Functional description and allocation of terminal pins Manual BG41/BG42/BG43
3-12 Rev. 99/49
3.2.3 20mA current loop/RS232C combination module (MD26)
3.2.3.1 General
The combination module serves as a point-to-point or a bus connection and has two line current
loops for transmission and reception at its disposal. In a bus operation the module, in a voltage-free
state, connects all loops in order to prevent the entire system from being impaired.
When operating a RS232C, only a point-to-point connection is possible.
The 20mA current loop/RS232C combination module in its delivery state is so adjusted, that when
operating as a 20mA current loop module the internal voltage supply is carried out by the back plane
bus. If the external voltage supply (25-pole SubD sockets) is switched to the 20mA current source,
the plug connectors X3 and X5 must be replugged.
The LED indicator is only effective, when the combination module is used as a 20mA current loop
module.
Transmit Data
(red LED)
Receive Data
(red LED)
30-pole Plug
26-pole Plug
Jumpers for 24 V Current Supply
(Internally through back plane bus or externally)
LEDs:
Continous Light : No Data Transmission
(Sleep Mode)
Flickering Light : Data Transmission in Progress
X3
X5
M24
INT
EXT
P24
INT
EXT
Fig. 3-10: Structure of a 20mA current loop/RS232C combination module
When using a 20mA current loop module at a baud rate of 38,4 Kbaud, a
twisted pair cable is required from a cable length of more than 50m, in order
to avoid the other wire pair from being influenced.
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