The 1977 Trinity - Which Computer is the Best Buy?

The 1977 Trinity, Courtesy of Wikipedia, Image by Timothy Colegrove

Consider that you have been transported back in time to the heady and hot summer of 1977. You are wanting one of these new "personal computers" that you see advertised in Popular Mechanics and Byte Magazine. Maybe you've seen an ad from Radio Shack. Perhaps you saw a flyer from that pocket calculator company called Commodore. Or you are a hobbyist frustrated with time sharing on the local college's PDP-10 and want a microcomputer of your very own. After you've seen Star Wars for the third time, you want to be part of the computer age. You have three choices, which will you chose? Let's break down the Apple II, Tandy Radio Shack TSR-80 and the Commodore PET 2001 and see how they might factor into a buyer's decision.

CPU

Apple II: 6502 @ 1.02MHz
TRS-80:  Z80 @ 1.774MHz
Commodore PET 2001: 6502 @ 1.02MHz

The Apple II and PET use the same CPU running at the same speed. The TRS-80 uses a Z80 but clocked at a rather lower speed compared to other machines with Z80s. The 6502 only has a 16-bit memory address bus, the Z-80 has both a 16-bit memory address bus and an 8-bit I/O address bus, so TRS-80 peripherals have some flexibility about where they can map their ports. 

Advantage: Apple or Commodore PET

RAM
Apple II: 4-48KiB
TRS-80: 4-16KiB
Commodore PET: 4-8KiB

An Apple II could be purchased at launch in any combination of 4KiB and 16KiB memory chips in three banks. Valid combinations are 4KiB, 8KiB, 12KiB, 16KiB, 20KiB, 24KiB, 32KiB, 36KiB, 48KiB and were priced to match. Each bank is socketed with a jumper block to configure the bank for 4KiB or 16KiB RAM chips. The system is very flexible and budget friendly. 

A TRS-80 could be upgraded from 4KiB to 16KiB by replacing the included memory chips, which should be socketed inside the system. The system could be additionally expanded to 48KiB with the addition of the Expansion Interface, with the RAM being an additional expense. The card edge cable connector between the TRS-80 and the Expansion Interface is not as robust as it could have been, which can lead to program crashes if the cable is nudged or gets dirty.

A PET came with either 4KiB or 8KiB of RAM internally, but it uses SRAM chips compared to the cheaper and easier to find DRAM chips of the TRS-80 and Apple II. Using SRAM saved Commodore from having to design a circuit to refresh the DRAM. The system can be upgraded to 32KiB with external memory expansion upgrades.

Advantage : Apple II

Keyboard
Apple II: 52 Keys
TRS-80: 53 Keys + 12 Keys
Commodore PET: 73 Keys

The Apple II uses an ASCII keyboard with 52 keys. The keyboard does not autorepeat a key when held down, you must press the REPT key to repeat a key. This keyboard has left and right cursor movement keys but no up and down keys. The Apple II's keyboard is the only keyboard of these systems which uses a dedicated keyboard controller, which means that the Shift and Ctrl keys are not readable from the keyboard. These keys only modify the scancode the controller reports to the CPU. The Reset key is not really part of the keyboard at all, it acts like a software reset switch and would soon have a mod which only makes it work when the Ctrl key is pressed at the same time.

The TRS-80 originally came with a 53 key keyboard with the system's name badge on the side of the enclosure. The keyboard matrix is decoded through generic logic, so every key is distinct except for the shift keys, which are wired together. This keyboard was replaced with a better keyboard with a numeric keypad, a much welcome feature for BASIC programming. Older machines could upgrade to the numeric keypad keyboard. The numeric keypad keys are duplicates of the corresponding keys on the main keyboard and wired together. Pressing Shift + either 1 key will give you a ! on the screen. The keys do not autorepeat in BASIC. In my opinion the layout of the TRS-80's keyboard is slightly better than the Apple II's layout. Multiple switch technologies were used in the Apple II and TRS-80 keyboards but the best of the bunch use Alps SKCC switches.

