Hi Dennis

I understand that for the temp. measurements you will use a thermocouple so you need a transmitter, which output 4 to 20 mA.
The same with pressure.
So you need to convert 6 signals of 4 to 20mA to 8 bits. If this is correct I can suggest for a parallel option the National ADC0808 ( it is very easy to interface with 8051). http://www.national.com/ds/AD/ADC0808.pdf

For a serial solution MAX1038
http://dbserv.maxim-ic.com/quick_vie...qv_pk=3397

With anyone of these you don't need sample and hold ( conversion time to fast for your signal BW). Also use an instrumentation resistor of 250ohms 0.1% to convert the signal current to 1 to 5 V.

Good luck
Daniel