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Dear forum, Many thanks for your replies. I had the data switch working with a MAX RS232 converter and switching the TTL levels using an analogue switch as suggested. As well as the RX TX lines there are two handshake lines used (CTS RTS). My main concern is with the effect of the act of switching (transition time blips) and how the external devices would react / interpret these blips. With the HC11 I can write code to disregard any useless information but I have no control over the operation of the external devices. An easy solution would be if there was a specific chip to do this job, but in the absence of this I shall press on with the above solution. With thanks, PFC

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