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Twinaxial Cable
Description of Twinax Cable
- IBM Midrange Systems (AS/400, S/36, S/34, S/32, etc.) Use a special cable,
called Twinaxial cable (twinax for short), to connect peripheral devices
(Terminals and printers) to the host. The term Twinax is used both for the
physical cable and for the protocol used to transfer data between the host and
the peripheral devices.
- The Twinax line is a two-lead shielded cable, designed to operate at speed
of 1 Million bits per second and to connect up to 7 devices on the same line.
Information passes through the cable as pulses of 3.5 volts to .2 volts (at
the end of a long cable). The peripheral devices are designed to work with
pulses that are less than 25 percent of the low .2 volts signal.
- The maximum permissible cable length is 5,000 feet. Each device is
assigned an address in the range of 0 to 6 (i.e. 7 total per line), set by a
physical or logical switch located at the device. Physical switches, typically
toggle three switches, are used for older machines and twinax printer
converters. Newer devices utilize a setup mode to assign the device address.
Each device connected to a Twinax loop must be assigned a unique address.
- The last device on a Twinax line must be terminated by a pair of 55 Ohm
resistors (a twinax “T” or “Y” cable is also acceptable).
Quick note
- If the terminal cursor is in the upper right corner of your screen you
have a cable problem! Cable Failures
- The Twinax cable is especially problematic at connecting points.
- Installation of the connector is difficult and should not be performed by
a novice. Non-professional connections will cause problems all along the line,
affecting devices other than the one with the poor connection. An improperly
engaged locking nut will allow the cable to rotate inside the connector. The
following problems may be due to poor installation of connectors.
- Please note that if you used Coaxial cable cutters on the twinax cable
ends they will not work!
- Short circuit between conductors or total line failure.
- Disconnection of one or two conductors, (no continuation-no termination
or half termination), to devices that are closer to the host.
- Poor grounding
- Intermittent problems (to some or all of the devices on the line) when a
device is moved.
- Terminal cursor displayed only in the upper right corner.
Lightning Protectors
- These devices protect both the host and the peripheral device(s) from
lightning damage. Normally, these devices are installed whenever these is a
need to put a Twinax line outside a building or underground. The disadvantage
is that these devices can aggravate any inherent communication problem.
Lightning protectors are ALWAYS installed in pairs to protect both host and
peripherals. Installation of more than a single pair on the same Twinax line
is not permitted. The design of some lightning protectors crosses the
communication line in order to enforce the use of two protectors
Splicing, Connections and Maximum Cable Length
- Each connecting point in the Twinax line reduces the signal quality. IBM
permits 14 connections per line. This permits the attachment of the maximum 7
devices times 2 connections each, at the most. When the Twinax line is not
long, one can add connections, if needed.
- Twinax Doublers
A Twinax doublers is an electronic device that enables the passage of 2 ports
(14 addresses) along a single Twinax line. This device contains transformers
which invert the signal of one of the ports. A pair of Twinax cables at the
computer or communications controller are connected to the Twinax doublers.
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