What is the Application for the TX300s' RJ11 RS232 Serial Port?

The optional RJ11/RS232, available in certain TX300s configurations, was originally used to discipline its Atomic Clock, using legacy GPSDO references, via serial port connection.

When configured with a single test module, the TX300s platform offers an additional (legacy) RS232/RJ11 port that was originally used to connect external GPS-disciplined oscillators (GPSDO), which used to have a DB9 serial port output for their 1PPS signal and ToD. Although external GPSDOs or their ToD information are no longer used, this auxiliary 1PPS port is still connected to the disciplining input of the built-in Atomic Clock. However, this is seldom used in present applications due to their accuracy limitations (their serial port components were most likely meant for microsecond-level applications). Also, common BNC or SMA 1PPS reference output ports (from modern PRTCs) can be fed directly to the test set's CLK IN (SMA) ports.

1. Potential Adapter Cables

If you are still interested on using the RS232/RJ11 1PPS input, as a last resort, because the only reference clock available on-site comes from a serial port, consider the following options. 

1.1 RS232/V.24 (DB9, RJ45, RJ11, etc.) to RS232 (RJ11)

If an RS232 port with 1PPS output is the only option available from an older switch, GPSDO or PRTC clock source, a simple passive adapter cable could be built by using a UTP pair to connect the device's 1PPS output signal to pin 2 of the TX300s' RJ11 plug and ground to pin 4, using one twisted pair. However, this is not often recommended since these 1PPS signal may have slow rising edges that may affect their overall accuracy.

Make sure that the 1PPS output amplitude is between 3.5V to 5V. Limit the cable length to 2m (6ft).

1.2 RS422/V.11 (RJ45) to RS232 (RJ11)

Network elements with V.11 ToD Serial Interfaces generate differential +1PPS and -1PPS signals (balanced) which are not compatible with the inputs of RS232/V.24 serial interfaces. One may think that connecting the +1PPS output to pin 2 and GND to pin 4 should be ok, however in many V.11 ports the +1PPS and -1PPS signals low levels may not reach zero volts (or same as GND), as they tend to have a voltage offset. For that reason, active converters are often required, but they insert an unwanted (sometimes unknown) phase delay.

Use an oscilloscope to verify the 1 and 0 voltage levels of the +1PPS signal, to make sure 0 = 0V = GND, before experimenting with a passive twisted pair adapter.

1.3 SMA or BNC (coax) to RS232 (RJ11)

Some users have asked about how to use an external 1PPS reference signal (50Ω SMA or BNC coaxial) from a modern PRTC to discipline the test set's Atomic Clock directly. However, this is not recommended.

Although not technically correct, if required, a fair approximation can be made by soldering a twisted pair cable directly to the SMA(m) or BNC(m), to minimize the effects of reflections due to impedance mismatch and coax capacitance. Connect center pin of a SMA/BNC to pin 2 and ground (shield) to pin 4 on the RJ11 plug.

RS232 signals are monopolar NRZ, similar to standard unbalanced 1PPS clocks, however, the TX300s' RJ11 has an impedance of about 9.6kΩ. That is why it is recommended to make the transition from coax to UTP right at the SMA/BNC connector, to minimize distortions due to impedance mismatch, signal reflections and capacitance. 

This type of external 1PPS source is still not recommended for modern precision timing applications, since the 1PPS output signal from the RS232 port may have slow rising edges that affect their overall accuracy.

Make sure that the 1PPS output amplitude is between 3.5V to 5V. Limit the cable length to 2m (6ft).

1PPS_Clock_Adapter_Cable_SMA-to-RJ11Example of an SMA-to-RJ11 (non-terminated) adapter cable

2. The RJ11/RS232 Port Pin Out


Although most test applications in the TX340s only use the 1PPS reference input, the NMEA input is no longer used. The full RJ11 pin-out is shown below:

TX-series RJ11 RS323/1PPS (Female)
Pin Signal
2 1PPS IN (V = 3.5 to 5 Vp)
5 Rx (NMEA ToD)
6 Tx

The TX and RX data lines had the following fix configuration: 9600 bps, 8 bits, 1 stop bit, no parity.

Caution: The TX300s platform currently does not offer cable delay compensation for the adapter cables suggested here, so this should be adjusted at the source (if possible) or by using 1PPS reference and test cables with similar delays (for them to cancel out).

3. Verifying the Adapter Cable

Besides checking the continuity with a multimeter, there is no easy way to verify the dynamics of the adapter cable once connected (e.g., signal level, distortion, etc.). However, the TX300s Atomic Clock's Phase Graph can provide some indications about the health if the 1PPS at its disciplining input.

Since the reference 1PPS signal comes from a PRTC device, it should be considered very stable. For this reason, the Atomic Clock can be configured with a tight disciplining window (e.g., Time Constant = 600 to 900s and Threshold = 10 ns). The expected variations should be wandering around a couple of nanoseconds.

Relative phase between the Atomic Clock's 1PPS Reference input vs. 1PPS outputTX300s Atomic Clock Relative Phase (Output vs. Ref.) with good 1PPS Reference

If there is large overshoot ringing due to impedance mismatch, other distortions due to capacitance, low voltage levels, etc., the TX300s Atomic Clock may have trouble locking onto the rising edge of the 1PPS and show a noisy Phase Graph.

TX300s Atomic Clock Relative Phase (Output vs. Ref.) with bad 1PPS Reference

You may also use an oscilloscope, in Hi-Z mode, to measure the resulting pulses level at the RJ11 plug.

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