Everybody probably knows the SIO2PC interface, which is used for data transfer between an 8-bit Atari and a PC. However, the RS232 serial interface has not been a standard feature on PCs for a long time, instead we usually find only USB. It has been switched to SIO2PC USB for this reason. However, the most commonly used FTDI FT232 chip is not cheap and recently it is not even in stock at vendors. I figured that since Arduino development board clones can successfully use Chinese CH340 chips for data transfer, it should work also with our 8-bit Atari.
The datasheet for the CH340 describes multiple versions of the chip, and the CH340G and CH340C were under consideration. The advantage of the C variant is that it does not need an external crystal to function. Unfortunately, I could not find it in any available store. CH340G can be bought for about 30,- CZK.
The interface schematic is based on the recommended wiring from the datasheet. But at first the data transfer towards the Atari did not work. This was eventually solved by adding a pull-up resistor to the Rx signal.
The CH340G chip has all handshake signals available. The Command signal from the Atari can be connected to one of three possible DSR, CTS or RI signals. It depends on used application.
I use AspeQT application to transfer data between the Atari and the big computer. Under the Linux operating system the communication runs right away without any problems, because the driver for the CH340 chips is included in the standard installation. Under Windows, the driver has to be installed.
Interestingly, with Mac OSX it is not possible to use the hardware handshake, it seems that it is not implemented in the driver at all. If the NONE option is selected in the settings of AspeQT the communication works. The data transfer doesn't work with the SOFTWARE option. I only tested the driver that is included in OSX, I found an alternative one later and haven't tried it yet.
You can find the interface schematic here in Flop. The circuit is simple, except for the actual chip you need a crystal and a few small components. A problem could be that the chip is only available in SMD.
I think the SIO2PC USB with the Chinese CH340G chip is an interesting alternative to FTDI and worth trying.
-jk-