Connector Geek here on DesignSpark has a good two-part series on mil-spec connectors, and after a quick read of the series, it looked like these were all part of the MIL-DTL-26482 Series 1 family. Now we know the connector series we can begin to shop around to find all the components we will need such as backshells and the actual connector assembly itself.
Selecting The Connectors
A note on measuring twice, buying once
MIL-DTL-26482 Series 1 connector features two keying methods — the angle of the bayonet locking pins on the outside, and angle of the internal keyways within the connector. These are specified in degrees from a datum point, so it is worth getting a protractor (or other means of measuring an angle) to measure the angle. A useful reference for the keying is this document provided by Glenair.
We found this out the hard way and inadvertently ordered the wrong keying for the USB plug, but luckily this fit in COM2 and so all was not lost.
Picking The Right Parts
Now we know the connector series and the keying angles, we can select the correct parts. The last things to determine are the shell sizes, which we know are size eight and twelve, and the connector arrangement, which is a case of matching what is provided in the datasheet.
In our case, the part numbers we used are Amphenol PT06A-8-4S(SR) Souriau offer connectors that follow the standard, and thus should be interchangeable too., Amphenol PT06E-12-10(SR) and Amphenol 62GB-56T12-10PC for the USB connector. Note the part number is different to the rest — 62GB is another series offered by Amphenol that meets MIL-DTL-26482 standards, and is thus interchangeable. Other manufacturers such as
Different backshells can also be provided, depending on the environment that the connectors are to be used in. It is best to consult the manufacturer of your particular connector to see what connector accessories they provide. In our case, we used plain strain relief backshells, as we do not need to worry about terminating shielding.
With all the parts now delivered, it is time to assemble the connectors. It is important to remember to slide the connector backshell if using a pre-terminated cable — in our case, we had cables that already had USB-A female connectors and a male DB9 soldered on. The order of assembly is shown in the image above.
The first step was to figure out the pinout of all the connectors on the laptop. Using a combination of a multimeter, oscilloscope, current limited bench power supply , some intuition and some trial-and-error we figured out the pinouts for all the connectors.
The first pinout to be found was for the power connector, which will enable us to find other pinouts once the laptop has booted. We started by identifying which pins connected to the chassis, which was a good assumption of ground. As it is a four-pin connector, we assumed the other pins must be the positive supply, and slowly ramped the applied voltage up to 12V at which point the laptop started up to a BIOS screen.
Next, the USB connector pinout was found — this was a bit trickier as there are ten pins, which implied two USB connections (there are four lines necessary for USB, and it would not make sense to only have one USB connection on a ten-pin connector). Once 5Vdc and ground were established, the next step was to try to identify the differential data pairs. This boiled down to mostly trial and error, involving connecting the data lines to a USB memory stick to random pins on the connector and seeing which ones moved on the oscilloscope.
During this process, we discovered the laptop could actually boot into Windows 2000, which helped in finding the pinout for the serial port — we could install Realterm and manually wiggle pins on the serial port to see which ones corresponded to which signal.
Now with all the pinouts found, it was time to move onto soldering the connectors.
Soldering the cable onto the connectors can be a bit fiddly, especially if the cable is thick and fairly resistant to bending. We found it useful to have a helping hand to hold the cable in position, and a pair of tweezers to help guide the conductors to the right solder bucket terminal.
With all the connections made, the backshell was slid down the cable and tightened onto the connector. This is made easier by mating the connector with the laptop as more torque can be applied without risking damaging the connector itself. The screws for the strain relief can then be tightened down — it is worth noting that the strain relief should grip the outer insulation of the cable and none of the conductors. In our case, this required a bit of a shove to fold the inner conductors up a bit, as we’d stripped slightly too much of the outer insulation off.
We only have one connector left to source, which is for COM1, but as we have already dipped our toes into the world of mil-spec connectors and finding unknown pinouts this process is a lot easier.
To Wrap Up
In this post, we have taken a brief look at how to identify an unknown connector, and then how to assemble a common mil-spec connector.