Circuit Note

CN-0575

analog.com

Rev. 0 | 2 of 7

CIRCUIT DESCRIPTION

FIELD DEVICES

10BASE-T1L is one of the physical layer device (PHY) specifica-

tions used in single pair Ethernet that provides a 10 Mbps Ethernet

connection over a single twisted pair, up to a length of 1 km. It was

defined and developed to be a migration path for the many open

and proprietary Fieldbus variants used in process automation such

as HART, IO-Link, and Modbus. This allows field instruments and

remote I/O devices (for example, sensors, actuators, cameras, ma-

chines) to seamlessly communicate with high level control systems

via Ethernet. 10BASE-T1L also supports power delivery via SPoE

for high power applications, with power levels from 1.23 W at 24 V

to 52 W at 54 V.

The CN0575 is a platform for developing Ethernet-capable field de-

vices and applications. Raspberry Pi and Linux provide a rich pro-

totyping and development environment with support for precision

data converters, temperature, pressure, flow, acceleration, angular

rate sensors, actuators, displays, and I/O devices. This allows a

wide variety of field devices to be prototyped and later migrated

to a more optimal controller and power solution for the particular

application.

MEDIUM DEPENDENT INTERFACE

The single pair Ethernet (SPE) cable, power coupling, filtering, and

data coupling network to the MAC-PHY forms the medium depend-

ent interface (MDI) of the CN0575. For 10BASE-T1L systems that

support SPoE, this circuit is usually comprised of a mechanical

connector, common-mode and differential-mode interface chokes,

coupling capacitors, and data isolation transformers. Figure 2

shows a simplified diagram of the MDI used in the CN0575. Proper

selection of the MDI components is critical to minimize losses and

to preserve the integrity of the data signals.

Figure 2. CN0575 Medium Dependent Interface Circuit

The common-mode choke (CMC) in the MDI suppresses common-

mode noise that may be coupled into the SPE cable from the envi-

ronment, while passing the differential data and power. As such, it

is important to select a device that has a high common-mode impe-

dance and a low differential-mode impedance in the 10BASE-T1L

frequency range (0.1 MHz to 20 MHz). Since the common-mode

choke is placed in series with the SPE cable, the device must be

rated to handle the maximum cable current specified for the PD

class being used (that is, 632 mA for Class 12; and 231 mA for

Class 13).

The core capability of the 10BASE-T1L is a full duplex, DC bal-

anced, point-to-point communication scheme with pulse-amplitude

modulation 3 (PAM-3) at a 7.5 MBd symbol rate with 4B3T coding.

It supports two amplitude modes: 2.4 V peak-to-peak for a cable

up to 1000 meters and 1.0 V peak-to-peak at a reduced distance.

Power and data are separated via inductive (DC) coupling and

transformer (AC) coupling, respectively. SPoE power is extracted

by a differential mode coupled inductor (DMI). Similar to the com-

mon-mode choke, it must also have a maximum current rating

appropriate for the PD class in use, and low DC resistance to

minimize power loss. Data is coupled to the PHY by an isolation

transformer.

The isolation transformer's inductance is in parallel with the DMI's

inductance; the total inductance must be high enough to avoid

significant loading at frequencies of 7.5 MHz and higher. The IEEE

802.3dd-2022 amendment specifies a maximum signal droop of

25%. The DMI inductance in the CN0575 is 220 µH and the

isolation transformer's inductance is 2200 µH, or 200 µH total. This

results in a maximum droop of 10%, maintaining signal integrity and

easily meeting the 25% specification.

10BASE-T1L MAC-PHY CIRCUIT

The ADIN1110 provides the 10 Mbps, full-duplex, single-pair Ether-

net connectivity in the CN0575. As shown in Figure 3, this device

combines a 10BASE-T1L PHY and its associated subsystems with

an Ethernet MAC that can directly communicate with the host

processor via SPI. This allows the ADIN1110 to be used with low

power processors that have no built-in MAC. The SPI bus can

operate up to 25 MHz, enabling the full 10 Mbps bandwidth of the

10BASE-T1L interface, with margin.

Figure 3. ADIN1110 Functional Block Diagram

The MDI pins, TXP, TXN, RXP, and RXN, connect the ADIN1110

PHY to the Ethernet network via the 10BASE-T1L cable. For proper

operation of the ADIN1110, an external hybrid must be placed

between these pins and the cable coupling network. Figure 4 shows

the required topology and values specified in the ADIN1110 data

sheet for this hybrid.