DATA-LINC GROUP
9
MDL500 User Guide
PN 161-10003-002B
rev 8/03
General Operating Notes
The MDL500/MR utilizes tuned circuits to achieve high performance. These circuits have been carefully adjusted at the
factory to operate over several miles of transmission line. In order not to degrade performance, it is important not to disturb
these adjustments. In particular, coils T1, T2, T3, T4 and T5 and potentiometers R18, R20, R31 and R57 should not be
adjusted. If these adjustments have been inadvertently changed, please contact the factory.
Non-Loaded Transmission Line
Proper operation of MDL500/MR requires that the transmission line be non-loaded (free from inductors and capacitors on the
line). The presence of inductors, capacitors or excessive resistance (i.e., bad connections) or shorts between the wires can
seriously degrade performance or render the data transmission inoperable.
Loop Back Test
A Loop Back test is suggested to confirm the proper operation of MDL500/MR’s in point-to-point operation, as follows:
A PC running a communication program in full-duplex can be connected to one of the MDL500/MR’S in a point-to-point
configuration. A jumper wire (a bent paper clip works very well) is placed between pin sockets 2 and 3 of the DB25 on the
other MDL500/MR. Data sent to the first MDL500/MR will be looped back from the second and appear on the PC screen. If
the characters are correct, the MDL500/MR’s are functioning.
AE485 Interface
The MDL500 is also available with an RS485 interface for connection to multiple Remote devices. All operating functions and
instructions are the same as for the RS232 full-duplex version, except that the digital interface through the DB25 connector
uses pin 12 as the -DO/RI connection and pin 13 as the +DO/RI connection. The RS232 connections (pins 2, 3 and 7) are
not functional.
Note: Do not connect anything except an RS485 signal to positions 12 and 13 on the DB25 or damage
to the circuit could occur.
The Data-Linc Group AE485 (Auto Enable) circuit contains a special feature that eliminates the need for a separate enable
signal or line to command the 485 transmit and receive functions. Data will pass through the MDL500 RS485 port
transparently in either direction. The MDL500s are operated in full-duplex point-to-point mode (no carrier control) when using
the AE-485 interface. Up to 32 Remote RS485 devices can be multi-dropped off the MDL500 AE485 port at distances of up
to 4000 feet and at data rates of 9600 baud. A 100 ohm termination resistor should be connected at the ends of the 485 line.