Simpson Amik 300, Operating Manual, Page: 35 / 72

11. RS 485 ( ModBus ) Output :
This instrument supports MODBUS (RS485) RTU protocol ( 2-wire ).
Connection should be made using twisted pair shielded cable. All "A" and "B" connections
are daisy chained together. The screens should also be connected to the “Gnd” terminal.
To avoid the possibility of loop currents, an Earth connection should be made at one point
on the network. Loop (ring) topology does not require any termination load. Line topology
may or may not require terminating loads depending on the type and length of cable used.
The impedance of the termination load should match the impedance of the cable and be at
both ends of the line. The cable should be terminated at each end with a 120 ohm
(1/4 Watt min.) resistor.
RS 485 network supports maximum lenght of 1.2km. Including the Master, a maximum of 32
instruments can be connected in RS 485 network. The permissible address range for the
instrument is between 1 and 247 for 32 instruments. Broadcast Mode (address 0) is not allowed.
The maximum latency time for the instrument is 200ms i.e. this is the amount of time that
can pass before the first response character is output.
After sending any query through software ( of the Master) , it must allow 200ms of time to
elapse before assuming that the instrument is not going to respond. If slave does not
respond within 200 ms, Master can ignore the previous query and can issue fresh query
to the slave.
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