Tech Briefs Magazine - March 2021 - ST-8

Single Pair Ethernet
SPE Implementation Scenarios
IIot Services
(Monitoring & Optimization)

Engineering
Workstation

Control
System

Asset Management
Station

Public/Private
Cloud

Integration
TCP/IP &
Ethernet

Industrial
Ethernet

SPE

Migration

Industrial
Ethernet

Addition

SPE

SPE Adoption Timeline
Figure 2. (Image courtesy of TE Connectivity)

means to deliver power on the same
Ethernet cable as the data. Supporting the delivery of up to 25.5 W at 48
VDC, PoE allowed for powering
remote devices such as surveillance
cameras and wireless access points. A
later version is known as Power over
Data Lines (PoDL - IEEE 802.3bu2016), which allows for delivery of up
to 50 W at 12, 24, or 48 VDC. PoDL
was developed specifically for the
xBASE-T1 SPE market and allows for
both data and power to be delivered
via a single wire pair.
The xBASE-T1 standards can be further
broken down into 10BASE-T1L (IEEE
802.3cg), 100BASE-T1 (IEEE 802.3bw),
and 1000BASE-T1 (IEEE 802.3bp). The
following table summarizes the primary
features of these variants:

The " L " in the 10BASE-T1L stands for
" long reach " owing to its 1 Km length
(many implementations can actually
exceed 1 Km depending on cable quality
and connector types). One of the many
advantages to the SPE specification is
that it can use the existing single twistedpair fieldbus cable runs. This is a huge
savings for industrial applications. And,
with the addition of PoDL, the remote
device can support communications and
be powered over the same cable segment. As an added benefit, the 10
Mbit/sec data rate is significantly faster
than the fieldbus implementations it is
meant to replace.
Whereas the T1L variant is targeted at
point-to-point applications, there is also
a " short reach " variant (10BASE-T1S)
that is wired as a multi-drop implemen-

Standard

10BASE-T1L (IEEE
802.3cg)

100BASE-T1 (IEEE
802.3bw)

1000BASE-T1 (IEEE
802.3bp)

Bandwidth

10 Mbps

100 Mbps

1000 Mbps

Cable Reach

1000 m (2.4V p2p)

50 m

15 m

Power dissipation

< 110 mW

< 220 mW

< 600 mW

Communication

Full Duplex

Full duplex

Full Duplex

Media Access
Control Interface

Media Independent
Interface (MII),
Reduced MII

MII, RMII, Serial Gigabit
MII (SGMII), Reduced
Gigabit MII (RGMII)

SGMII, RGMII

Table 1: SPE Category Specifications

www.techbriefs.com

Cov

ToC

tation to replace common fieldbus versions such as 20 mA current loop and
CAN. The reach of the T1S flavors is significantly shortened to 25 m. But, the
use of multi-drop allows for a single
cable run with a single port interface for
the media access PHY.
In order to support the multi-drop
access and avoid potential contention
on the cable, there are two approaches.
The first is using Carrier Sense Multiple
Access/Collision Detection (CSMA/
CD) that harkens back to the original
Ethernet implementations from the
1980s. In this approach, each station
first listens for traffic on the bus before
transmitting. In the event that multiple
stations attempt to transmit at the same
time, the collision is detected, each station stops transmitting, waits a short random amount of time, and then the
process repeats with listening for an idle
bus and trying the transmit again. This
is a simple approach, but it adds a stochastic element to the communications,
which will introduce latency.
If the application is particularly latencysensitive, an alternative Physical Layer
Collision Avoidance (PLCA) mechanism
can be added to the CSMA/CD, where
one station is designated the master, which
sends out a beacon that allows only the station designated in the beacon to transmit
(somewhat like a token ring implementation). This facilitates determinacy by providing a designated slot for each station
and avoiding the potential for collisions as
the number of stations increases to the 31station maximum. It should be noted that
PoDL is not yet defined for use in multidrop applications.
Ethernet is Ethernet
One of the significant advantages to
SPE is that in the final analysis, it is
Ethernet. So, as far as protocol stacks are
concerned, SPE is just like every other
Ethernet segment. This means that you
can easily use IPv4/IPv6 protocols on
top of the SPE implementation. This is a
huge software development time-saver,
as the software teams can use standard
IP-based APIs for communications.
There is no requirement for protocol
conversion from one of the fieldbus variants to IP and back, thereby reducing
latency and eliminating the cost of the
protocol converter device.
Auto speed detection in the SPE
switch is also a possibility just like the
typical four-pair Ethernet switch.
Therefore, one switch could handle
Sensor Technology, March 2021

http://www.techbriefs.com

Tech Briefs Magazine - March 2021

Table of Contents for the Digital Edition of Tech Briefs Magazine - March 2021