MDR24X User’s Manual
10/26/2006
12
Figure 7- DCE to DCE Interface
Note: The Straight Through Cable provided with the MDR24X has female connectors on both
ends and the Crossover cable has a male connector at one end and a female connector at the
other end. Please use a male-to-male adapter or female-to-female adapter wherever required.
Using a null modem adapter with a Crossover cable is equivalent to using a Straight Through
cable.
2.2
RS-485 (2-wire Half Duplex)
The RS-485 interface uses a Differential Data Transmission that can help nullify the effects of
ground shifts and induced noise signals that can appear as common mode voltages on a
network.
The MDR24X implements a RS-485 (2-wire Half Duplex) multi-drop interface. Typically, a RS-485
bus will consist of a master and several slaves. The nodes will have unique addresses and can
send addressed packets to specific nodes. Because the bus is half duplex, no two nodes should
try to talk at the same time. The MDR24X does not have a RS-485 address, therefore, it will
transmit all RS-485 traffic over the RF. Conversely, as soon as a MDR24X receives a packet over
the RF, it will transmit the packet over the RS-485 bus.
Note: When using RS-485 (2-wire Half Duplex), a RS-485 to RS-232 converter is required to
configure the unit. Black Box recommends a B&B Electronics 4WSD9R converter to translate
RS-485 to RS-232.
CTS
DCE
DCE
Crossover Cable
CTS
RTS
RTS
RXD
TXD
TXD
RXD
GND
GND
DCD
DTR
DSR
DSR
DCD
DTR
OEM Host
MDR24X Modem
NC
NC
RI
NC
RI
NC
