Tape Topics

PACTL Poker

In the last two issues we discussed cassette tapes for the ATARI 410 Program Recorder and began to investigate its inner workings. This time we will delve a bit further into the playback circuit. I'll also explain why you have to press [RETURN] twice for a CLOAD or CSAVE.

Figure 1 shows a functional block diagram of the playback circuit in your 410 Recorder. The playback circuit uses an "active filter" network to separate the 3995 Hz (ZEROs) and the 5327 Hz (ONEs) and send each kind of signal down two parallel lines. The signals are then cleaned up by passing them through a simple rectifier and a filter. This "dry cleaning" leaves you with a signal that is essentially DC (constant voltage). Each of these DC signals is fed into opposite sides of a comparator, where output is further filtered and passed on to a "driving" transistor which drives the data line to your computer.

This circuit converts the Frequency Shift Keying (FSK) signals that were recorded on your tape into a binaryserial data stream for input to your computer.

The reason why two [RETURN]s must be pressed for every CLOAD or CSAVE is that the first [RETURN] signals the Operating System (OS) to accept your command. When the OS receives a cassette LOAD or SAVE command it jumps to a special part of the OS called the "cassette handler." Up to this point, no signals are sent out from the Recorder. Atari designed the handler this way to give you a chance to prepare the tape before the Recorder is activated.

The cassette handler will sit there until it gets a second [RETURN]. When your computer is first powered up, the Motor Control Line (pin 8) of the serial bus connector is initialized in a "low" state (zero voltage). This line is connected to the motor ON/OFF control in your 410 Recorder. When you press the second [RETURN], this line is fed a positive voltage that activates the Recorder motor.

You can fool the Operating System into thinking you have already pressed [RETURN] twice. If you use a CLOAD or CSAVE command in your program, all you have to do to eliminate the second [RETURN] is to precede the 1/ ) command with POKE 764,12. This POKEs a [RETURN] code into the "last key pressed" buffer. So, when the CLOAD command is encountered and the cassette handler is called up, the [RETURN] is already there for the cassette handler. The computer will beep at you normally, but the CLOAD or CSAVE operation will begin immediately.

Another trick POKE lets you turn the cassette motor ON and OFF under software control. POKE 54018,52 will turn the Recorder's motor ON and POKE 54018,60 will turn it back-OFF. Location 54018 is called PACTL. This PACTL trick will allow you to listen to a cassette tape without actually trying to CLOAD it. I use this capability to locate the start of a file on a tape. For example, I had one adventure game program that I could not set to load. Finally I tried the PACTL trick to see if there even was a file on the tape! I found the file way out at a tape counter value of 68, and it loaded fine once I found it. You could also POKE PACTL to listen to music: over your TV speaker, or even to activate a special controller to operate household appliances.

Next lssue I will give you the schematic diagram for the 410, along with a parts list, and identify a few key components that you can replace to improve the reliability of your CLOADs.

Your letters have been pouring in. There is not enough time to answer all of you individually. Those of you who have really serious problems, l will try to answer with a letter.

Tangle Angles

Dear Mr. Evans:

You asked for comments regarding the 410 Program Recorder. I have one, which at present is not operating. The left side of the lid, where the arm extends back into the machine, cracked when the lid was raised with the eject button. A call to Chicago revealed that they wanted $40.00 as a flat charge and would not send a new lid for me to change. l think this is unnecessary and unreasonably high-priced. Because of that, my lid is still broken.

Thomas A. Mabel, M.D.
Noblesville, IN

Finding replacements for 410 Recorder parts is no easy task. I finally located one source at the Foothill Computer Center in Upland, California. They are an authorized Atari Service Center and should be able to supply you with virtually any Atari component you might need. I asked them specifically about your broken lid and they said they would send you onefor about $7 plus shipping. Their address is:

Foothill Computer Center
Attn: Mr. Jim Degner
949 W. Foothill Blvd.
Upland, CA 91786
(714) 985-3278

If you, or any of you other readers, know of a good source for parts near to you, then send their name, address, and phone number to me so I can spread the word.