ROTRONIC HM4 - Manual (Page 27)

E-M-HM4-V0_08
ROTRONIC AG
Bassersdorf, Switzerland
Document Code Unit
HM4 Humidity and Temperature Probe with
Heated Sensor
Instruction Manual
Document Type
Page 27 of 41
Document Title
© 2014; ROTRONIC AG E-M-HM4-V0_08
7.2 RS-485 Interface
The HM4 has a serial RS-485 port. If the number of USB or TCP/IP network ports available is not enough, the
serial RS-485 interface can be used to connect up to 64 devices in a subnet (as shown in Figure 6). ROTRONIC
offers the RS-485 T-junction box AC3021 as suitable accessory for this.
To read out the measured values, it is also possible to access the HM4 directly without the HW4 software using
digital commands. See the manual E-M-AC3000-CP for the main AirChip3000 commands.
Important:
The RS-485 network address of an HM4 is factory set on "0". The integration of a new factory-configured RS-
485 device in an RS-485 network is performed by the HW4 software. The new device is identified by detection
of the address "0", which is then overwritten with the first free RS-485 address (i.e. the first address not yet used
in the network). This automatic mechanism prevents malfunctions from double allocation of addresses and use
of the invalid address "0" in the network.
Figure 6: Example of a RS-485 network with master (e.g.: HF5, HF8) and HM4.
Remarks:
The start and end of the bus line must both be terminated with a resistor of 240 Ω.
All devices must have the same baud rate of 19,200 baud (symbols/s).
It is possible to use a HF5, HF8, AC3011 cable (Ethernet-to-RS-485 converter) or AC3010 cable
(USB-to-RS-485 converter) as master in the RS-485 network.
The HF5 and HF8 must then be allocated a RS-485 address. The AC3011 and AC3010 cables do
not need a network address.
The HM4 cannot be supplied with power from the network; it needs a separate power supply of
15…24 VDC.
USB or TCP/IP
Master
ADD 1
HM4
ADD n
240 240 Further RS-485 devices
DRAFT