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PW250
3.3 RS485 interface
A 2-wire RS485 interface will be connected to the RS485 terminals (A = + and B = -) in the terminal
compartment. Please note that this is a transparent interface without addressing on the RS485 bus. The
direction control of the half-duplex 2-wire connection is realized that the PW250 is passive in the idle state
(receiver on, transmitter off) and becomes on transmitted data from M-Bus meters (receiver off, transmitter
on). The level converter remains active on the RS485 for approximately 50ms after the last transmitted space
(0-Bit) of the slave. The master software should not start a new request to the connected meters during this
period.
The RS485 bus is terminated at the factory with a 120Ω resistor. This can be disabled if necessary by removing
the jumper "RS485 TERM" on the board in the terminal compartment. The terminating resistor is designed for
permanent overvoltage up to a value of 7 VDC on the RS485 bus. If a higher overvoltage protection is required,
the jumper should be removed and a proper dimensioned external resistor be connected between terminals
A and B.
3.4 Repeater interface
The PW250 can be used as a so-called Repeater, as a network extension for existing M-Bus systems, if the
maximum number of meters or the maximum cable length has been exceeded for the installation. Up to 250
devices and up to 4km cable (1 x 2 x 0.8 JYSTY) at 2400 baud transmission speed can be connected to a
repeater.
The M-Bus line of the existing master or level converter is connected to the terminals labeled with M-Bus IN
(Repeater). The polarity does not matter - as usable and standardized for M-Bus slaves. The signal at the
M-Bus output of the PW250 is reconditioned to connect a complete M-Bus network. An almost unlimited
number of repeaters can be cascaded or connected in parallel due to the automatic measurement of the
maximum transmission speed and the recovery of bit times (bit recovery) by the digital repeater. This allows
nearly unlimited M-Bus networks.
PW250 · User Manual Version 1.1 – 11 / 2019
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