This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
IIe and earlier Apple II's: These require a serial card which usually plugs into Slot 2. In general, the card determines maximum connection speed for these computers. Many early serial cards max out at 9600 baud. Apple's Super Serial Card (SSC) and compatible serial cards max out at 19,200 baud. (Lightning Systems made a "Turbo ASB board" add-on for the SSC which extends speed to 230,400. A 1MHz Apple II with the SSC and Turbo ASB should be good for 57.6k baud.) IIc and IIc+: These machines have built-in serial ports which are generally compatible with software specified as requiring a Super Serial Card. Original IIc releases use a cheap method of generating the clock frequency for the serial ports. Most implementations based on the 6551 chip use a 1.8432 MHz crystal, which gives exact baud rates; but, these IIc's take the master system clock (14.31818 MHz in an American IIc) and divide it by eight to produce 1.7898 MHz. The 3% decrease in clock frequency produces a 3% drop in the baud rate, which is enough to prevent operation with some serial devices, particularly intelligent modems running at 1200 bps or faster. This is not always a problem, and I have successfully used one of these IIc's with a ZyXEL U-1496E modem and a direct connection to a IIgs at 9600 bps. I have had problems in other cases. Later motherboards use a crystal, (and, some original motherboards may have a crystal installed) resulting in much better behaviour. You can make a reasonable guess at whether you have the original motherboard by checking which firmware version is installed. From the BASIC prompt enter PRINT PEEK(64447) and check the displayed value against this list: 255 Original firmware- probably original motherboard 0 UniDisk 3.5 support- may be original or revised motherboard 3 or 4 Memory expansion card- revised motherboard In theory, the IIc's maximum baud rate is 19,200. Whether it can actually keep up with that rate is another question. 9600 should be fine. IIc+ baud rate generation is reliable and max speed is 19,200 baud. IIgs: Although the firmware IIgs Control Panel allows a top setting of 19,200, maximum speed for the built-in IIgs serial ports is about 230k baud. Spectrum and a few other telecom products directly control the ports and allow 57.6k transfers. By: Gareth Jones