The Commodore PET 2001 has the greatest number of unique keys at 73 but its keyboard is the worst of the three by far. The keys are tiny, not spaced in a layout convenient to type on and have a pair of legends on each key. They also use rubber domes over a PCB, Apple II and TRS-80 use mechanical switches. The cursor keys only move the BASIC cursor down and right unless you hold a shift key. The keyboard is controlled by a 6520 PIA and scanned via the Kernal ROM firmware. The keys are unique and most have a PETSCII graphics character printed above the main key.

Advantage: TRS-80

Ports & Expansion
Apple II: Cassette In/Out, Video Out, Internal Speaker, RF Header, 8 Expansion Slots, Game I/O
TRS-80: Monitor, Cassette In/Out, Expansion Connector, Reset Switch
Commodore PET: Built-in Display & Cassette Drive, External Cassette Interface Port, IEEE-488 Port, User Port, Memory Expansion Port

The Apple II has relatively minimal built-in expansion, but the cassette input and output uses 3.5mm cables instead of something proprietary. It is the only computer which has a built-in speaker for audible sound output. It also has a header on the mainboard for an RF modulator for hooking up to a TV and another socket for a joystick or pair of paddles. The video out uses a standard composite video RCA jack. The computer's main strength lies in its eight expansion slots, seven of which are more-or-less general purpose and permit full access to the CPU bus. The eighth (slot 0) is generally devoted to a 16KiB Language Card memory upgrade but can be used to swap out the BASIC firmware ROMs with a Firmware Card.

The TRS-80 has the least amount of expansion and the connectors are all proprietary. The cassette port supports motor remote control, something the Apple II does not. The video port is designed for a special monitor sold by Radio Shack (which was just a TV with its RF circuitry removed). These ports use the same DIN5 connector as does the power port right next to them, so one has to be very careful how one plugs in things. 

Further TRS-80 expansion is handled by the Expansion Connector. This was intended for an Expansion Interface, which added the ability to upgrade the RAM to 48KiB, a floppy disk drive interface, a second cassette port, a card edge parallel port interface, a timer circuit and a connector for an optional RS-232 serial interface. It also had a passthrough connector to plug in another device which relied on the Expansion Connector, such as the Voice Synthesizer or VOXBOX. The The TRS-80 can output sound through the cassette output jack and several programs support audio when this jack is connected to a powered speaker.

The Commodore PET 2001 has perhaps the best built in I/O of the three systems. The most obvious inclusions are its 9" black and white display and cassette tape drive. It may be the first all-in-one computer ever made. The IEEE-488 connector was used to connect to disk drives and can be daisy chained to work with compatible printers. Unlike the disk drives of the C64 and VIC-20, the disk drives of the PET used the parallel bus form of IEEE-488 and thus were not crippled in speed by a slow serial bus connection. The User Port could be used for modems. The PET supports a second Commodore 1530 cassette drive through Commodore's proprietary card edge interface. 

Like the TRS-80 the PET has no dedicated speaker but can output audio through the User Port and an adapter for a speaker. The audio is controlled by a programmable timer in a 6522 VIA chip, so music can be generated while the CPU can do other things like update the screen display. The Apple II and TRS-80 have to rely on CPU-timed speaker toggling to generate audio. 

Advantage: Commodore PET for built-in ports, Apple II for expandability

Disk Drives

Apple II: 116KiB-143KiB x 2
TRS-80: 85KiB x 4
Commodore PET: 170KiB x 2

While none of these computers were advertised with a disk drive at launch, they all had floppy disk drive options available within a year or so. Apple offered the Disk II drive with an interface card which plugged into the computer. These drives could format disks initially with a 116KiB capacity per side but with improved firmware PROMs would format to a standard 143KiB capacity. These drives were single sided and two could be connected to an interface card. With Applesoft firmware or an Autoboot ROM the Apple II could boot disks directly from power on. Through the use of GCR encoding, Apple could offer double density capacities on single density drives.

