PC adapter FLLBOB


This adapter allows a PC with one or two serial ports to non-intrusively monitor a 25-way RS232 interface connection.

It is designed for use with our FSA32 PC based serial data analyser, when running on a PC with two serial COM ports; but is also suitable for similar applications.

It differs from our other PC adapters by providing extra connections to the PC serial COM port. These allow the PC application to transmit to line, and to monitor and set the most commonly used RS232 control lines.


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  • Allows a PC with one or two serial COM ports to non-intrusively monitor a 25-way RS232 interface
  • Provides a connection for the PC to transmit data to line
  • Provides connections for monitoring DCD, DSR, CTS and RI control lines
  • Allows PC to set RTS and DTR control lines
  • Provides full 25-way through connections to the interface being being monitored
  • Has a 2-way switch to allow RD line to be switched to a second COM port, allowing "full-duplex" monitoring
  • Has three 3-way switches to allow TD, RTS and DTR lines to be left floating or switched to alternative COM port output positions

Further information

For PC's with one COM port, one of our FLL221 PC adapters may be more suitable (click here).

 What's included

FLLBOB adapter, two DB25 way male to DB9 way female cables (for connection to PC ports), 12 months warranty, lifetime support by phone or email

Order code:  FLLBOB 125.00 + VAT (UK) Discontinued  

 Application notes

Monitoring an RS232 interface

The FLLBOB adapter is connected to the monitored serial line via the RS232 IN/OUT connectors, shown at the top and bottom of the picture above. (That at the top is a DB25 female connector, the other a DB25 male connector). These connectors have a full 25-way through connection and do not affect the monitored line.

PC with one COM port - this is connected to the "PRIMARY PORT" connection, shown on the right of the picture above. In this case either line 3 (monitored line RD) or line 2 (monitored line TD) may be switched through to the primary port for monitoring, but not both. However it does allow the control lines listed above to be monitored.

PC with two COM ports -  connection is made both to the "PRIMARY PORT" connection, shown on the right of the picture above, and to the "SECONDARY PORT", shown on the left hand side. In this way both line 3 (monitored line RD) and line 2 (monitored line TD) are connected to the PC.

Transmitting to line

This is usually carried out in "Active" mode, when one COM port of the PC is directly connected to one equipment. Using the FLLBOB adapter rather rather than a direct connection is useful in this in this mode, because the switches on the FLLBOB can be used to provide a "crossover" connection, where the transmit line from the equipment is connected to the receive line on the PC, and vice-versa.

Table of FLLBOB PRIMARY PORT connections

FSA/PC signal In/out Connected to pin
RD In 2 or 3
DCD In 8
DSR In 6
CTS In 5
RI In 22
TD Out 2 or 3 or floating
RTS Out 4 or 5 or floating
DTR Out 6 or 20 or floating

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                       New Forest Electronics Tel. +44 (0) 1425 650089                 Issue 3.108    12 March 2024