Twinaxial cable

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

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 System i (formerly known as 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.

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.

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