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MODBUS Communications Driver

Cabling

Pin

Direction

Signal

1

Shield Ground

2

Output

Transmit Data

3

Input

Receive Data

4

Output

Request to Send

5

Input

Clear to Send

6

Input

Data Set Ready

7

Signal

Ground

8

Input

Carrier Detect

9

Output+

Transmit Current Loop

11

Output-

Transmit Current Loop

18

Input+

Receive Current Loop

20

Output

Data Terminal Ready

22

Input

Ring Indicator

25

Input-

Receive Current Loop

Normally, your ACS software will be supplied with a cable suitable for connecting the IBM PC or compatible to the MODBUS device.

However, some of our customers find that they need to make their own cables. This section describes the cable and pinouts at each end of the connection. The serial port pinouts are included for reference, since they are not often described in computer manuals.

Note: Only strictly IBM-compatible serial ports implement the 20ma current loop interface.

PC Serial Port

The IBM PC serial port is a DB25M (25-pin Male) connector. Here are its pinouts (pins not listed are No Connection):

Revision 2.210

June 6, 1998

Page 3

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