hi,
device which has a fixed protocol of 4800bd,1stop bit, 7 data bits, Even parity and 2 Stop bits

So one or two stop bits? I think do you meant 1start bit? So no problems: use standard UART of 51: it detects start bit, receives 8 bits of data (yeah, last one is first stop bit) and then stop bit. Some notes:
- check last bit for is it "1": you know now that it is first stop bit;
- use modern C51 which provides "Frame Error" bit: by it you may use hardware detection of second stop bit here. If such part is not available for you then switch UART to "9-bits mode" and set RB8 bit of SCON register: this way C51 still be able to detect second stop bit itself;
- parity bit may be detected by software; it is not hard: you need to load received and masked value (remember that it is 7 bits wide only!) to ACC and then check P-flag of PSW.

Regards,
Oleg