10BASE-T1S is an Ethernet physical layer, standardised by the Institute of Electrical and Electronics Engineers (IEEE), designed to provide 10 Mbps communication over a single twisted pair of wires. This makes it particularly suitable for environments where space and weight constraints are critical, such as in automotive networks and industrial automation systems.
For signalling, it utilises the Biphase-Mark variant of Differential Manchester Encoding (DME) where the presence or absence of transitions in the middle of the bit period indicates logical values (zero or one). As a result it eliminates any baseline wandering, and data is immune to noise and polarity inversions due to the use of transitions, not absolute voltage levels.
It utilises Physical Layer Collision Avoidance (PLCA) on a multidrop, half duplex network up to 25m where up to 255 node IDs can be assigned. All nodes on the bus are assigned a node ID, where ID 0 is the coordinator. It operates using time slots, so the network needs configuring to the correct number of nodes or the time slots will be out of sync. The transmission cycle begins when the coordinator sends a beacon, then each node in turn has a Transmit Opportunity (TO) – a node will forfeit its TO if nothing is sent within the TO window (32 bit-times by default). Nodes operate using a round robin schedule where they count TO’s; they don’t handshake with one another.
10BASE-T1S utilises 4B/5B encoding and also uses a 17-bit scrambler in order to improve EMC performance. If you use our Differential Manchester decoder to view the data, you will only be able to see the scrambled, 5B transmitted data – it can’t do the descrambling or 5B -> 4B conversions; that’s where our 10BASE-T1S decoder comes into play. Some of the 5B data represents ‘special functions’ such as Beacon, Silence, Sync/Commit, SSD (start of stream delimiter) and ESD (End of stream delimiter). There are a few types of ESD’s defined in IEEE 802.3cg which can be sent in 10BASE-T1S to indicate various states. The special functions don’t have a corresponding 4B value, so it’s not possible to translate it into a 4B binary sequence – all other valid 5B data can be translated into a 4B value.
There are several types of packets which we can expect to see on a 10BASE-T1S network. These are:
- Beacon Indication
- Silence Indication
- Heartbeat event
- Standard packet
- Burst mode packet.
Beacon Indication #
A sequence of at least two consecutive ‘N’ symbols can be regarded as a Beacon message. We have typically seen 5 beacons together.
Silence Indication #
Used by a node to communicate to other nodes that they have nothing to transmit. Defined as symbol ‘I’ in IEEE 802.3cg.
Heartbeat Event #
Represented using symbol ‘T’, it is used during the `Auto-Negotiation for single differential pair media` process. This may or may not be implemented by particular PHYs. If Auto-Negotiation is implemented, 1000BASE-T1, 100BASE-T1, and 10BASE-T1S PHYs shall support HSM (High speed mode) and may optionally support LSM (low speed mode). When performing Auto-Negotiation in high-speed mode, DME pages are transmitted at a nominal rate of 16.667 Mb/s. In low speed mode, DME pages are transmitted at a nominal rate of 625 kb/s.
Standard Packet #
When a node has a packet to transmit, it will begin transmitting 2 commit special functions (J), followed by 2 SSD special functions (H) so we end up with JJHH. We can then expect to see a standard ethernet frame (up to 1530 bytes) with an ESD appended. If the packet is fine, we can expect to see the heartbeat symbol ‘T’ (ESD / HB), followed by symbol ‘R’ (ESDOK / ESDBRS).
If the ESD is ‘T’ followed by ‘K’ then it means there has been a transmit error detected. However, if we see ‘T’ followed by ‘S’ then a jabber condition has been detected. This is a fault case in ethernet where the frame size exceeds the maximum allowed size.
Burst Mode Packet #
10BASE-T1S also supports a mode of operation called burst mode. This occurs when a node wants to share more data than the standard packet allows, and it allows the node to hog the bus and utilise several transmit opportunities to send more than one packet.
To indicate that a burst mode is coming, the ESD at the end of the first packet is modified so it contains two ‘R’ symbols. The next packet will begin with a series of commits ‘J’, followed by two SSD symbols ‘H’ followed by a standard ethernet frame. If, after this, there are no more packets to send in this burst sequence, the ESD will appear as ‘T’, then ‘R’. If there is another packet to send, it will send two ‘R’ symbols in the ESD again and the sequence repeats. The last packet sent should contain ‘T’ then ‘R’ as the ESD as this indicates the end of the message.
Capturing and analysing 10BASE-T1S communications with PicoScope #
This guide will show you how to use the 10BASE-T1S decoder in the PicoScope software.
Select Serial decoding from the tab on the left. If it doesn’t appear on the main page, select the More… tab and you’ll find it there.
Next, select 10BASE-T1S from the list of available protocols.
In the Configuration tab, select the corresponding PicoScope input channel for Data and provide the appropriate values for the relevant 10Base-T1S fields.
In the Display tab, select the desired Graph and Table display format options to display 10BASE-T1S packets in the appropriate locations.
Double-click a packet in the graph view to see the same packet in the table view, and vice versa, or use the zoom feature to focus on the appropriate areas of the decoder packets.