Campbell Scientific MD485 RS-485 Multidrop Interface - Manual (Page 15)

MD485 RS-485 Multidrop Interface
The base MD485 is connected to the computer's COM port with a serial cable.
A transformer supplies +12 VDC power to the MD485.
The MD485 at the computer is connected to one or more remote MD485s with
the CABLE2TP cable.
The MD485 at the datalogger is connected via an SC12 cable (supplied with
the MD485) and is powered from the datalogger CS I/O port.
Each MD485 includes (2) green 3-pin terminal blocks to allow for cable
connections.
5.1 Transparent Communication
When configured as a transparent device, the MD485 simply passes serial data
from one interface to the other as quickly as possible without translation. If
communication rates are different, the MD485 can buffer up to 1000 bytes.
Functioning as a transparent device, the MD485 becomes somewhat protocol
independent. It acts as a way to get from one physical interface to another. In
this mode the MD485 uses what is called “send-data” transmit control. The
485 transceiver is normally set to receive. When data needs to be transmitted
on the RS-485 link, the driver is enabled, and stays enabled for 1 character tone
after the last byte is sent. After this time, the transceiver switches back to
receiving.
When using the EZSetup Wizard provided in PC400 and LoggerNet, select
Direct Connect for the Connection Type. In transparent mode, the MD485
does not need to be represented in the network map of LoggerNet or PC208W.
Figure 7 shows an MD485 being used as an RS-232 to CS I/O interface
converter. The MD485 is powered by the datalogger through the CS I/O
interface. RS-232 and CS I/O are chosen as the active ports in the MD485
Setup Menu. In this configuration, the MD485 acts as a full-duplex device,
fully compatible with all datalogger protocols and CS I/O configurations.
RS-232
CS I/O
FIGURE 7. RS-232 to CS I/O Conversion
Figure 8 would allow the longer distance characteristics of RS-485 to be taken
advantage of, while still providing an RS-232 interface for the PC and using
the CS I/O interface on the datalogger. A transformer is required to power the
MD485 closest to the PC. The base MD485 has RS-232 and RS-485 chosen as
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