Auxiliary and Analog Interfaces

RTC Coin Cell Interface

Overview

The RTC Coin Cell Interface provides coin-cell backup power to the OEM modem's real-time clock, allowing calendar time to be retained across main-battery power cycles.

Electrical Connections

All signal directions are relative to the OEM modem. Refer to the digital board connector locations figure for the connector's physical location.

RTC Coin Cell Interface schematic connections.

RTC Coin Cell Interface Electrical Pinout

Pin Number	I/O	Pin Name	Notes
1	P	RTC_VBAT	RTC battery backup voltage supply, 1.0–5.0 V, nominal 3.0 V
2	P	GND	Board ground

Hardware Components

The RTC Coin Cell Interface uses a Molex PicoBlade connector, P/N 0510210200, or equivalent. The recommended contacts are Molex P/N 0500798000, suitable for 26–28 AWG wire.

RTC Coin Cell Interface User-Side Molex Connector.

RTC Coin Cell Interface Components and Part Numbers

Part Number	Manufacturer	Description
0510210200	Molex	PicoBlade 2-position connector receptacle, 1.25 mm pitch
0500798000	Molex	PicoBlade 26–28 AWG sockets
2002181900	Molex	PicoBlade hand crimp tool
2149202211	Molex	Pre-crimped PicoBlade lead, 75 mm length

Hydrophone Input Interface

Overview

The two Hydrophone Input Interface connectors digitize acoustic data from up to eight hydrophones in addition to the standard acoustic transducer interface.

Electrical Connections

Each hydrophone input is single-ended; the AINxN returns are tied internally to board ground. Each input is AC-coupled through a 10,000 pF capacitor with a 158 kΩ resistor, producing a 100 Hz high-pass cutoff and approximately 158 kΩ pass-band input impedance.

On PMM6081 units manufactured before mid-2026, odd-numbered inputs (AIN1, AIN3, AIN5, and AIN7) default to 43 dB gain and even-numbered inputs default to 12 dB. On units manufactured after mid-2026, all hydrophone inputs default to 43 dB gain.

The full-scale ADC input is 5.32 V peak-to-peak. This corresponds to 37.3 mV peak-to-peak at a 43 dB input and 1.34 V peak-to-peak at a 12 dB input. Contact info@popotomodem.com for other gain configurations.

Refer to the digital board connector locations figure for the connectors' physical locations.

Hydrophone Input Interface schematic connections.

Hydrophone Input Connector 1 Interface Electrical Pinout

Pin Number	I/O	Pin Name	Notes
1	I	AIN4P	Hydrophone input
2	P	AIN4N	Hydrophone return
3	I	AIN3P	Hydrophone input
4	P	AIN3N	Hydrophone return
5	I	AIN2P	Hydrophone input
6	P	AIN2N	Hydrophone return
7	I	AIN1P	Hydrophone input
8	P	AIN1N	Hydrophone return
9	P	VSYS_5V	+5 V out when unit is powered up
10	P	GND	Board ground

Hydrophone Input Connector 2 Interface Electrical Pinout

Pin Number	I/O	Pin Name	Notes
1	I	AIN8P	Hydrophone input
2	P	AIN8N	Hydrophone return
3	I	AIN7P	Hydrophone input
4	P	AIN7N	Hydrophone return
5	I	AIN6P	Hydrophone input
6	P	AIN6N	Hydrophone return
7	I	AIN5P	Hydrophone input
8	P	AIN5N	Hydrophone return
9	P	VSYS_5V	+5 V out when unit is powered up
10	P	GND	Board ground

Hardware Components

Each Hydrophone Input Interface uses a Molex iGrid connector, P/N 5016461000, or equivalent. The recommended contacts are Molex P/N 5016471000, suitable for 22–26 AWG wire. They can be crimped with Molex hand tool 0638192300; pre-crimped leads may also be used.

Hydrophone Input Interface User-Side Molex Connector.

Hydrophone Input Interface Components and Part Numbers

Part Number	Manufacturer	Description
5016461000	Molex	iGrid 10-position connector receptacle, 2.0 mm pitch
5016471000	Molex	iGrid 22–26 AWG sockets
0638192300	Molex	iGrid 22–26 AWG hand crimp tool
2174912225	Molex	Pre-crimped iGrid 22 AWG lead, 450 mm length

Mating Cable Notes

• Route all hydrophone connections carefully to minimize noise pickup.
• Attach or detach the interface only while power is disconnected from the OEM modem to avoid electrical damage.

Analog Line Input Interface

Overview

The Analog Line Input Interface accepts an analog signal into the receiver pathway. The transducer receiver is normally digitized at separate 43 dB and 12 dB gains; using analog line input disconnects the transducer from the 12 dB pathway.

Move the Analog Input Selection Jumper from pins 1/2 to pins 2/3 before using this interface.

Electrical Connections

The interface is single-ended and AC-coupled through a 10 µF capacitor with a 10 kΩ input resistor, producing a 1.59 Hz high-pass cutoff and approximately 10 kΩ pass-band input impedance. Gain is −1 V/V with an intentional 123 kHz low-pass roll-off. The full-scale ADC input is 5.32 V peak-to-peak.

Analog Line Input Interface schematic connections.

Analog Line Input Interface Electrical Pinout

Pin Number	I/O	Pin Name	Notes
1, SMA center	I	LINE_IN	Analog line input
2–5, SMA shell	P	GND	Board ground

Hardware Components

The Analog Line Input Interface accepts any standard SMA cable. One example is TE Connectivity P/N CSE-SGAM-305-SGAM.

Analog Line Output Interface

Overview

The Analog Line Output Interface provides an analog signal from the transmitter pathway.

Electrical Connections

The transmitter pathway contains a two-channel stereo DAC. The left channel drives the Transducer Interface and the right channel drives the Analog Line Output Interface. The line output is single-ended and DC-coupled, with a minimum supported load of 32 Ω and a full-scale output of 1.58 V RMS.

Analog Line Output Interface schematic connections.

Analog Line Output Interface Electrical Pinout

Pin Number	I/O	Pin Name	Notes
1, SMA center	O	LINE_OUT	Analog line output
2–5, SMA shell	P	GND	Board ground

Hardware Components

The Analog Line Output Interface accepts any standard SMA cable. One example is TE Connectivity P/N CSE-SGAM-305-SGAM.