Atart interface is an interface among Atari computer, PC computer and Atari casette dataset. It allows to transfer data between any two of the named devices. You must do some little modification of Atari dataset to enable data transfer from the Atari to the PC. Data transfer from PC to dataset is on development now only.
Atart interface works with normal Atari casette software and handler. It means that when data are being transfered between Atari and PC, PC is simulating an Atari dataset. When data are being transfered between dataset and PC, PC is simulating an Atari computer SIO interface. Atart interface needs the Atart program running on the PC to work. Atart interface can also work as SIO2PC interface.
Named functions are useful in two cases especially. The first case is if you want transfer some data or program from Atari casette and save it to the PC file. The second case comes on if you want to use some Atari program which cannot work with disk and it can work with casette only. In this case you can use normal functions of this program for data reading and writing to casette. But data will be stored and loaded not from the casette but from the PC file.
First warning: The connector CN2 is designed for a dataset only. Do not connect any other device here. If you want to have other device connected (disk drive, printer, etc.) to an Atari, connect it directly to the Atari and connect CN2 to the second SIO connector of this device.
Second warning: If you do not use an AFMS board in the dataset, check if pin 4 of its SIO connector is connected to ground and pin 6 is disconnected OR change wiring of the pin 6 on CN2 to the ground.
Theory of operation
If SW2 is switched to SIO2PC, interface is wired as SIO2PC interface and the dataset is normally connected to Atari. In this case you can normally use any disk emulation software on PC and you can use the dataset for normal data reading and recording from or to the Atari computer. SW1 changes serial port pin for command line. The SW1's right position depends on used software for disk emulation.
If SW2 is switched to Atart, PC output line TxD is connected via U1 to Atari data input. Atari data output is connected to dataset input and dataset output is connected via U1 to PC serial input RxD. PC RTS line is used for switching dataset motor on and off. U1 changes voltage levels, T1 works as electronical switch for motor control line. It is needed because that line has load current larger than some models of U1 can supply. D1 simulates an open collector output for prevent conflicts with other devices connected to Atari SIO.
It was said that when data are being transfered between Atari and PC in Atart mode, PC is simulating an Atari dataset. When data are being transfered between the dataset and PC, PC is simulating an Atari computer SIO interface.
Atari uses FSK encoding for data storing on casette tape. A FSK encoder is a part of the pokey chip installed in the Atari and Atari data output is FSK encoded in a casette mode. FSK decoder is a part of dataset electronics. It means that when Atari is writing to the casette (or to the PC in Atart mode), it is sending FSK encoded data. But if Atari is receives data from the casette (or from the PC in Atart mode), it is receives FSK decoded data in normal TTL level.
It is clear now that in Atart mode, it is no problem to transfer data from PC to Atari (Atart mode 2B). Also there is no problem with data transfer from dataset to PC (Atart mode 1A). But Atari output is FSK encoded in the casette mode and must be FSK decoded if you want to link it to a PC (Atart mode 2A). In this case, FSK decoder in a dataset is used. I have developed a device called AFMS board to allow using dataset FSK decoder for this job. One additional wire between an Atart interface and a dataset is used for automatic switching between "normal" dataset mode and "FSK" dataset mode. In a normal mode the dataset works normally. But in FSK mode the dataset receives data from its input (where an Atari output is always linked), FSK decodes them and link them to dataset output, where PC is connected if SW2 is switched to Atart. Note that in older Atart interface version a manual switch was used for switching between normal and FSK modes. AFMS board does this automatically.
I have said about a additional wire (let us name it as FSK control line) between an Atart interface and a dataset. I have used pin 6 of SIO connector for it. It is not problematic if you know, what you connect to CN2. Normal XC12 dataset models have this pin disconnected and that is why you can connect they to CN2. But check before connecting. Dataset with AFMS board you can directly connect to an Atari or other normal Atari device without any risk. The pin 6 of SIO connector is linked to the ground in this cases, for a dataset with an AFMS board the pin 6 of a SIO connector is an input line. Dataset will stay in normal mode and that is what you want in this case. Function of the FSK control line is very simple. If SW2 is switched to SIO2PC mode, FSK control line is disconnected. In Atart mode it tranfers the state of the Atari SIO motor control signal. FSK control line transfers logical 1 only if SW2 is switched to Atart mode and Atari SIO motor control line is on. It is exactly the case when an Atari transmits data and PC receives data.
Normally an Atart interface and a dataset are powered from an Atari computer. But you may want to save data from casette on PC and you cannot or you don't want to use an Atari computer in this work. In this case, you can connect any +5V DC power supply to CN5. Check its polarisation! A plus pole must be connected to the pin 1 and a minus pole (ground) to the pin3. Otherwise modify wiring. Do not connect Atari and external power source at once.
You can connect CN4 to an audio amplifier input or to a little earphone with high impedance(!) for hearing the sound from casette and better seeking. You need not do it (but you can do), if you have the Atari computer connected. In this case you can hear the casette sound from device (TV, AV monitor or audio amplifier) which you use for hearing sounds from Atari.
If you want to save data from the PC to the casette, you must have a FSK encoder with right frequencies and output level. I am sorry I have not developed and tested any. If you want to try some, connect its input (I suppose it is a TTL input) to U1 pin 12. Disconnect dataset input (SIO pin 5) from everything and connect it to the FSK encoder output. Note that large FSK encoder's output voltage level can damage the dataset and you can make this experiment you can do at your own risk only.
| Signal | SIO pin | Cannon-15 pin |
| Clock in | 1 | 7 |
| Clock out | 2 | 14 |
| Data in | 3 | 6 |
| GND | 4 | 13 |
| Data out | 5 | 5 |
| GND | 6 | 12 |
| Command | 7 | 4 |
| Motor control | 8 | 11 |
| Proceed | 9 | 3 |
| +5V | 10 | 10 |
| Audio | 11 | 2 |
| +12V/NC | 12 | 9 |
| Interrrupt | 13 | 1 |
| FSK control | 6 | 8 |