The TRS-80 offered disks with 89KiB per side, an Expansion Interface contained the disk drive controller. Four disk drives could be connected, daisy-chain, to the Interface. These disk drives are truly single density and use FM encoding.

Commodore offered a variety of disk drives for the Commodore PET 2001. These include the CBM 2040 and 4040 dual floppy drives and the single floppy 2031 and 4031 drives. These disks can be formatted to a 170KiB capacity. Even though Commodore's drives use 35 tracks like Apple and TRS-80 drives, through the use of zone constant angular velocity they can fit more sectors on outer tracks  where there is more room compared to inner tracks. Commodore also used its own form of GCR encoding to offer greater densities of bits compared to FM.

Floppy drives were expensive at first but many businesses and some consumers were willing to pay those prices because they were far faster and more reliable over cassette storage. Apple suggested retail price was $595 for a Disk II drive and interface and $495 for a second drive. Radio Shack charged $499 for each TRS-80 disk drive. Prices for the PET drives were competitive, the 2041 was being advertised at $595 and the dual floppy 2024 for $1095.

Advantage: Commodore PET (capacity), Apple II (price).

BASIC
Apple II: 8KiB Integer BASIC or 12KiB Applesoft BASIC
TRS-80: 4KiB Level I or 12KiB Level II BASIC
Commodore PET: 14KiB Commodore BASIC 1.0-2.0 + KERNAL

Apple II's initial BASIC was Integer BASIC, which did not offer floating point support. Packed into this firmware was also a mini-assembler and the SWEET16 interpreter. The Apple II was never sold with Applesoft BASIC in ROM, which was an adaptation of Microsoft BASIC and came with floating point support, but the ROMs could be purchased to upgrade the machine when Applesoft BASIC was released. At this point the machine would be an Apple II+. Both Integer BASIC and Applesoft BASIC came with a system language monitor. You could also load Applesoft BASIC from tape or disk at the cost of the loading time and almost 12KiB of RAM.

The Apple II could be a bit intimidating to an unseasoned hardware hacker. In the first revision of the mainboard the system would not even show anything on the screen on power up until you pressed reset. Then you would see a screen of random text characters, some of them flashing and others in reverse video, with one clear line after an asterisk. This was the system monitor and commands using it were cryptic and required a command of hexadecimal numbers and the basics of how a computer works.

The TRS-80 originally came with only the very simple Level I BASIC. Within a year it was selling systems with the floating point capable Level II BASIC. This BASIC was licensed from Microsoft and could be retrofitted into a Level I BASIC machine. The TRS-80 had something of a machine language monitor which could load machine language programs.

The Commodore PET came with Microsoft BASIC from day 1. The BASIC improved over time, v1.0 was somewhat limited in its capabilities and the PET could be upgraded to v2.0 with just a ROM swap. The Kernal added a full screen editor and various routines programmers could call and also handled keyboard reading.

Advantage: Commodore PET (built-in) Apple II or TRS-80 (upgrade)

Display Modes
Apple II: 40x24 text mode, 40x48 graphics (15 colors), 280x192 graphics (6 colors)
TRS-80: 64x16 text, 32x16 text, 128x48 semigraphics (monochrome)
Commodore PET: 40x25 text mode (monochrome)

All three systems use discrete logic to construct their video displays, no fancy graphics chips here, that would have to wait until the Atari 8-bit computer systems. 

