Apparatus for transmitting a data stream between cards connected by a midplane breaks the data stream into a plurality of channels. The apparatus comprises a serializer for each channel. Serialized data is transmitted across the midplane to a deserializer at the destination. Sequential cells are sent in different channels. Cell ordering is preserved by staggering the start of cell transmission in the channels. The invention permits data to be transmitted across a mid plane using a reduced number of data lines. The data stream on each card may be carried in a wide bus at a relatively low clock rate. Apparatus according to the invention may be used to transmit an OC-192 data stream across a pin-limited interface.