A really good way of doing this is to use the "linear" opto-couplers from H-P (or whatever they're called this week). The HCNR200 application datasheet has excellent circuits for 4 - 20 mA TX and RX. The only omission in these designs are the RFI suppression and decoupling capacitors that you'll find necessary in the "real world". The accuracy and linearity possible with these devices is remarkable. Hope this helps Chris PS: H-P Semiconductors is now called "Agilent" ----- Original Message ----- From: "Tony K|bek" To: Sent: Tuesday, November 19, 2002 10:37 AM Subject: [EE]: RS232 <-> 20mA current loop designs Hi, I have some designs for an 20mA current loop i/f that we use on some of our products, however these are designed for TTL i/f as they are internal. Now I need to make an external version, my questions are: Are there any designs available on the net ? Are there any 'driver chips' available ? I need to cater for both active and passive loops. Thanks in advance. /Tony -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics