science

Twinaxial cabling

Twinaxial cabling is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Twinaxial cabling rather than just read about it. In short: Twinaxial cabling, or twinax, is a type of cable similar to coaxial cable, but with two inner conductors in a twisted pair instead of one. Due to cost efficiency it is becoming common in modern (2013) very-short-range high-speed differential signaling applications.

Key takeaways

Twinaxial cabling belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Twinaxial cabling to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Twinaxial cabling from memory before moving on to harder problems.

Reference excerpt

Twinaxial cabling, or twinax, is a type of cable similar to coaxial cable, but with two inner conductors in a twisted pair instead of one. Due to cost efficiency it is becoming common in modern (2013) very-short-range high-speed differential signaling applications.

Legacy applications

IBM Historically, twinax was the cable specified for the IBM 5250 terminals and printers, used with IBM's System/34, System/36, System/38, and IBM AS/400 midrange hosts, and with IBM Power Systems machines running IBM i. The data transmission is half-duplex, balanced transmission, at 1 Mbit/s, on a single shielded, 110 Ω twisted pair. With twinax, seven devices can be addressed, from workstation address 0 to 6. The devices do not have to be sequential. Twinax is a bus topology that requires termination to function properly. Most twinax T-connectors have an automatic termination feature. For use in buildings wired with Category 3 or higher twisted pair there are baluns that convert Twinax to twisted pair and hubs that convert from a bus topology to a star topology. Twinax was designed by IBM. Its main advantages were high speed (1 Mbit/s versus 9600 bit/s) and multiple addressable devices per connection. The main disadvantage was the requirement for proprietary twinax cabling with bulky screw-shell connectors.

Physical layer Signals are sent differentially over the wires at 1 Mbit/s (1 μs/bit ± 2%), Manchester coded, with preemphasis. The signal coding is only approximately differential and not completely differentially balanced. In general, one of the two signal lines is driven to −0.32 V ± 20%, while the other carries 0 V. This, itself, could be considered as two differential signals of ±0.16 V superimposed on a −0.16 V common mode level. However, to provide preemphasis, for the first 250 ns (1/4 bit time) after a signal is driven low, the negative signal line is driven to −1.6 V. During this time, the common-mode voltage is −0.8 V. This signal is designed to provide a minimum of ±100 mV at the end of 152 m (500 feet) of cable. The two wires are denoted A and B. To encode a 0 bit, A>B for the first half of the bit time, and A<B for the second half. A 1 bit is the opposite. Thus, each signal line is driven low for either 500 or 1000 ns at a time, of which the first 250 ns is emphasized. The plug consists of two pins of the same gender.

Data link layer A message begins with five normal 1 bits (A driven low for 500 ns, then B driven low for 500 ns) for bit synchronization, followed by a special frame sync pattern, three bit times long, that violates the usual Manchester encoding rules. A is driven low for 1500 ns, then B is driven low for 1500 ns. This is like a 1 bit sent at 1/3 normal speed (although the preemphasis pulses remain 250 ns long). This pattern is followed by up to 256 16-bit data frames. Each data frame consists of a start bit of 1, an 8-bit data field, a 3-bit station address, and an even parity bit (which includes the start bit, so is equivalent to odd parity over the data and address fields only). This is then followed by three or more fill bits of 0. The bits within each frame are sent lsbit-first. All messages are sent between the controller (master) and one slave device. The first frame in a message from the controller contains the device's address, from 0 to 6. The address field of following frames can be any value from 0 to 6, although is usually set to the device's address as well. The final frame in a message includes an address of 7 (all ones) as an end-of-message (EOM) indicator. A single-frame message does not have an EOM indicator. When a command calls for a response, the device is expected to respond in 30 to 80 μs. A device's response also consists of up to 256 frames, and includes its address in all frames but the last. In this case, a single-frame response includes the EOM address, and the controller assumes it comes from the device it most recently addressed. Generally, the first frame in a message is a command byte, and following frames are associated data.

MIL-STD-1553 MIL-STD-1553 specifies that the data bus should have characteristic impedance between 70 and 85 ohms, while the industry has standardized on 78 ohms. Likewise the industry has generally standardized on the cable known as twinax cable that has a characteristic impedance of 78 ohms.

Current applications

Networking (Direct-Attach Copper)

Direct-Attach Copper (DAC) is a type of standard cabling used in Small Form-factor Pluggable (SFP) Ethernet, initially defined with SFP+ Direct-Attach Copper (10GSFP+Cu), which provides 10 Gigabit Ethernet over either an active or passive twinax cable assembly and connects directly into an SFP+ housing. An active twinax cable has active electronic components in the SFP+ housing to improve the signal quality; a passive twinax cable is mainly just a straight "wire" and contains few components. Generally, twinax cables shorter than 7 meters are passive and those longer than 7 meters are active, but this may vary from vendor to vendor. SFP+ Direct Attach Copper (DAC) is a popular choice for 10G Ethernet reaches up to 10 m due to low latency and low cost. One major application is connecting network hardware through their SFP+ interfaces. This type of connection is able to transmit at 10 gigabits/second full duplex speed over 5 meter distances. Moreover, this setup offers 15 to 25 times lower transceiver latency than current 10GBASE-T Cat 6/Cat 6A/Cat 7 cabling systems: 0.1 μs for Twinax with SFP+ versus 1.5 to 2.5 μs for current 10GBASE-T specification. The power draw of Twinax with SFP+ is around 0.1 watts, which is also much better than 4–8 watts for 10GBASE-T. As always with cabling, one of the consideration points is the bit error ratio (BER). Twinax copper cabling has a BER of better than 10−18 according to Cisco, and therefore is acceptable for applications in critical environments.

Cables must not be bent below their minimum bend radius, which depends upon cable size as expressed in AWG. The table on the right summarizes minimum values typically admitted for SFP+ sustained bend radiuses. This SFP+ twinax DAC is also referred to as "10GBASE-CR" or "10GBASE-CR1" by some manufacturers, even though there is no IEEE or other standard with that name.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Twinaxial cabling

Start with the simplest possible case. Write down what Twinaxial cabling claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Twinaxial cabling before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Twinaxial cabling ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Twinaxial cabling

In research: Twinaxial cabling appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Twinaxial cabling in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Twinaxial cabling is common in secondary-school and first-year university syllabi. It links to neighbouring topics Signal cables, so understanding it makes those chapters shorter.
In everyday life: Look for Twinaxial cabling outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Twinaxial cabling in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Twinaxial cabling means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Twinaxial cabling out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Twinaxial cabling in simple terms?

Why does Twinaxial cabling matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Twinaxial cabling?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Twinaxial cabling.