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Commodore Peripheral Ports

Ottis R. Cowper, Technical Editor

Commodore computers provide their users with a variety of methods for communicating with the outside world. The devices from which the computer receives input or to which it sends output (or both) are generically called peripherals, and the connectors where peripherals are attached to the computer are referred to as ports. Each of the several ports has distinctive characteristics that make it suitable for particular applications. For some, the computer's operating system the ROM which controls the machine's functions provides routines that handle much or all of the "dirty work" of communicating with devices attached to those ports. To use other ports, you must program all the necessary support routines yourself. That task can range from very easy (for example, reading a joystick) to quite complex (interfacing with a parallel printer through the user port, for example). Here's a rundown of the features of all the ports:

The Serial Port

For most users, the serial port is the major data artery of the computer. As the connection point for disk drives and printers, it's the port through which most information exchanges take place. This is the one port that is the same on the VIC, 64, Plus/4, and 16. Well, almost the same - there are some signal timing differences. The VIC transfers data at a slightly faster rate than the others, which is one of the reasons the VIC is listed as incompatible with the 1526 printer, and why the original 1540 disk drive was only for the VIC. Obviously, this port is bidirectional - data can flow both in and out with equal ease.

The signal format used to exchange data over the six lines provided through this port is unique to Commodore. The format should not be confused with the more standard RS-232 serial communications format used by numerous peripherals; RS-232 communication is handled through the user port (see below). The serial port is essentially a stripped down version of the parallel IEEE-488 port used for most data communications in Commodore's earlier PET/CBM models. As the term serial implies, data can be transferred only one bit at a time (and in only one direction at a time, either in or out). Three of the other lines control the direction of data flow, and whether the signals on the data line are to be interpreted as data or as commands to the peripheral device. The computer's RESET line is also present at this port, which explains why the disk drive resets whenever the computer is turned on or off.

The operating system fully supports communications through this port. By addressing a peripheral attached to this port with a device number, and using OPEN, CLOSE, PRINT#, INPUT# and related routines provided by the operating system, you can avoid worrying with the details of controlling the individual signal lines. Any peripheral addressed with a device number between 4 and 31 (the highest device number allowed) is assumed by the computer to be connected to this port.

Commodore has established several standards for device numbers: Printers are usually device 4, although some can be changed to device 5, the 1520 Printer/Plotter is designed to be device 6, and device numbers 8 and above usually refer to disk drives. Device 8 is the default number for the disk drive, and almost all software assumes the disk drive will have this device number; device 9 is the most common choice for a second drive. Commodore 1541 drives allow you to select any device number via software, or numbers 8 - 11 via hardware.

The use of a unique signal format for communication with the disk drive is not unusual; almost all computer manufacturers use a proprietary disk interface compatible only with their own products. What is unusual is that this same nonstandard format is also used for communications with printers. Since so much software assumes that printers will be connected through the serial port (as device 4), most third-party interfaces for non-Commodore printers also attach to this port. These interfaces act as interpreters, reading the Commodore-format serial signal from the port and converting it to the more standard parallel (eight bits at a time) format used by most printers.

The Memory Expansion Port

This is often referred to as the cartridge port, since ROM cartridges are the peripherals most often attached through this connector. In the VIC, this is also the port through which RAM memory expanders are added. The lines available at this port include most of the address, data, and control lines of the microprocessor chip that is the heart of the computer. Thus, any peripheral which needs to be intimately tied to the workings of the computer - for example, ROM that must be addressed by the microprocessor - is connected through this port. The operating system does not support any devices through this port; in essence, anything attached here is no longer a peripheral, but part of the computer itself.

This port is present on the VIC, 64, Plus/4, and 16. Many of the same lines are available on corresponding pins of the expansion port connectors used in the VIC and 64, but the connectors themselves are different sizes, so cartridges designed for the 64 cannot be used on the VIC, and vice versa. The Plus/4 and 16 have identical 50-pin connectors for this port (as opposed to the 44-pin connectors used in the VIC and 64), so presumably there will be some compatibility of cartridges for these models, although no VIC or 64 cartridges or memory port peripherals can be used.

Examples of other types of devices which attach through this port are the CP/M cartridge for the 64, which contains a second microprocessor that takes complete control of the 64's RAM and input/output chips, and the Magic Voice speech module, which is attached through this port because it contains additional ROM to allow the computer to support voice output.

