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Careful stripping of cable ends and proper tightening of the terminal screws is of key importance for proper bus
operation. Intermittent connections may cause hard to troubleshoot problems. However, open, short or miswired
connections can be diagnosed using a simple digital multimeter.
Switch off the bus power supply (supplies).
1.
Measure resistance between A and B terminals at both ends of the bus, where the 120 Ohm
2.
terminators are connected. The value of resistance must be between 60 to 90 Ohms. The lower value is
valid for short buses of a few tens of meters, the higher value for buses of significant total length of several
hundreds of meters.
If the resistance is lower than 60 Ohms, there is a short between the A and B signal wires somewhere
along the line, or the terminators are of lower than the required 120 Ohm value, or more than two
terminators are installed by mistake.
If the resistance is higher than 90 Ohms, one of the A or B signal lines is disconnected along the bus, or
the terminators are of higher than the required 120 Ohm value. Note, that if the incoming and outgoing
wires are connected to different unit terminals (incoming to A, B, R, SH and outgoing to A', B', R', SH', or
vice versa), the signals are connected through inside the units, so if a unit is removed, the bus remains
open at that point.
If needed, a quick check can be performed at any unit along the bus: resistance between A and B
terminals should be between 60 and 110 Ohms.
Measure resistance between B and R terminal at the hub (in case of multiple hubs on the bus, at the
3.
hub where bias jumpers are installed). It should be somewhat below 270 Ohms, the exact value depends
on the number of units connected and the exact topology of the network. If it is higher, the B bias jumper in
the hub is not installed. Now measure resistance between B and R terminal at the far end of the network, it
should be below 470 Ohm. Resistance above this value indicates that the R signal is open along the bus.
Switch on the bus power supply. Make sure that the bus is idle, i.e. there is no activity at the connected
4.
SMDs, rotators don't turn, etc. Idle bus is indicated by the STATUS LEDs on the units being permanently
off.
Measure voltage between A and B terminals at any unit. There should be around 0.5V between A and
5.
B, in case of long buses this value may be higher at the hub (up to 0.75V) and lower at the far end of the
bus (down to 0.25V).
If the voltage is lower than that, one of the bias jumpers in the hub is missing, or the power to the hub is
disconnected.
If the voltage is higher, there are multiple hubs on the bus and bias jumpers are installed in more than one
of them.
General rule is that the voltage at A terminal must always be positive relative to B terminal at every unit. If
it is not, wires are miswired somewhere along the bus.
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Basic uLINK bus troubleshooting
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