24

®

2400 baud and above by a proportionate amount. For example, shielded

cable with 50pF/ft (164pF/m) would reduce the distances by 50%.

For baud rates of 1200 and below, distances are limited by DC voltage

drop. For 2400 baud and above, distances are limited by pulse

distortion. The use of low-capacitance cable can extend the distances

shown. Belden 9182 and 9184 are, respectively, single and dual

twisted-pair cables that are especially designed for high-speed data

communications applications. With these cables the distances can

be extended by 50%. However, the DC-resistance-limited distance

given under 1200 baud may not be exceeded.

Cable capacitance for individually shielded wire pairs is usually given

by manufacturers as capacitance between wires and capacitance from

each wire to the shield. The effective transmission line capacitance

is approximately the interwire capacitance plus one-half of the wire-

to-shield capacitance.

Installation

Installation of the LDM485 consists of attaching it to its mating 25-pin

connector on the terminal of the host computer, either directly or through

a cable. Optional mounting screws and screw jacks are provided.

The DCE/DTE (data-communication equipment/data terminal

equipment) switch must be set to be complementary to the terminal

or computer port (DCE connects to DTE and DTE to DCE). Since the

LDM485 is a communications device, its normal setting is DCE.

In the event that the host port is not known, the LED indicators may

be used to find the proper switch setting. The transmit and receive

LEDs will be off during a MARK, which is the normal, or standby,

condition when no data is being transmitted. Set the switch to the

position which allows these LEDs to be off. The field wiring as shown

in Figure 2 must be correct. It is sometimes useful to tie the RS-485

data output back to the RS-485 input during initial check-out.

The four other LEDs indicate the status of various control conditions and

will be on when these functions are asserted. An open circuit to Request

To Send will be interpreted as assertion, allowing convenient operation

with equipment not supporting this function. An open circuit on the

Receive Data line of the RS-485 circuits will be interpreted as a MARK.

For 2-wire half-duplex and for 2-wire multi-drop installations, echo

suppression is available by strapping P1 pin 16 to P1 pin 17. Then RTS

asserted enables echo suppression and forces RD (P1 pin 3) to a MARK.

Data Terminal Ready, DTR, must be asserted before the LDM485 can

transmit data. This is normally done by the host computer. For

situations where the host equipment does not have the capability of

supplying a DTR signal, RLSD may be used to automatically assert

DTR. On the RS-232 connector P1 of each LDM485, simply connect

RLSD pin 8 to DTR pin 20. This connection is not appropriate for

multi-drop installations.

Figure 2. Field Wiring, LDM to LDM

Unit 1

Unit 2

Function

Function

Terminal

Terminal

12VAC

PWR RTN

(local)

A

B

A’

B’

A

B

A’

B’

RTS

Output

CTS

Input

TD

Output

RD

Input

SIGNAL RTN

SIGNAL RTN

12VAC

PWR RTN

(local)

A’

B’

A

B

A’

B’

A

B

SIGNAL RTN

SIGNAL RTN

1

2

3

4

5

6

7

8

10

11

9

12

1

2

3

4

5

6

1

2

5

6

3

4

10

11

7

8

9

12

7

8

9

10

11

12

Note: Signal return

not required. Shield

connection if used.

Notes: (1) For data channels MARK is A negative relative to B. (2) For control lines Assertion is A

positive relative to B. (3) RTS = Request To Send, CTS = Clear To Send, TD = TransmittedData, RD

= Received Data. (4) Recommended wire sizes are 14AWG to 22AWG. Belden 8442 or Signal 1172

are typical low cost, nonshielded, twisted pair cables for use with LDM485. (5) Signal return is not

required. Cable shield, if used, should be connected to SIGNAL RTN. (6) LDM485 units are suitable for

use in RS-422/485 applications.

For multi-drop installations, the following points should be considered

(see Figure 3 for multi-drop wiring connections):

1. If the LDM485 is not powered, it releases the transmit bus so

other RS-485 devices may use the bus.

2. Local equipment connected to the RS-232 connector must not

leave DTR in the asserted state.

3. An open circuit or zero volts on the RS-232 connector pin 20

(DTR) is equivalent to disassertion.

4. The RS-485 line should be terminated at both ends using internal

line termination DIP switches (see Figure 3). Stub length off the main

line should be as short as possible.

5. DTR should be asserted at least 5.0µs before start bit and

disasserted at end of last stop bit. This disables the RS-485 line after

6.0µs min.

WARNING!

WARNING!

WARNING!

WARNING!

WARNING! Because PWR RTN and RS-232 GND (P1-7) and Shield (P1-

1) are common, when powering more than one unit from the same

transformer, wire all units' 12VAC's together to one side of the secondary

and all PWR RTN's together to the other side of the secondary.