This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
I use an AE Serial Pro in my IIe to connect to my ImageWriter II. All of the DIP switches are set to OPEN (switch down). The switch block next to the printer interface connector is for hardware handshaking signals. (Copied without any permission whatsoever from the AE Serial Pro User's Manual.) o Switch 1, when closed, select pin 4 (Request to send) as the flow control handshaking line. Some printers which use this line are: Data General TP2; Heath H-25; Olympia ESW102/103; QUME Sprint 5; and Smith-Corona TP1 o Switch 2, when closed, selects pin 11 which is, according to RS-232- C specifications, undefined and is used by some serial printers as a printer- ready signal. Some Centronics, Texas Instruments, and Epson serial printers may use this pin. o Switch 3, when closed, selects pin 19 (Secondary Request to Send) as the handshaking line. Some of the printers that use this pin are the Anadex DP8000/9000, Bell TP-1000, Lear Seigler 310, NEC 3500/7700, and Digital Equipment (DEC) LA-series serial printers. o Switch 4, when closed, selects pin 20 (Data Terminal Ready) as not only the device-available handshaking line but also as the data-flow- control line. Some Diablo, C.Itoh, Okidata, QUME, Tectronics, or Xerox printers may use this handshaking signal. o When all of the switches are open, Data Terminal Ready (Dsub-25 pin 20) is the only line monitored as the hardware handshaking line from your printer. This supports most popular serial printers. The second set of DIP swithces (close to the front of the card) is for generating Maskable (IRQ) and Non Maskable (NMI) interrupts from the 6551 Asynchronous Communications Interface Adaptor (ACIA) chip and the 6818 clock chip. The swithces select the type and source of interrupt request. Normally all switches are in the OPEN position. o Switch 1: IRQ from 6551 o Switch 2: NMI from 6551 o Switch 3: IRQ from 6818 o Switch 4: NMI from 6818 By: Cyrus Roton <firstname.lastname@example.org>