GE IC754CSF15CTD Hardware User's Manual page 54

4
electrical standards specify a differential signaling technique that provides
high data rates, long distances and good noise rejection. The standards do
not address signal encoding (protocol), connectors, or cabling. However,
certain characteristics these devices' interfaces should be considered to
ensure reliable connections.
Connections
Interconnect
media
Termination
4-12
15" QuickPanel* View & QuickPanel Control Loaded Color TFT
Connect nodes in a daisy chain fashion. Do not connect in other
arrangements, especially "star." The standards do not specify the maximum
number of nodes or devices that can be connected to a TIA/EIA 422 or 485
network. Instead, the standards limit the number of electrical connections by
specifying that a maximum of 32 unit loads may be connected. The
QuickPanel View or QuickPanel Control presents one unit load.
Always use twisted pair cabling and group complimentary signals into
conductor pairs; TXA with TXB, for example. Use a cable with a characteristic
impedance of 100 ohms to 120 ohms. A wire gauge of 24 AWG is commonly
used. Maximum cable length is 4,000' (1,219.2m), but may be less due to
cable impedance, connection quality, data rates, and other factors.
Always provide proper termination at each end of the 422/485 network. The
QuickPanel View provides built-in termination resistance when pin #9
(TRMRXB) is connected to pin #10 (RXA).
Do not terminate every node. Only terminate the end nodes.
– December 2011
GFK-2402D