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Wiring polarity
Not all serial communication protocols require wiring polarity to be consistent over a network, such as LonWorks. The
commonly used Modbus and BACnet MS/TP do need the A(-) and B(+) terminals to match throughout the daisy-chain
wiring.
How to troubleshoot: using the UART errors counter you will see the number increasing if one or more devices have
reversed polarity wiring on the A(-) and B(+) terminals. The data being transmitted or received is reverse the voltage
expected by the device or link. Typically wire color will be used to maintain polarity across the communication link. If
this is not clear by comparing the terminal wiring on other devices, simply swap the wiring polarity at your device and
determine if the error counter stops increasing or at least slows down significantly, see Figure 7.
Bias, too much or too little
Known as the failsafe state, having a bias circuit in the RS-485 network pulls the transceivers to known high and low
voltages. The failsafe prevents transient voltage levels from being interpreted as network traffic. Newer designed
transceivers, like those used in the ACH580, are internally biased and do not require networking biasing. However,
legacy products or devices with different transceiver types may still require there to be a bias circuit to ensure this
failsafe state.
Biasing may become an issue if no device on the network is set to bias the link or if too many are installed causing
potential ground loops or over loading with too many pull up or down resistors. This is possible with internally biased
transceivers then adding, or enabling built-in, bias circuits. The number of transceivers will dictate whether network
biasing will be required.
How to troubleshoot: check the UART errors counter and if increasing there is
potentially problems with the data packets on the network being affected by the
amount of biasing. Confirm if any device includes a dip switch that have enabled or
disabled biasing resistors. If no biasing is enabled on the comm link, identify a device
that could have this biasing resistor enabled, preferably at the end of the comm link.
The ACH580 includes a separate dip switch for bias as well as terminator whereas the
ACH550 included one dip switch for both bias and terminator. In this case you would
need the ACH550 to be the last device on your comm link to act as End of Line
terminator as well. Confirm if the controls system manufacture requires biasing
through their supervisory controller or other specific devices installed on the link.
Number of devices per network
Many controller manufactures will specify the device limit per network link. This is determined using an assumption of
transceiver type that will be connected to their controller. The latest BACnet guide specification states "no more than
32 devices on each MS/TP segment." This refers to the RS-485 standard requiring complaint transceivers to drive a
network with up to 32 unit loads (UL). As more devices are added to a communication link it is possible to overload the
network with too much load impedance thus increasing voltage drop and reducing signal strength.
However, not all transceivers are a full unit load device. When troubleshooting a network, determine the total number of
devices connected to a specific link. If there are approximately 100 devices, the designer may have made the
assumption all devices will utilize a quarter-load (1/4) transceiver, or this is what their controller allows. It is possible by
adding enough other transceiver unit loads this will cause your network to become overloaded. For example, the
ACH580 uses a half-load (1/2) transceiver which would be equivalent to two devices the network is capable of driving. If
the network contains existing ACH550 or ACS320 which are full load transceivers, each drive would be equal up to 4
devices on their network count, see Table 1.
LVD-EOTKN129U-EN REV A
Effective: 2023-03-31
Figure 7: UART error counter increasing
Figure 8: ACH580 bias and
terminator dip switches
4
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