Phoenix MicroATX Express, Installation Manual

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Phoenix MicroATX Express – Installation Guide
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.
RS-485 is Differential and Multi-Point Connected
Differential Transmission
Please, read the Differential Transmission explanation in the previous RS-422 section.
Termination Resistors
Follow instructions in the previous RS-422 and RS-485 sections. The termination resistors available are rated to
120
Ω
.
Ground Connections
All 422- and 485-compliant system configurations shown up to this point do not have incorporated signal-return
paths to ground. Obviously, having a solid ground connection so that both receivers and drivers can talk error free is
imperative. The figure below shows how to make this connection and recommends adding some resistance between
logic and chassis ground to avoid excess ground-loop currents. Logic ground does not have any resistance in its path
from the driver or receiver. A potential problem might exist, especially during transients, when a high-voltage
potential between the remote grounds could develop. Therefore, some resistance between them is recommended.
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