Retro Computer Build Part 3 – Console Port

So far we have discussed getting the Micro operational with the basic control lines, some ROM to hold the firmware, and RAM for the storage of data.

What about communicating directly to the Micro? How are we going to interact with it?

One of the reasons why this Micro was chosen, was because it has a built in UART. A UART is a port, that allows communications to and from another computer in a serial manner (one piece at a time). Serial communications are the most common form of communication methods used for computer to computer communications.

Initially, we will use the console port to interact with the Micro, as its primary interface. Over this interface, we will be able to mentor the internal registers of the Micro, write code, and perform some basic functions that will all help get the Micro operational.

There are different forms of serial communications. Some of these include USB, SATA, ADSL, Wi-Fi, and Ethernet. Each of these have their own advantages and disadvantages and specialised use cases. We will be using the RS232 serial communication on this console port.

In its day RS232 was a very common communication standard (it’s been around since 1962, it’s older than me). It’s not found in many new computers any more, as it has been replaced by USB. To enable RS232 for this Micro, we need to add a transceiver, which will convert the serial signals on the Micro to RS232 compliant levels.

As only some modern PC’s have an RS232 port, we may want to provide an alternative method, to allow communication over USB. If we bring out the serial connections from the Micro to a header, we could then connect this to a TTL to USB transceivers. This  now gives us the ability to plug this USB cable into a USB port on a PC or even an Apple Mac.

This is the approach that I’m thinking of taking, as if I don’t want to communicate over RS232. I could just pop-out the RS232 transceiver and plug in a TTL to USB transceiver into a header on the PCB. Then I could plug the transceiver into a USB port only computer and with a terminal program, I can start talking to the Micro.

We will be adding a full RS232 port to the Micro at a later date, as well as an RS485 port for industrial communications(more on that later in the build).

I plan to use a Maxim MAX222 transceiver, which gives us two transmitter & receiving lines that can be connected to the Micro. We are only going to use one of each of these. We might use the other two unused lines for control signals. The nice thing about this chip is that it has a sleep mode, where the chip will go to sleep when the sleep line is tied low. This device generates both the positive and negative supply rails for the RS232 port.


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