Comments on: What is RS-232?

By: Murel Warren

Murel Warren — Tue, 03 Nov 2020 16:38:44 +0000

When I started in the “Information Systems/Data Processing” industry in the mid-80s working at American Red Cross, I recall receiving a book on RS-232C. We built DB-25 pin connectors for terminals connected to our Tandem Non-Stop II system, then eventually for HP3000. I recall only pins 2, 3, and 7 were actually needed for those various terminals.

We also used 15 pin connectors for our early LANs running on Novell Netware that were daisy chained maxing out at 8 PCs per server card. Depending on the devices added on we would have to figure out which wires needed to be switched or removed.

Eventually all those old connectors were being replaced with RJ11 (for old modems and multiplexers) and RJ45, but by that time I had moved into programming. I missed those old challenges which took some knowledge to figure out and some skill to make those connectors quickly.

By: Jason Bucata

Jason Bucata — Tue, 03 Nov 2020 01:33:29 +0000

So in particular, “RS” does *not* stand for “Radio Shack”, as many of us who liked to browse catalogs from technology shops might have come to assume…

By: Shirley Dulcey

Shirley Dulcey — Mon, 02 Nov 2020 15:13:10 +0000

PCI Express x1 is yet another example of a serial interface. Slightly wider versions like x2 and x4 are hybrids; they use more than one line in parallel, but the amount of data transferred in parallel is smaller than the native size of data on the system. Some of the recent USB standards are also hybrids in the same sense as they use two parallel signal lanes.

We often see TTL level serial ports on microcontrollers such as the ones used on Arduino and Raspberry Pi products. Those use the RS-232 signaling standard, but at TTL logic levels (High = 2.4V or more, low = 0.8V or less) rather than the standard RS-232C voltages. Generating the standard voltages requires either plus and minus 5 volt power supplies, or incorporation of a charge pump to create them.

Modern microcontrollers have feature sizes on the IC that are too small to handle that much voltage so incorporating them on the chip would be difficult, and a charge pump would take up a lot of valuable real estate. That’s why level translation to RS-232 is usually relegated to a separate IC when it is needed. The separate chip also allows for higher output current than a microcontroller port can usually provide.