Issue
Are 120 Ohm terminating resistors required on the IOU bus?
Environment
IOU Bus
Cause
Occasionally, customers have reported that unreliable communications with RS485 versions of IO Modules can be corrected by removing the 120 ohm bus termination resistors. The Continuum IO Module Install Sheet and the Continuum IO Systems Reference documents state that these resistors are required because it is recommended by the RS485 standard and by Echelon documents. These documents assume a perfect bus topology where there are no stubs, cable impedance is uniform throughout the bus, shielding is continuous, and each node's transceiver circuit presents the same load.
Resolution
The reality is that each installation of a Continuum IO bus is a little different with respect to these parameters and does not represent an ideal bus. A typical installation usually consists of clusters of modules plugged directly together and then connected to each other and the NetController by various lengths of shielded cable. Additional variations are introduced by the fact that multiple vendors and multiple generations of RS485 transceiver ICs are used in the Continuum modules. This means that each installation is in fact a little different and it is possible that the added load of the termination resistors can cause the signal levels on the bus to be ambiguous to some transceivers when the bus is idle. In these cases, removal of the resistors will make the bus more reliable. In summary, if you are experiencing IO Module communications problems, first check all connections per the installation instructions and ensure that the NetController and modules are properly grounded. If you are still experiencing problems try removing the 120 ohm resistors and check to see if reliable communication is restored.
