PowerLogix COM128 installation and operation guide Introduction

COM128 Installation and Operation Manual

1 INTRODUCTION

The COM128 is used to interface a single RS-232 communica- tions line to up to four RS-485 communications buses.

The RS-485 standard restricts each RS-485 bus to supporting

32 or fewer devices connected to it. The standard also dictates a 4000 foot (1219 m) maximum overall cable length on an RS- 485 bus. The COM128 is ideal for installations that contain more than 32 remote devices, or that have devices positioned at distances greater than could be reached with a 4000 foot cable.

The COM128 has four physically independent RS-485 ports, allowing communications with up to 128 devices. Data is transmitted to all four RS-485 ports simultaneously (ie. the ports are not addressable).

The COM128 provides many features which make it flexible, easy to use, and reliable:

TWO METHODS OF RS-485 BUS CONTROL

RS-232 is a duplex communications standard, allowing signals to be transmitted and received simultaneously. RS-485 is half duplex standard, allowing only one device connected to an RS- 485 bus to transmit at any given time.

The COM128 has two ways to determine whether it should be transmitting onto the RS-485 buses, or listening to them:

1)The COM128 can use the RTS signal generated by a computer or other device connected to the RS-232 port. When the RTS line is asserted, the COM128 will convert the RS-232 data stream to RS-485 signal levels and transmit onto the RS-485 buses. When the RTS signal is de-asserted, the COM128 will listen for data on the RS- 485 bus and pass it through to the RS-232 port.

2)Some types of modems and computer hardware/software do not support RTS line operation. For these applications, the COM128 has an Auto RTS mode which monitors the RS-232 bus for data signals. When data is present, the COM128 automatically enables the RS-485 transmit mode and begins to transmit onto the RS-485 buses.

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DTE OR DCE OPERATION

The COM128 is switch-selectable to act in DTE or DCE mode. An RS-232 port is provided for each mode, eliminating the need for null-modems.

In DTE mode, the COM128 supports a CTS-WAIT mode in which received RS-485 packets will be buffered until the CTS line indicates the RS-232 device is ready to receive the data.

This feature is useful when interfacing radio modems or slow data logging equipment to RS-485 devices.

AUTOMATIC BAUD RATE DETECTION

The microprocessor within the COM128 continuously monitors the data being received on its RS-232 port, and determines the correct baud rate. No jumper or switch settings are required.

HIGH ELECTRICAL ISOLATION

RS-485 buses are generally quite long, and are often run through electrically noisy environments. The risk of exposure to transient voltage surges and spikes on an RS-485 line is substantially higher than on an RS-232 line. The COM128 uses optical isolation between RS-485 and RS-232 sections to provide 750 VDC electrical isolation, and includes transient suppression circuitry on its RS-485 ports.

MICROPROCESSORCONTROL

Unlike most converters, the COM128 is controlled by a microprocessor. COM128 features are determined by the processor, not by its electrical circuitry. As new features become available, they can be added to the COM128 by a simple firmware upgrade. This process requires the replace- ment of a single chip, allowing upgrades to be performed in the field in a matter of minutes.

LINE DRIVER COMPATIBILITY

Short-haul modem/line drivers, such as the Gandalf mLDS, draw their power from the RS-232 DTR line. The COM128 will support these devices by means of a simple process that requires the movement of a jumper within the COM128 case. The process requires only a screwdriver and should take no more than a few minutes to perform. Please refer to Chapter 5 (Page 12) "Supplying Power to the RS-232 DTR Pin" for details.

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