Hardware manual

If the serial port buffer contains 64 bytes ready to send when it receives a flow control signal to stop sending,
it will send out the 64 bytes anyway in violation of the stop request. There is no stopping it since it doesn't
know about flow control. If the buffer was large, then many more bytes would be sent in violation of flow
control's request to stop.
18.5 UART Model Numbers
Here's a list of some UARTs. TL is Trigger Level
8250, 16450, early 16550: Obsolete with 1-byte buffers•
16550, 16550A, 16C552: 16-byte buffers, TL=1,4,8,14; 115.2 kbps standard, many support 230.4 or
460.8 kbps
•
16650: 32-byte buffers. 460.8 kbps•
16750: 64-byte buffer for send, 56-byte for receive. 921.6 kbps•
16850, 16C850: 128-byte buffers. 460.8 kbps or 1.5 mbps•
16950•
Hayes ESP: 1k-byte buffers.•
For V.90 56k modems, it may be a several percent faster with a 16650 (especially if you are downloading
large uncompressed files). The main advantage of the 16650 is its larger buffer size as the extra speed isn't
needed unless the modem compression ratio is high. Some 56k internal modems may come with a 16650 ??
Non-UART, and intelligent multiport boards use DSP chips to do additional buffering and control, thus
relieving the CPU even more. For example, the Cyclades Cyclom, and Stallion EasyIO boards use a Cirrus
Logic CD1400 RISC UART, and many boards use 80186 CPUs or even special RISC CPUs, to handle the
serial IO.
Many 486 PCs (old) and all Pentiums (or the like) should have at least 16550As (usually called just 16550's)
with FIFOs. Some better motherboards today (2000) even have 16650s. For replacing obsolete UARTs with
newer ones in pre 1990 hardware see the Appendix: Obsolete ...
19. Pinout and Signals
19.1 Pinout of 9-pin and 25-pin serial connectors
The pin numbers are often engraved in the plastic of the connector but you may need a magnifying glass to
read them. Note DCD is sometimes labeled CD. The numbering of the pins on a female connector is read
from right to left, starting with 1 in the upper right corner (instead of 1 in the upper left corner for the male
connector as shown below). --> direction is out of PC.
___________ ________________________________________
\1 2 3 4 5/ Looking at pins \1 2 3 4 5 6 7 8 9 10 11 12 13/
\6 7 8 9/ on male connector \14 15 16 17 18 19 20 21 22 23 24 25/
------ -----------------------------------
Pin # Pin # Acronym Full-Name Direction What-it-May-Do/Mean
9-pin 25-pin
3 2 TxD Transmit Data --> Transmits bytes out of PC
2 3 RxD Receive Data <-- Receives bytes into PC
7 4 RTS Request To Send --> RTS/CTS flow control
8 5 CTS Clear To Send <-- RTS/CTS flow control
6 6 DSR Data Set Ready <-- I'm ready to communicate
Serial HOWTO
18.4 Why FIFO Buffers are Small 70