There is more to the termination at the Host PC receiver. The additional resistors
ensure that when all units are tri-state, the differential line rest in an 'idle' state and
therefore do not risk detection of spurious data due to noise or slight offsets in the
differential inputs.
3.5.3 CONNECTING MORE THAN 32 UNITS
RS485 has a drive limitation of 32 receivers. If additional instruments are required
(there is a logical maximum of 99 units ) it is necessary to buffer the Host PC
transmitter as shown in the diagram below.
3.5.4 GROUNDING PROBLEMS
Each instrument has an internal link which connects the comms 0v to unit ground. If
this causes any problems, it may be removed.
Warning Do not remove the Earth strap from the instrument as this could
potentially leave the instrument in an unsafe condition.
COMMUNICATIONS
Page 24
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