Aaeon MAX-Q670A, Installation Manual

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Appendix C: Communication Devices
61
TIA/EIA-485
Historically, TIA/EIA-422 was on the market before TIA/EIA-485.
Due to the lack of bi-directional capabilities, a new standard adding this
feature was created: TIA/EIA-485 . The standard (TIA/EIA-485-A or
ISO/IEC 8284) defines the electrical characteristics of the
interconnection, including driver, line, and receiver. It allows data rates
in the range of 35 Mbps and above and line lengths of up to 1200 m. Of
course both limits can not be reached at the same time. Furthermore,
recommendations are given regarding wiring and termination. The
specification does not give any advice on the connector or any protocol
requirements.
Electrical
TIA/EIA-485 describes a half-duplex, differential transmission on
cable lengths of up to 1200 m and at data rates of typically up to 35
Mbps (requirement similar to TIA/EIA-422, but tr<30% of the bit
duration, there are also faster devices available, suited for higher rates
under certain load-conditions). The standard allows a maximum of 32
unit loads of 12 k
Ω
, equal to 32 standard nodes or even higher count
with increased input impedance. The maximum total load should not
drop below 52
Ω
. The common-mode voltage levels on the bus have to
maintain between –7 V and 12 V. The receivers have to be capable to
detect a differential input signal as low as 200 mV.
RS-485 is terminated at both sides of the common bus, even if only
two stations are connected to the backbone.
Protocol
Not applicable/none specified; exceptions: SCSI systems and the DIN-
Bus DIN66348. The Raptor MicroATX requires transmitter enabled by
the RTS signal.