The P-NET standard specifies the use of shielded cable with 1 x twisted pair; this cable type is widely used. However, using a cable with an additional pair of wires results in optimal noise immunity and a faster installation, which both compensates for the extra cost of the cable.
When using a shielded cable with 2 x twisted pairs the extra pair is used for connection of the ‘S’ terminals. Both wires of the extra pair must be connected to the ‘S’ terminal. This releases the shield for use for shielding purposes only. The shield is terminated as shield only and is not connected to any terminal.
The communication pair must have a characteristic impedance of 100 to 120Ω and a core cross section of minimum 0.22mm² / 24 AWG. To ensure maximum signal integrity, it is recommended that the core cross-section be increased for long bus cables.
|Bus length||Recommended core cross-section|
|0 - 100m||Minimum 0.22 mm² / 24 AWG|
|100 - 400m||Minimum 0.34 mm² / 22 AWG|
|400 - 1200m||Minimum 0.50 mm² / 20 AWG
Both 1 x twisted pair and 2 x twisted pair cable are valid installation techniques, though the 2 x twisted pair installation is recommended due to the extended noise immunity and the more convenient installation.
Recommended cable (1x twisted pair)
IBM part No.: 7362211. 1 x 2 x 0.50mm² / 20 AWG, characteristic impedance: 105Ω +/- 5Ω, 51pF/m.
Belden part No.: 3105A. 1 x 2 x 0. 34mm² / 22 AWG, characteristic impedance: 120Ω, 33pF/m.
Recommended cable (2 x twisted pair)
Belden part No.: 3107A. 2 x 2 x 0,34mm² / 22 AWG, characteristic impedance: 120Ω, 33pF/m