RS485 – The PC side

RS485 with Automatic Transmit Enable
Here is a simple RS232-485 converter for the PC serial port I developed with the help of a Circuit Cellar article by Jan Axelson about RS485 interfacing.
It uses a 555 as a monostable to enable the transmit mode only when “sending”.
It’s a pretty basic circuit and nothing special is really happening here other than the portion involving the LM555 timer. The termination jumpers allow the 120 OHM termination on the master node as well as the balancing termination resistors connected to +5V and GND.
There were only minor changes throughout development. For example, originally the indicator LED’s were ON unless sending since they were tied to GND. Now then go LIT when sending, which is more intuitive, I suppose.
Here is the circuit:

Overall this was an Easy Project.
And yes, this is a HOMEBREW toner transfer method PC board. The DRAWING package is NOT EAGLE but rather ABACOM SPRINT LAYOUT and ABACOM SPLAN.

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