PC adapter FLL221


This adapter allows a PC with only one serial port to non-intrusively monitor a 9-way RS232 interface connection.

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


  • Allows a PC with only one serial port to non-intrusively monitor a 9-way RS232 interface
  • Works by connecting signal lines 2 (9-way RD) and 3 (9-way TD) to the PC port RD line (click here for circuit)
  • Actively combines the two signal lines using a line powered circuit
  • FLL221 version suitable for a monitoring half-duplex data (both lines permanently connected through)
  • FLL221S version also suitable for monitoring either RD or TD line if full-duplex data (either RD or TD line or both can be switched through)
  • Designed for use with our FSA32 PC based analyser running on a PC with only one COM port
  • Also allows a PC application which can only handle one COM port to view half-duplex data
  • May be used for 25-way interfaces, if 9/25 way converters are used

 Further information

For PC's with two COM ports, our FLL222 PC adapter may be more suitable (click here).

This adapter does not allow the PC to transmit back to line, or to monitor control lines, for that see our FLLBOB adapter (click here).

The FLL221 is powered by the RS-232 line that is being monitored and will therefore load the line by taking a small amount of current. With RS-232 lines that follow specifications this will not be a problem.

The FLL221 is suitable for half-duplex transmission lines such as RS-232-lines that control an RS-485 network or other systems that are controlled by a RS-232 half-duplex protocol. Please note that if data is transmitted in both directions simultaneously (full-duplex) then:
(a) FLL221 - the signals will be combined, so will not be correct
(b) FLL221S - the signal seen will depend on the switch settings, at best only one of the two signals can be viewed.

 What's included

FLL221 or FLL221S adapter, 12 months warranty, lifetime support by phone or email

 Order code: FLL221 86.00 + VAT (UK) In stock  Order Now
Order code:  FLL221S 93.00 + VAT (UK) Discontinued  Order Now

 Application notes

Click on diagram to return

The single COM port problem when using a PC based serial data analyser

When attempting to non-intrusively monitor the signals on an RS232 interface lead, a problem arises when the monitoring PC has only one COM port. This is because the RS232 lead carries two serial data signals, (RD on line 2 and TD on line 3 if 9-way) but a PC serial COM port has only one input, its own RD line on position 2.

The problem can be partially resolved using an adapter such as the FLL221. This combines the RD and TD signals into one, as shown here. This is fine when the RD and TD signals are not present at the same time, as is the case during "half-duplex" working. However should both RD and TD signals occur at the same moment (which can occur with "full-duplex" working), then with this method the combined signal passed to the monitoring PC will result in corrupted data being seen by the PC.


The FLL221S adapter helps the full-duplex situation by enabling the the user to switch either the RD or TD signals though to the monitoring PC as required. This is done by means of two slider switches on the body of the adaptor, as shown opposite. If required, both can be switched through at the same time, in which case the adapter acts in the same way as the FLL221 above.

However, to fully cope with full-duplex data, a PC with two COM ports is needed, using an application which can cope with two COM ports (e.g. our FSA32 PC based analyser) and a corresponding adapter (e.g. our FLL222) or by using the FLL221 on one COM port and our RS232-USB adapter cable to create a second.

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