Specifications

RS-422/485 Application Note 21
© Copyright B&B Electronics -- Revised 2506
B&B Electronics Mfg Co – 707 Dayton Rd - PO Box 1040 - Ottawa IL 61350 - Ph 815-433-5100 - Fax [phone]
B&B Electronics Ltd – Westlink Comm. Pk – Oranmore, Galway, Ireland – Ph 353-91-792444 – Fax [phone]
Losses in a transmission line are a combination of AC losses (skin effect),
DC conductor loss, leakage, and AC losses in the dielectric. In high quality
cable, the conductor losses and the dielectric losses are on the same order of
magnitude. Figure 3.2 is included in this application note to point out the
significant difference in performance of different cables. This chart shows
Attenuation versus Frequency for three different Belden cables. Note that the
polyethylene cables offer much lower attenuation than PVC cables.
Figure 3.1
22 RS-422/485 Application Note
© Copyright B&B Electronics -- Revised 2506
B&B Electronics Mfg Co – 707 Dayton Rd - PO Box 1040 - Ottawa IL 61350 - Ph 815-433-5100 - Fax [phone]
B&B Electronics Ltd – Westlink Comm. Pk – Oranmore, Galway, Ireland – Ph 353-91-792444 – Fax [phone]
Figure 3.2
Another approach to choosing transmission line is the “E-GRADE
Program,” which has been established by Anixter Bros. Inc. Anixter is a
worldwide distributor of wiring system products. Under this program, Anixter
divides data interface cables into four categories as follows:
E-GRADE 1 LIMITED DISTANCE
E-GRADE 2 STANDARD DISTANCE
E-GRADE 3 EXTENDED DISTANCE
E-GRADE 4 MAXIMUM DISTANCE
Simple charts are used to help the user select the proper cable without any
technical understanding of the cable parameters. This program divides the
usage categories into EIA-232-D, EIA-422-A, and EIA-423-A. When using
this literature, use the EIA-422-A charts for choosing RS-485 cable.