CyberResearch® Serial Communications

COMT 366 Series

4.0 Operation

Operation in an RS-485 environment requires all devices attached to the network to have some level of intelligence in order to establish an orderly flow of data on the single twisted pair. The COMT 366 offers three possibilities for hardware data flow control. The user selects the mode which avoids the contention problem. Unless contention is resolved, there is the possible collision of data caused by two devices attempting to transmit simultaneously. The software control is the responsibility of the user.

The RTS (Pin 4) and CTS (Pin 5) signals of the EIA-232 connector are jumpered together. This combined signal (referred to as RTS) is used to control the transmitter and, depending on the mode, can also control the receiver. The COMT 366 can only transmit data on the RS-485 port if RTS is asserted.

Note: On the COMT 366 only: if the RTS signal cannot be controlled, then carefully open the covers and locate JP1 and move the jumper to the adjacent pin. This allows the

TD

signal

to

control

the

transmitter.

Positions 3 and 4 of the dipswitch control the I/O circuits of the RS-485 port as indicated below:

Mode

3

4

Transmitter

Receiver

1

Off

Off

Controlled by RTS

Disabled

2

On

Off

Controlled by RTS

Enabled when no

 

 

 

 

RTS

3

Off

On

Controlled by RTS

Enabled

4

On

On

Disabled

Enabled

An analysis of the operating modes reveals that Mode 2 or 3 are the most useful in the majority of installations. Mode 3 allows the converter to "listen" to the line at all times, including its own transmission. This allows observation of data collisions if they occur.

The RTS indicator displays the status of the RTS/CTS signal applied to the control ports of the RS-485 transmitter and receiver as set by the Mode switches. Mode 4 disables the transmitter completely, regardless of the RTS/CTS signal; the RTS display reflects the disabled status.

CyberResearch, Inc.

5

25 Business Park Drive

P: (203) 483-8815; F: (203) 483-9024

Branford, CT USA

www.cyberresearch.com