The match, a one leg affair - the game will be settled on the night, will take place probably on Tuesday 23rd August but that is to be confirmed.
Colchester boss Phil Parkinson is urging his players to concentrate on league matches before Cardiff roll into town
"It's a decent draw for us, but certainly a tough game against a team who will not be the easiest side to take on," he told CU-FC.com the official Colchester website.
"It is one to look forward to, but the players will have to concentrate on the four league games before they start to think too much about Cardiff."
City last went to Layer Rd in December 2002 when the Bluebirds won 2 - 1. The return match at Ninian Park in April 2003 was a 3 - 0 win for United.
The full draw is
Preston v Barnsley
Chesterfield v Huddersfield
Blackpool v Hull
Mansfield v Stoke
Rotherham v Port Vale
Nottingham Forest v Macclesfield
Sheffield United v Boston
Derby v Grimsby
Leeds v Oldham
Wrexham v Doncaster
Scunthorpe v Tranmere
Stockport v Sheffield Wednesday
Bury v Leicester
Rochdale v Bradford
Hartlepool v Darlington
Wolves v Chester
Lincoln v Crewe
Burnley v Carlisle
Reading v Swansea
Gillingham v Oxford
MK Dons v Norwich
Plymouth v Peterborough
Shewsbury v Brighton
Cheltenham v Brentford
Northampton v QPR
Millwall v Bristol Rovers
Watford v Notts County
Rushden & Diamonds v Coventry
Bristol City v Barnet
Colchester v Cardiff
Torquay v Bournemouth
Ipswich v Yeovil
Southend v Southampton
Crystal Palace v Walsall
Swindon v Wycombe
Leyton Orient v Luton