The Apple II beats the other systems hands down in terms of overall graphics capabilities because it offers not only color graphics but high resolution graphics, and it is the only one of these systems to support color. High resolution graphics require a 16KiB machine. In low resolution graphics modes 15 colors can be displayed. In high resolution graphics modes 6 colors can be displayed and a monitor or TV screen may sometimes be able to show the illusion of more colors through careful use of dithering. The Apple II offers a 40x24 text mode with a 7x8 pixel character cell size. Its 40x48 graphics mode assigns one color to the top four lines of a text cell and a second color to the bottom four lines of a text cell. The Apple II has the ability to show a mixed text/graphics mode with either graphics mode, giving four rows of text cells on the bottom of the screen. 

The Apple II character generator ROM has no provision for lowercase characters and can only show 64 unique character glyphs. The characters are set in a 5x7 pixel matrix. These glyphs can show in reverse video or flashing depending on the byte written to the text memory location. Obtaining lowercase text outside of the high resolution graphics mode would require either an 80-column text card or a non-standard modification to add lowercase to 40-column text modes (involving a replacement character generator ROM). A modification to allow the CPU to read the state of the shift key (the shift key mod) is required to designate lowercase text even if the display will not show it for many programs. 80-column text  cards would not be available for some years following the introduction of the Apple II and tended to be legible only on monochrome displays (at least until the IIe's color display).

The Apple II shares system memory with the video display hardware, the latter refreshes the mainboard RAM by accessing it 60 times per second. Certain memory locations areas are dedicated to graphics and these areas cannot be relocated. Text and low resolution graphics modes use 960 bytes of RAM and high resolution graphics modes use 6,720 bytes of RAM.

The TRS-80 offers two text modes, 64x16 or 32x16, with a 6x12 character cell size. The characters are set in a 5x7 pixel matrix. Each cell can select between text mode and semigraphics just by the byte written to the memory location. In semigraphics mode a cell becomes a 2x3 block of pixels, each pixel can be individually turned on and off. This has an effective resolution of 128x48 pixels. The TRS-80 has its own 1KiB of SRAM dedicated to video display. This 1KiB allows each cell to select its character glyph in text modes, but because there is a missing bit in the RAM bank it can only select from 64 glyphs. Adding the missing RAM chip and (depending on the age of the machine) updating the character generator ROM will allow the display of lowercase text. Radio Shack offered this modification to customers. As the video circuitry has no way of halting the CPU during contending video memory access, there will be flickering horizontal lines when the CPU writes to video memory outside of the vertical or horizontal blanking intervals.

The Commodore PET only offers a 40x25 text mode, with an 8x8 character cell size. It has lowercase characters built into its character generator ROM, unlike the other systems. The character generator ROM can generate 256 characters but there are many duplicates. There are many line and graphic characters in PETSCII, unlike the more rigid ASCII of the TRS-80 and Apple II. This permits the system a certain graphic-like ability through clever use of text characters. The PET has a separate 1KiB of SRAM for the text display. Similar to the TRS-80, when the CPU accesses video memory during the time when the video hardware is drawing the screen there will be "snow". The Apple II avoids snow by interleaving CPU and video accesses to memory on opposite halves of a CPU clock cycle. The two cannot conflict with each other.

Advantage: Apple II (graphics), Commodore PET (built-in text), TRS-80 (upgradeable text)

Printers
Apple II: Serial or Parallel
TRS-80: Parallel through Expansion Interface
Commodore PET: IEEE-488 Parallel

A printer is an extremely important device for any computer back in the early days of home computing. Apple had serial and parallel cards designed for certain printers in the early days but the official Apple solutions tended to revolve around serial printers. Other companies sold cards like the Grappler interface which worked with many parallel port printers.

Radio Shack sold many printers and almost all supported the card edge connector of the TRS-80, usually with a special cable. Third party printers were less likely to support the TRS-80 but if they had a parallel port there was a good chance that Radio Shack had a cable which could connect to it.

Commodore also sold printers for its systems but as the bus was standardized, many IEEE-488/GPIB printers should work. They tended to be expensive.

