It has been used for a lot of low speed input devices like barcode scanners, magnet card readers, advanced joysticks, numeric keyboards, often designed to be place between the regular keyboard and the PC. There is two way communication and power, this could have been the future communication standard, but there was some serious limits on data speed, especially from the PC to the keyboard, it was (mostly) limited to sending lamps (Num, caps, scroll lock) on/off messages. First a 5 pin DIN connector, then a 6 pin mini DIN connectors. 5 volt) and with a usb converter that gives 5 volt DC together with Rx and Tx signals (Outside the device it will look like a regular usb device and on the PC it is a virtual com port).īecause the keyboard needed power, IBM could not use RS232 (At that time it was not possible to make a keyboard with low enough power consumption), instead they made another connector with power and data connections. Today the serial port is still used, but often at TTL level (i.e. It has been fairly standard in many small devices (Like mouse and magnet card readers) to use the status signals for power, it is possible to draw 5-10mA. Due to the 'high' voltage levels special chips was nearly always required to drive these signals. One data input and one data output and a couple of status pins. For the PC it was cut down to a maximum of 9 wires and a smaller d-sub connector. Most low speed peripheral communication was done with RS232, it is/was a very simple interface that could be done with as little as 3 wires or as much as 25 wires. I know of hardware keys that were used to lock software to a specific computer. it was not possible to draw much power (<1mA), but a few devices succeeded in doing it.

Later on the data pins got bidirectional, making it possible to use the port for external disks.Īll signals was fairly low level, i.e. This connector is rather large and the PC got a 25 pin d-sub connector instead with a lot fewer ground connections.Ĭommunication was mostly from computer to printer, but with the status input pin it was possible to receive data on the computer and at a fairly decent speed (I.e. I will start with a short history of the connectors that was used before usb.Īt the time the PC was first designed by IBM the most used connectors on printers was a 36 way Centronics connector with half the pin connect to ground and the other half for data and signaling (Using half the wires for ground is good practice for high speed cables). This article is about usb power and all the different standards.

Usb 2 vs usb 3 charging