22
LD485S-MP
Figure 5-2. Digital Loopback Test.
In the line loopback mode, the RS-485 receive data is not actively received and
retransmitted. In effect, the length of the line is doubled. This may cause cause
reception of errors. If you suspect that the length of the line is the source of the
errors, press the DLB pushbutton at the remote end.
Test Procedure
Proceed as follows to perform a test:
1. Check for the simple problems first: Is power applied? Are the line drivers
configured correctly?
2. Press the LLB button on the units that are not operating properly. Press the
Test button. As each unit’s Test button is pressed, the RD indicator should
flash, then go off. This tests the internal circuitry of the line driver. If a unit
does not pass this test, it is defective.
3. In a point to point application, observe the indicators at the local unit. In a
Multipoint configuration, observe the indicators at the master unit. Have
someone press the DLB pushbutton only at the remote unit(s) in question.
The RD indicator on the local unit should flash, then go off. If the RD
indicator remains lit or flickers, any of the following could be causing the
problem:
• The baud rates of the local and remote units do not match.
• Incorrect RS-232 connections.
ENCODE
DECODE
PIN 2
PIN 3
RS-232
RX A, B
TX A, B
RS-485
DIGITAL LOOPBACK
RS-485
RECEIVER
RS-485
TRANSCEIVER