Advantage : Apple II

Miscellaneous

The Apple II is incredibly easy to get inside. The top cover just pops off and is held in place with velcro-like hard plastic strips. The PET is also easy to open, a few screws need to be removed and the main unit can be lifted up on a hinge with a metal arm to keep it open while you work on it as if you were working under the hood of a car. The TRS-80 is slightly more involved, six screws need to be removed. You would not likely need to get into the system more than once or twice to upgrade the ROM or RAM.

The chassis of the PET is mostly metal, so it is very durable. Apple used some very thick plastic for the top over a metal bottom, so it too is exceptionally sturdy. The shiny painted plastic of the TRS-80 is not in the same league. Larger businesses could appreciate the PET's sturdy all-in-one design, put it on a desk, plug it in and you are ready to go.

I do not know where else to mention it but the Apple II has an internal power supply that connects via a standard 3-prong AC cable. The TRS-80 uses an external power brick with a DIN5 connector which plugs into the system. The Commodore PET has a hardwired power cord and an externally accessible slo-blo fuse. The power supply of an Apple II may be riveted together, so getting inside it is difficult if you need to replace a blown fuse. The TRS-80's power supply is encased in sonically welded plastic, so a cutting tool would be required to get at its fuse.

The Apple II came with a pair of paddles, cassette recorder cables and a Preliminary Reference Manual which would quickly become the Apple II Reference Manual, a.k.a. "the Redbook" (January 1978). It also came with demonstration cassettes, Breakout/Color Demos for a 4KiB system. Star Trek/High Resolution Graphics Demo and eventually Applesoft/Floating Point BASIC Demo were added with a 16KiB system. The Applesoft Reference Manual was also included when Applesoft was added to the package.

Radio Shack charged extra for its reference manual but included a cassette with Blackjack/Backgammon with the system. The User's Manual for Level 1 BASIC was included in every TRS-80, those who bought machines with or upgraded to Level II BASIC also received the LEVEL II BASIC Reference Manual.

Commodore may not have sold a reference manual for the PET, it may have confined that precious information to dealers and authorized repair shops. It also initially shipped with "A Brief Introduction to Your Commodore PET" and then "An Introduction to Your New PET Personal Electronic Transactor"

Launch Price

Apple II: $1,298 (4KiB) - $2,638 (48KiB)

TRS-80: $599 (4KiB)

Commodore PET: $595 (4KiB), $795 (8KiB)

The price of the three systems reflects their capabilities. The Apple II with its color, high resolution graphics, speaker and expansion slots was priced the highest. You could buy the computer in kit form for $598, but you would need to solder it yourself, build an enclosure and buy a keyboard and power supply. Unlike the other two computers, the Apple II uses a switching power supply, so you just can't wire up some generic brick you bought at Radio Shack. Either way you still needed to find a monitor, but with an affordable RF modulator like the Sup'r II Mod, you could use any display that would accept a TV antenna signal.

The TRS-80's initial cost of $399 (RSC-01) did not include its special video display, which was really just an RCA TV with its TV tuner components removed. That cost $199 and the CTR-41 cassette tape recorder Radio Shack offered was an additional $49. It offered $50 off the price if you ordered all these things at once.

The TRS-80 and the PET were very competitive in their initial pricing. However the $299 Expansion Interface was required to expand the TRS-80 to accept the most important peripherals whereas the PET had various ports built in. Once you started upgrading the TRS-80, whatever savings you were looking to achieve would fade away into Radio Shack's cash registers.

Conclusion

In 1977-1978, it is clear that the Apple was the best buy but also the most expensive buy. If you could afford one, you should have gone with it. The PET and TRS-80 were more competitive with each other between Commodore's expanding dealer networks and Radio Shack's 3500 stores. The TRS-80 outsold both the PET and the Apple II throughout the 1970s for a reason. The PET had a better version of BASIC built in at first but Radio Shack had a stronger software library and a much better keyboard. Tandy might get you on the extras but its low price was very attractive to first-time users.