60mA Teletype Current Loop Interface - Design notes

John Nagle
nagle@animats.com
February, 2009
V2.0

Files:
	converter.sch		ExpressPCB schematic file
	converter.pcb		ExpressPCB board layout
	converter.xls		Parts list and pinouts
	converter.jpg		A photo of a completed, functional board.
	NOTES.txt		This file.

This is a simple level converter for connecting a Teletype using a
60mA current loop to a computer's serial port.  Conversion is provided
for both input and output.  The device requires a 120VDC power supply
for the current loop.

Boards can be ordered using the ExpressPCB service ("http://www.expresspcb.com").
The board is designed to their "MiniBoard" standard, and as of 2009, 
three boards cost $51.  We have no connection with ExpressPCB other than
as a customer.

All parts for the board are available from Digi-Key ("http://www.digikey.com")
and the ordering information is in the "converter.xls" spreadsheet.
OpenOffice 3.0 will open this spreadsheet, as will Microsoft Excel.

The RS-232 connection and the high voltage loop are fully optoisolated and
do not share a ground.  The high voltage loops are snubbed and the optoisolators
have high enough voltage ratings to handle the inductive kickback from Teletype
selector magnets. 

Notes:
- The Teletype is assumed to use a standard 55 ohm selector magnet coil
  requiring 60mA.  Other coil resistances may require recalculation of R8,
  which is a trimming resistor on the large 10W resistor R7.
- R7, the 10W resistor in the current loop, gets hot.  This board requires
  reasonable ventilation.
  Do not enclose it in a case without vents.
- The program driving the RS-232 port must turn on DTR if keyboard input is to work.
  Power for the RS-232 Receive Data signal comes from the DTR line.
- For receive-only applications, IC2, D2, C2, R2, R3, and R6 can be omitted.
- This board is full-duplex only.  The keyboard attaches to J1 pins 1/2, and
  the printer to J1 pins 4/5.  Connecting the keyboard to pins 3/4 and
  disconnecting the serial port J2 will allow a simple local loopback test,
  but half duplex is not supported.  There is no provision for hardware local
  echo; echo must be done in software on the host computer.
- The RS-232 interface is not fully RS-232 compliant.  Receive Data only
  swings from 0 to the level provided from DTR; it will not go negative.
  Transmit Data, from the computer, must be RS-232 compliant, and must 
  go negative to drive the printer. 
- The board has been tested with a desktop PC running Windows using the
  "Heavy Metal" program ("http://www.buzbee.net/heavymetal/Intro.htm").

This design is offerered as a hobbyist project, with no warranty.
