Interfacing the Serial/RS-232 Port
Interfacing the Serial/RS-232 Port
Interfacing the Serial/RS-232 Port
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<strong>Interfacing</strong> <strong>the</strong> <strong>Serial</strong> / <strong>RS</strong><strong>232</strong> <strong>Port</strong> V5.0<br />
AT's have a fast bus speed which <strong>the</strong> 8250 series of UART can't handle to well thus it is very<br />
unlikely to be found in any AT. However if you wish to test for <strong>the</strong>m as well you can follow <strong>the</strong> same<br />
test as above to distinguish 16550's or 16550A's from <strong>the</strong> rest. If no FIFOs are enabled <strong>the</strong>n a possible<br />
UART is <strong>the</strong> 16450, 8250, 8250A or 8250B. Once it is established <strong>the</strong> it could be one of <strong>the</strong>se four<br />
chips, try writing a byte to <strong>the</strong> scratch register and <strong>the</strong>n read it back and compare <strong>the</strong> results. If <strong>the</strong> results<br />
match <strong>the</strong>n you must have a scratch register, if <strong>the</strong>y don't you ei<strong>the</strong>r don't have a scratch register, or it<br />
doesn't work to well.<br />
From <strong>the</strong> descriptions of <strong>the</strong> UART above if you read back your byte from <strong>the</strong> scratch register<br />
<strong>the</strong>n <strong>the</strong> UART must be a 16450 or 8250A. (Both have scratch registers) If you don't read back your byte<br />
<strong>the</strong>n it's ei<strong>the</strong>r a 8250 or 8250B.<br />
The 16750 has 64 byte FIFO's, thus <strong>the</strong> easiest way to test for it's presence is to enable <strong>the</strong> 64<br />
byte buffer using <strong>the</strong> FIFO Control Register and <strong>the</strong>n read back <strong>the</strong> status of <strong>the</strong> Interrupt Identification<br />
Register. However I have never tested this.<br />
<strong>Interfacing</strong> <strong>the</strong> <strong>Serial</strong> / <strong>RS</strong><strong>232</strong> <strong>Port</strong> V5.0 Page 32