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guides:virtual_modem:section_2

Section 2 - Shopping list

Raspberry Pi 3 Model B/B+
Raspberry Pi Zero W or newer

$15-30
Virtual modem has been tested
with Raspberry Pi 3 model B and Raspberry Pi Zero W.
Other models should work, but are not supported.
8GB or larger MicroSD memory card
$5 eBay
USB Power Adapter
$5 eBay
Prolific USB-to-Serial adapter
$8 eBay. Get the one with the
Prolific chipset (explanation below).
Serial RS-232 Null Modem cable FEMALE/FEMALE or a
Null Modem F/F Adapter
$5 eBay
Total project cost ~ $38-$53

You can also choose to use a Raspberry Pi Zero which will save you some $15 from the total price, but for the sake of simplicity, we'll be using the Raspberry Pi 3 Model B on this tutorial. I've only tested these two Raspberry Pi's, but I see no reason why Virtual Modem wouldn't work with other Raspberry Pi models.

Important note about USB-to-Serial adapters and pre-Pentium era computers

I have found in my testing, that the cheapest USB-to-serial adapters do not fare well with some older, pre-Pentium era computers with certain UART chips. The cheaper “molded blue plastic” USB-to-serial adapters would not work at all with a 386-class system with an 8250 UART, even at 9600 baud. However, it would work fine with my 486 system which had a 16550AF UART running at 56 kbit/s. If you're unsure which UART chip you have, you can check it easily in DOS 6 by running the MSD tool. In the DOS prompt, type MSD to run the program, go to Com Ports (Press key C) and it will show the UARTs available. Commond UARTs are 8250, 16450 and 16550.

If you are planning on using a 486 or older computer with either a 8250 or a 16450 UART, you can work around this limitation by using a genuine Prolific USB-to-Serial adapter, or upgrade the computer by adding a serial I/O card with a 16550AF UART chip. Please note that 8250 and 16450 UARTs will still be limited, and cannot handle much faster transfer speeds than about 9600 bit/s. To verify the chipset under Windows 10, plug in the adapter, open up Device Manager. Under “Ports” you will see the Prolific adapter identify as “Prolific USB-to-Serial Comm Port”.

Optimal configuration for use with the Virtual Modem. A Prolific adapter and computer that has a 16550 UART.


Two types adapters compared. Top one is a Prolific adapter, with a Null-Modem adapter attached, and the bottom one is a generic “CH 340” USB-to-Serial adapter. This is only indicative. USB-to-Serial adapters come in all shapes and sizes. If you buy a Prolific adapter from eBay, make sure the item description specifically states it's a Prolific adapter. That way you'll have buyer protection in case the adapter you receive turns out to be something else.


On to the next chapter!

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guides/virtual_modem/section_2.txt · Last modified: 2021-06-24 19:00 by omolini