Hello, I have a USRP E310 that I'm trying to synchronize to an external 1 PPS input. My goal is to trigger the SDR to begin receiving samples on the rising edge of the external PPS, so I'm using a pulse generator to generate a square wave with 5V amplitude and 25% duty cycle, 1 second period and connecting that to the sync port on the e310. The problem is that I can't tell if the device is actually seeing that external PPS signal, when I run the test_pps_input utitlity it passes regardless of whether the external PPS is connected or not. It seems like the test_pps_input should throw an error if there is no external PPS present, correct? The Checking ref_locked sensor...FAIL! happens even with the external PPS connected. Here are the outputs from the test_pps_input utility: *************************************************************************************************** root@ni-e31x-30FB1A6:/usr/lib/uhd/examples# ./test_pps_input --source "external" Creating the USRP device with: ... [INFO] [UHD] linux; GNU C++ version 9.2.0; Boost_107100; UHD_4.6.0.0-0-g50fa3baa [INFO] [MPMD] Initializing 1 device(s) in parallel with args: mgmt_addr=127.0.0.1,type=e3xx,product=e310_sg3,serial=30FB1A6,name=ni-e31x-30FB1A6,fpga=n/a,claimed=False [INFO] [MPM.PeriphManager] init() called with device args `fpga=n/a,mgmt_addr=127.0.0.1,name=ni-e31x-30FB1A6,product=e310_sg3'. [INFO] [0/Radio#0] Performing CODEC loopback test on channel 0 ... [INFO] [0/Radio#0] CODEC loopback test passed [INFO] [0/Radio#0] Performing CODEC loopback test on channel 1 ... [INFO] [0/Radio#0] CODEC loopback test passed Using Device: Single USRP: Device: E300-Series Device Mboard 0: e310_sg3 RX Channel: 0 RX DSP: n/a RX Dboard: A RX Subdev: E3xx RX Channel: 1 RX DSP: n/a RX Dboard: A RX Subdev: E3xx TX Channel: 0 TX DSP: n/a TX Dboard: A TX Subdev: E3xx TX Channel: 1 TX DSP: n/a TX Dboard: A TX Subdev: E3xx Attempt to detect the PPS and set the time... [INFO] [MULTI_USRP] 1) catch time transition at pps edge [INFO] [MULTI_USRP] 2) set times next pps (synchronously) Success! Product requires verification of ref_locked sensor! Checking ref_locked sensor...FAIL! *************************************************************************************************** Again, this output does not change whether my external PPS is connected or not. I'm using UHD 4.6, any help would be greatly appreciated.