please dont rip this site Oricom Technologies
P.O. Box 68, Boulder, CO 80306 - (303) 444-9776
e-mail: -

Return to: App Notes Index

A Simple Inexpensive Network-Compatible RS-232 Interface

[RS-232 I/F]

Several simple RS-232 interfaces have appeared in various magazine articles, built up on bipolar transistor circuits, but most have several shortcomings. The circuit shown here, however, is small, simple, inexpensive, compatible with the RS-232 standard (re voltage levels and also in having inverters in both RxD and TxD pathways), and works in full-duplex operation, which most simplified interfaces do not.

The circuit "steals" its negative voltage from the host serial port, and draws no power when not in operation (except for transistor leakage currents), so it is useful for low-power applications. It costs approximately $1, about 1/3 the cost of a MAX232 interface, and saves the 5 mA quiescent current draw of that device.

The circuit is based on that used in the Parallax BS2. Q1 is the receive inverter, and Q2 the transmit inverter. TxD and RxD go to the host serial port and have RS-232 compatible levels (+5, -12v), while Tc and Rc go to the local cpu with 0 & 5V levels. By adding diode D1 and capacitor C1 to the Parallax circuit, we get one where C1 charges to the -12 volts or so on the RxD line and holds it during positive RxD transitions, effectively isolating the TxD and RxD lines, and allowing normal full-duplex operation. Most general-purpose bipolar transistors should work here, and resistor values are not critical - values 2-5x larger should work fine, except for RN1c. For RN1 and RN2, we use 3-resistor SIP networks for easy layout. We have used this circuit consistently at baudrates to 115,200 through 12' foot cables.

Further, for networking applications, the TxD and RxD channels of several of these circuits can be wired "unmodified" in parallel. In this case, however, the Q2 inverter can load down the network if the circuit driving Q2 is unpowered, but insertion of diode D2 between Q2's collector and RN1c (in place of the wire) will prevent this problem. Also, if more than a couple of these circuits are tied together, RN1c, D1, and C1 can be removed from all but one circuit, in order to reduce costs as well as lower the current it takes to drive the TxD line.


© Oricom Technologies, updated September 1998

file: /Techref/net/qwest/users/www/http/~oricom/an-ecd02/AN-ECD02.HTM, 3KB, , updated: 2001/5/25 14:41, local time: 2024/6/18 07:49,

 ©2024 These pages are served without commercial sponsorship. (No popup ads, etc...).Bandwidth abuse increases hosting cost forcing sponsorship or shutdown. This server aggressively defends against automated copying for any reason including offline viewing, duplication, etc... Please respect this requirement and DO NOT RIP THIS SITE. Questions?
Please DO link to this page! Digg it! / MAKE!

<A HREF=""> AppNote AN-ECD02</A>

After you find an appropriate page, you are invited to your to this massmind site! (posts will be visible only to you before review) Just type a nice message (short messages are blocked as spam) in the box and press the Post button. (HTML welcomed, but not the <A tag: Instead, use the link box to link to another page. A tutorial is available Members can login to post directly, become page editors, and be credited for their posts.

Link? Put it here: 
if you want a response, please enter your email address: 
Attn spammers: All posts are reviewed before being made visible to anyone other than the poster.
Did you find what you needed?

  PICList 2024 contributors:
o List host: MIT, Site host, Top posters @none found
- Page Editors: James Newton, David Cary, and YOU!
* Roman Black of Black Robotics donates from sales of Linistep stepper controller kits.
* Ashley Roll of Digital Nemesis donates from sales of RCL-1 RS232 to TTL converters.
* Monthly Subscribers: Gregg Rew. on-going support is MOST appreciated!
* Contributors: Richard Seriani, Sr.

Welcome to!