Multiply the number of devices*, by the length of the line in km, by the speed of operation in kBaud, by a “problem factor”, and keep the number below thirty.
The star (*) above on number of devices is to warn that one generally deals in quarter load or better devices. If you are using older full-load RS485 devices then consider each device as four devices.
The problem factor is a number which is usually unity (1) when all the line characteristics are well conditioned. Double it if there is poor biasing. Double it for poor termination. Double it again if cable runs are close to motors or transformers. Double it for improper capacitance. Double it for poor twisting of the cable. Double it for potential ground faults like poor/weak or multiple grounding.
BACnet MS/TP and many RS485 communications networks allow 250 addresses with data rates up into the 100 kBaud range and line lengths to a maximum of 1200m. But having all of these maximums at once is not practical.
The rule of thumb is key when connecting chains of Modbus RTU meters to Cimetrics B6030 EasyMap to BACnet or Cimetrics B2000 router to ModbusTCP; or when connecting chains of BACnet MS/TP devices for use with a Cimetrics B6000 BACnet/IP to MS/TP router.
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