Flow Control

Flow control should almost always be set for hardware. On the Macintosh, some cables do not support hardware flow control. If they don't then it is best to get one that does.

Data Bits, Stop Bits & Parity

These settings should stay at their defaults which are : Data Bits - 8, Stop Bit - 1, No Parity

Originally modems would sometimes only transmit 7 bits at a time, since 7 bits can fit all of the normal ASCII characters. They would then use the 8th bit as a parity bit to do basic error control, which would either be Odd or Even Parity. Parity is not really that important nowadays since it does not always catch errors and tends to slow the modem down. Error correction is now dealt with in separate protocols.

Every byte in a modem also has a start bit and a stop bit. This is why there are 10 bits to a byte with modems. It is possible to tell the modem to use more than one stop bit but this will slow the modem down and doesn't give any real benefit..

Receive & Transmit Buffer

This is related to how much the UART on the serial port can hold. These settings can normally be all the way at maximum. If an old UART is being used then changing these settings may help resolve some problems. The higher these settings are, the faster the modem will run. However, because of the faster speeds, the modem will have to do more error correction which can have the opposite effect and actually slow the modem down! If you're getting slow connection speeds, then sometimes lowering these two settings can help speed things up as the modem will have less error correction to do. It's a bit of a backwards way of doing things but it does often work.