The User Port

This port (sometimes called the RS-232 or modem port) was designed with the experimenter in mind. Just as the memory expansion port gives you access to a number of the microprocessor's control lines, this port gives you access to many of the control lines of one of the interface adapter chips (VIAs for the VIC or CIAs for the 64). Using these lines, a wide variety of peripherals could be connected, since both serial and eight-bit parallel communications are available.

Unfortunately, most of this flexibility goes unused since it isn't supported by the operating system. Most home computer users today are more interested in software than in tinkering with hardware projects, so this port is most frequently used for its one function supported by the operating system: RS-232 serial communications.

RS-232 is the name of the most common serial communications standard. If you use the operating system to address device 2, data directed to that device will be transferred through the user port in an approximation of RS-232 format. Actually, the signal format is true RS-232, but the voltage levels are different from those prescribed. The standard calls for voltage levels of -12 to +12 volts, and the user port only provides levels of 0 to +5 volts. Adapters are available - from Commodore and other sources - to convert the signal voltage to the proper levels. These adapters are not necessary if you're going to use Commodore's modems, but are required to use any standard RS-232 equipment.

The 24 pins of this port have a similar configuration on both the VIC and 64, so many devices designed to interface to this port can be used on either computer, although the software to run the devices will generally be different. The VICmodem and Automodem, for example, work with either model. The Plus/4 appears to have the same 24-pin connector, but the computer casing around the connector is smaller, so neither the VICmodem nor Automodem can be plugged into the Plus/4. (Commodore's new 1660 modem works with the VIC, 64, and Plus/4.) The Commodore 16 has no user port, so it is as yet unclear if or how a modem may be used with that computer.

Since eight-bit parallel data communications are available through this port, it might seem surprising that it's not commonly used for interfacing with printers. After all, it would appear on the surface to be simpler to write a machine language program to simulate the commonly used Centronics parallel format through this port, than to go to all the trouble of designing the hardware interface to convert the data from the serial port to the proper parallel format.

The reason this isn't often done is that almost all Commodore software expects the printer to be device 4 on the serial port, and in the long run it proves easier to seek a hardware solution to allow you to use the built-in operating system routines as provided in ROM. That way, you don't have to worry about having to load your printer handler routine into memory before you can use it, finding a safe place in RAM to store the handler routine, etc.

For more information on interfacing through the user port, see the article "Using the User Port" in COMPUTE!'s First Book of Commodore 64. VIC users might also be interested in two articles which show how to use the user port to provide a second joystick port: "Fighter Aces - Add A Second Joystick" (COMPUTE!'s Second Book of VIC) and "Tankmania: Adding A Second Joystick To The VIC" (COMPUTE!'s GAZETTE, April 1984).

The Control Ports

These ports (or port, in the case of the VIC, which has only one) are usually referred to as the joystick ports, since they are most commonly used for joysticks. In fact, on the Plus/4 and 16, these ports are labeled JOY1 and JOY2. The operating system ROM does not support any devices through these ports, so BASIC must communicate using PEEKs and POKEs.

In addition to joysticks, which are simple devices consisting of five switches - one for each of the four principal directions, plus one for the fire button - the ports can be used to read other devices that behave like joysticks, such as trackballs. Although the computer normally uses the five joystick lines strictly for input, it's possible to program them for output as well. We've never seen any peripherals for Commodore computers that attempt to output through this port, but there are printer interfaces for the Atari which use Atari's nearly identical joystick port. (Atari and Coleco joysticks are functionally identical to Commodore joysticks for the VIC and 64, and can be used interchangeably.)

A warning to those contemplating the purchase of a Plus/4 or Commodore 16: On those computers Commodore has abandoned the widely used DB-9 joystick connector in favor of a nonstandard connector, so existing joysticks cannot be used. Apparently, this was done to reduce radio frequency (RF) interference. The joystick mechanism is exactly the same, it's just the plug on the end of the cable that's been changed. And the new plug isn't one you'll be able to find easily, so - until someone comes out with an adapter plug - you'll be limited to buying only Commodore's joysticks. On the other hand, the improved BASIC in the Plus/4 and 16 includes a JOY function to read the joysticks, so the complicated PEEKs required to read joysticks on the VIC and 64 can be avoided.

In addition to the joystick, this port in the VIC and 64 can be used to read paddle controllers. (The Plus/4 and 16 have no circuitry for reading paddles.) Paddle controllers, which always come in pairs, are actually just variable resistors which provide variable voltage levels to two lines on the port, Circuitry within the computers (in the VIC chip in the VIC-20, and in the SID chip in the 64) calculates a digital value corresponding to the voltage level. The value ranges from 0 to 255 as the voltage on the lines changes from 0 to 5 volts. Other devices which operate like paddles - providing a varying voltage input - can also be read through this port; graphics tablets are a good example.

