Hello,

You can consider using TTL or CMOS latches for I/O expansion and send a serial frame.

You can implement many chips you want.
For example using three stremaed 4094 latches will provide you 24 digital output. You'll have to send a 24 bits frame to the 4094.
As far as I can remember the TTL chips are 74LS245 or 545 somethink like that.

Other solution is to use bi-directional I2C expander as PCF8574/8575. As most of 8051 have hardware I2C included, it's easy to do.

Regards
Stephane