Trusted, Communication interface t8151b, Signal ground – Rockwell Automation T8xxx Trusted Communications Interface User Manual
Page 54: A and b circuit designators
Trusted
TM
Communication Interface T8151B
Issue 21 Apr 10
PD-T8151B
54
A
B
RX
A
B
TX
A
B
RX
A
B
RX
A
B
RX
A
B
TX
A
B
TX
A
B
TX
Term
Term
Figure 22 Half Duplex Multiplexed Topology
The maximum number of nodes that the Communication Interface supports before hardware build C
for multi-drop applications is 6. From hardware build C, the module has internal terminations changed
to support the full 32 drop capacity.
6.2.4. Signal Ground
The topologies shown in the previous diagrams do not show the Signal Ground for reasons of clarity.
On all the above circuits, Signal Ground must be connected for correct operation. It is also important
not to connect the signal ground to the chassis ground. The Signal Grounds for each port on the
Communication Interface should be kept separate from each other to maintain the isolation between
ports. It is recommended that when specifying cables that an extra twisted pair is specified to carry the
signal ground connection. Signal ground should not be connected to the cable screen because the
cable screen should be used for EMI shielding and should be connected to chassis ground.
6.2.5. A and B Circuit Designators
The ‘A’ and ‘B’ circuit designators are often a source of confusion for systems integrators. This is not
helped by the use of non-standard ‘+’ and ‘-’ symbols on some equipment.
To determine which terminal is ‘A’ and which is ‘B’, set the line to the ‘Idle’ or ‘Off’ state. Connect a
multimeter on the volts range across the ‘A’ and ‘B’ terminals. When the multimeter reads a negative
voltage, the +ve terminal of the multimeter is connected to ‘A’ and the -ve terminal is connected to ‘B’.
The ‘+’ and ‘-’ symbols shown on some equipment are rarely very reliable at indicating which circuit is
which unless connecting equipment of the same make together. There is no standard for matching ‘+’
and ‘-’ to ‘A’ and ‘B’. It is therefore important to refer to the manufacturer’s handbook or to test the line
as in the previous paragraph.