Each paddle usually also has a button, but instead of being read like the joystick buttons, the paddle buttons are connected to the lines for two of the joystick directional switches. One paddle button corresponds to the joystick's right directional line, and the other to the line for reading joystick left. By convention, the paddle that uses the right directional line for its button is called the right paddle, and the one that uses the joystick-left line is the left paddle.

Unlike Atari joysticks, Atari paddles are not completely interchangeable with those made by Commodore. While Atari paddles can be used with the VIC and 64, they have a higher resistance and thus are less accurate for Commodore systems. (A half turn on Atari paddles corresponds roughly to a full turn on Commodore paddles.)

One additional type of peripheral - the light pen - can also be connected to this port. The pen contains a phototransistor that switches when it detects the electron beam of the video display sweeping past. A line is connected from the phototransistor through the port to the chip that generates the video signal (the VIC chip in the VIC, the VIC-II chip in the 64, and the TED chip in the Plus/4 and 16).

When the video chip receives the signal from the pen, it latches (stores) the current position of the raster (electron beam) in a set of registers (memory locations within the chip). The stored value can then be read, and the position where the pen is touching the screen can be calculated. (As with joysticks, Plus/4 and 16 owners will have to wait until someone manufactures a light pen with the proper plug to fit the unique socket used for this port by those computers.)

The Audio/Video Port

This connector is not really a port, since data cannot be transferred through it. Instead, it provides a connection point to the computer's video and audio signals. With the exception of an audio input line on the 64, all lines at this port are outputs only. Like the serial port, this port is compatible on the VIC, 64, Plus/4, and 16, but compatible doesn't mean identical. When attaching the computer to a video monitor, you need a cable that brings out two signals which are available at the same pins on all four models, a composite video signal and an audio signal. Corresponding video and audio inputs are found on most black and white or color monitors. The Commodore 1701/1702 color monitor also provides for the input of a luminance signal via the rear connections. Separating the luminance provides increased contrast, and drastically reduces color smearing.

Television users can attach a thin wire between the composite and luminance signals on the video port to improve television contrast, but some TVs can't take the extra luminance and produce a distorted screen. The more recent 64s use an eight-pin jack rather than the five pins used on the VIC and 64, but a five-pin monitor plug will still work. The eight-pin 64s have a purer color signal available for use with the rear connections of the 1701/1702, so you may want to use an eight-pin cable. The 64's audio input line attaches directly to the audio input on the SID chip. Be careful to feed in only low-level (unamplified) sound sources. There's no way to process the sound, but it can be mixed with the sound of the SID chip, and the SID chip's filters can be used as a programmable equalizer for the sound coming in.

The Cassette Port

This port is designed for one particular peripheral, the Commodore Datassette recorder. There are now two models of the Datassette, the 1530 (or equivalent C2N) for use with the VIC and 64, and the 1531, for use with the Plus/4 and 16. As with joysticks, the only difference between the two is the plug on the end of the connecting cable. Commodore has used a new and incompatible type of connector for this port on its new models.

Three of the six lines from this port are used for writing a signal to the tape, reading a signal from the tape, and testing whether a button is pushed. Note that since there is only one line (labeled Cassette Sense) to test the buttons, it's only possible to check whether any buttons are pressed, not which particular button or buttons are pressed. Thus, if you're supposed to press PLAY and RECORD and accidentally press only PLAY, the computer won't be able to detect the mistake. Other lines supply power to the tape motor (9 volts) and for the electronics in the Datassette (5 volts). Some other peripherals for example, several brands of printer interfaces - also make use of the 5 volt power source available here.

Communication through this port is fully supported by the operating system, with the Datassette being designated as device 1. Device 1 is the default storage device; unless you specify otherwise, all your SAVEs and LOADs will be directed to the Datassette. In addition to SAVE, VERIFY, and LOAD, the OPEN, CLOSE, PRINT#, and INPUT# statements provide all the features necessary for storing and retrieving data on tape, so programmers rarely need to worry about the intimate details of interfacing to this port, such as what sort of magnetic pattern is actually used to represent a byte of data on tape. Nevertheless, it's possible to program several of the individual lines of this port to achieve special effects; for an example, refer to the "TurboTape" articles in the January and February issues of COMPUTE!, the GAZETTE's sister magazine.


Published in COMPUTE!'s Gazette, Issue 21, March 1985. Copyright © 1985 COMPUTE! Publications, Inc.



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