As many as 26 external devices can be physically connected in "daisy-chain" style to the PET's IEEE-488 bus, although a maximum of 10 can be active at the same time. The beauty of this way of handling peripheral devices is that there is no programming overhead in operating these devices, i.e., communication to the printer, disk drive, etc. is handled with the same programming techniques. Commands as well as data can be transmitted, so that the external device can have its own "smarts." For example, you can command the disk peripheral to make a disk-to-disk copy with a single command; as soon as the command is sent, the disk system goes on its merry way to complete this several minute task, but the PET is free for any non-disk related task. IEEE-488 devices presently available from Commodore are line printer, dual floppy-disk, telephone modem, and recently, a speech synthesizer. Other IEEE-488 devices are available from a wide variety of manufacturers, but I would definitely advise the reader to specifically verify that they will work with the PET by contacting the manufacturer. On the negative side, IEEE-488 compatible devices tend to be somewhat more expensive than RS-232 devices.
The PET has an 8 bit USER PORT, and two "handshake" lines that are uncommitted, and therefore are completely under control of the user. The physical wires are accessible on an edge connector at the rear of the PET. Software control out of Basic is accomplished by PEEK'ing and POKE'ing to specific address locations. Utilization of the user port requires hardware experience. The lines can in general drive one TTL load, and would therefore need buffering to be used, for example, to turn on a power relay. I have personally used the user port in several ways, for example, to drive a digital-to-analog (DAC) converter to produce music, to communicate data at high speed (about 40,000 bytes per second) between the PET and a KIM or AIM, and to input high speed analog-to-digital data. One of the most common uses of the single bit "CB2" control line is to generate sound effects when connected to an audio amplifier and speaker. Not particularly well known is the fact that the user port has access to a fairly sophisticated timer (part of the 6522 integrated circuit which supplies the user port function) which again is under user control.
The final hardware item to be discussed is the EXPANSION BUS. These wires are accessible at the right side of the PET and communicate at nearly the lowest possible level to the 6502 microprocessor chip that runs the whole show. Available are buffered address lines 0-11, 4K selects for blocks 0-7 and 9-B,, the buffered data bus, the R/W, IRQ, RESET, and CLOCK signals. The expansion bus is not easily used, since it required detailed hardware