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Current loop to RS-232?

NathanAllan

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Hiya guys,

I have two devices that need to connect to a serial port, one is a teletype and one is a Unix terminal. I can find lots of information around for making serial work on current loop but not the other way around.

Bot the tele and the term are current loop and they output the 20mA that will fry a serial port on the laptop that they will be hooked to. Help??
 

vwestlife

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central NJ
The original IBM 8-bit ISA serial port card (and exact clones) supported current loop, using some of the pins not normally used by RS-232C. As Wikipedia says, "The original IBM PC serial port card had provisions for a 20 mA current loop", and "on the original IBM PC, a male D-sub was an RS-232-C DTE port (with a non-standard current loop interface on reserved pins)".

I found the pinout:
Code:
Rs-232 Serial (Com) Pc Port Connector Db-25
DB-25 PIN (Male) FUNCTION ABBREVIATION
1 ---------------------------- Chassis/Frame Ground GND
2 ------------------------------ Transmitted Data TX or TD
3 -------------------------------- Receive Data RX or RD
4 ------------------------------ Request To Send RTS
5 ------------------------------- Clear To Send CTS
6 ------------------------------- Data Set Ready DSR
7 ------------------------------- Signal Ground GND
8 ---------------------------- Data Carrier Detect DCD or CD
9 ------------------------- Transmit + (Current loop) TD+
11 ------------------------ Transmit - (Current Loop) TD-
18 ------------------------- Receive + (Current Loop) RD+
20 --------------------------- Data Terminal Ready DTR
22 ----------------------------- Ring Indicator RI
25 ------------------------- Receive - (Current Loop) RD-
NOTE!! Current loop technology was supported in the PC and XT interfaces.
Current loop was discontinued when the AT interface was introduced.
Transmitted and receive data are referenced from the data
device and not the modem.
 

Chuck(G)

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You can also check the PC Techref (online) for exact details on the jumpers and pinout.

There weren't very many serial cards that supported current loop; even the original Compaq cards didn't.

Also, in connection with current loop, take a look at some of the RS232-to-MIDI converters--MIDI isn't anything but current loop interface. See here for example. Of course MIDI is a 6ma interface, rather than 20, but that can be adjusted.

As a tidbit, early MIDI experimenters used the IBM serial card for MIDI experiments; the crystal was changed, because the bitrate is 31.25Kbit/sec and a resistor was added to drop the current from 20 ma. to about 5.
 

kiyotewolf

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Urbana, IL
A MiDi interface is just something driving an optoisolator on the other end of the wire.

Had a MiDi SENSOR, and all there was inside was a resistor and an LED.



~Paul

Which in that case, with your 20 mA current loop, I'd try a bi-color LED, one that's two in one, just reverse the current to go red to green, and a resistor, and see what happens.
 
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NathanAllan

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Bellevue, Colorado
Excellent information! Thanks, Chuck(G), that looks like a winner to me. I'm not using an IBM, but I will bear taht in mind if I ever get one. All I have are clones.
 

vbriel

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Aug 3, 2003
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SoCal
Check out the first book of KIM. In that book is a current loop to RS232 circuit and it can be done with 5V and ground only and it works. How do I know, well, I use this circuit in my Micro-KIM. The original KIM-1 had a TTY 20mA current loop interface and with just a few components you can convert that to RS232 levels. If you can't find the circuit, let me know I can dig it up. Very simple circuit to make.

Vince
 
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