Ace,
Your comments are noted - tnx for your reply.

Four of the five PORTA channels are the usual throttle, rudder, ailerons and ele' channels. The fifth channel is a so-called 'switching channel'. It shifts PORTA pulses to either four outputs on PORTB.0 to PORTB.3... or PORTB.4 to PORTB.7.
Only the latter four will be inverted.
Instead of aeroplanes, think along the lines of two working gun turrets - each requiring four servos. A sort of pseudo 8-channel Tx.

I'm loathe to buy such gadgets. I'd rather bung a few quid in the direction of someone who builds, rather than buy.


Thanks for the CMOS suggestion. Maybe Google will throw some light this way.