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Zonker's RS-232 Information Page

http://www.conserver.com/consoles/Clues/rs232.html
(updated: Nov 06, 2005, 10:32 pm)

Table of Contents
Basic Terminology
DTE and DCE Signals
RS-232-C and EIA-232
Cable Clues
Protocol Analyzer Software & Tools
Author Credits

Basic Terminology

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DTE and DCE

  • Legacy terms, that described the 'type of interface'
  • These terms also defined which pins could be expected to be "transmitters" (outputs)
    • Outputs are also called "Transmitters" or "Drivers"
    • Inputs are also called "Receivers"
  • Data Terminal Equipment (DTE)
    • typically hosts, terminals
  • Data Communications Equipment (DCE)
    • typically modems, printers

Data Signals (TD and RD)

  • These two wires (plus Signal Ground) are needed to pass two-way data between devices
  • Transmit Data (TD), is an output on DTE devices, and an input on DCE devices
  • Receive Data (RD), is an input on DTE devices, and an output on DCE devices

(Rarely, a vendor gets these backwards, so I always use a signal tracer to be sure.)

Flow Control Signals (RTS and CTS)

  • Signal leads used by devices to stop and start data flow on the 'other' device.
  • Referred to as "Hardware Flow Control", versus Software Flow Control (XON/XOFF).
  • Request To Send (RTS) is how DTE tells the DCE device to start/stop.
  • Clear To Send (CTS) is how DCE tells the DTE device to start/stop.
  • If you plan to use "Protocol Communications" on the serial connection, you will need to turn. Software Flow Control off, and turn Hardware Flow Control on.
    • This includes SLIP and PPP, as well as XMODEM/YMODEM/ZMODEM transfers.
    • This is because packet numbers can look like the XOFF character, and they can freeze your session by stopping communications, with no way to restart it!

Handshaking Signals (DTR, DSR, DCD)

  • Signal leads used to the other' device when it is powered and ready (versus flow control)
  • Data Terminal Ready (DTR) used by DTE device to tell DCE device it is ready
  • Data Set Ready (DSR) used by DCE device to tell DTE device it is ready
  • Data Carrier Detect (DCD) used by modems (DTE device) to tell DCE device a call is on-line

DTE and DCE Signals

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The RS-232 standards outline certain signal leads, with their common designations, should appear on specific pins of the DB-25 connector. These common designations appear below. The normal signals used for asynchronous serial communications are shown in boldface (pins 2, 3, 4, 5, 6, 7, 8, and 20).

Pin

Signal

DTE

DCE

1

Chassis Ground

-

-

2

Tx Data (TD)

output

input

3

Rx Data (RD)

input

output

4

Request To Send (RTS)

output

input

5

Clear To Send (CTS)

input

output

6

Data Set Ready (DSR)

input

output

7

Signal Ground

-

-

8

Carrier Detect (DCD)

input

output

9

positive test voltage (loop)

-

output

10

negative test voltage (loop)

-

output

11

not assigned

x

x

12

Secondary DCD (SDCD)

input

output

13

Secondary CTS (SCTS)

input

output

14

Secondary Tx Data (STD)

output

input

15

Transmitter Clock (TC)

input

output

16

Secondary RD (SRD)

input

output

17

Receiver Clock (RC)

input

output

18

Local Loopback

output

input

19

Secondary RTS (SRTS)

output

input

20

Data Terminal Ready (DTR)

output

input

21

Signal Quality Detect (SQ)

input

output

22

Ring Indicator (RI)

input

output

23

Data Rate Selector (DRS)

either

either

24

External Transmitter Clock (XTC)

output

input

25

Test Mode (TM)

input

output

Note 1: Basic data connections only require three leads: TD, RD, and Signal Ground.
Note 2: Modem connections also require: CTS, RTS, DSR, DTR, DCD.

RS-232-C and EIA-232

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Serial communications standards, both specifying mechanical and electrical information, and both represent one of the most common asynchronous serial standards today. Some of the specifications from these standards are;

  • +21V = logic '1', -21V = logic '0'.
  • Some devices use +5V / -5V signaling .
  • Between +3V to -3V = "No Man's Land" .
    • (long cables deplete the signals, giving unreliable output)
  • 50' length maximum between devices .
    • (though folks often exceed this...successfully)
    • Devices with +5V / -5V signaling have trouble using long cable runs!
  • DB-25 Connector Pinouts .
    • (IBM developed the de facto DB-9 standard for the PC-AT)

Cable Clues

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The RS-232 specification called for no more than 50 feet. However, I've had success with much greater distances in the past, depending on the devices being connected (driver voltages), cable length, cable type, shielded or not, etc.

The DigiCorp folks have posted a note on their support pages that recommends that the cable length be no more than 2500 picofarad in capacitance, where each connector has a 'cost' of 100 pf, and then you need to find out how many picofarads-per-foot your cabling is rated. More info is available on their Tech Support Specifications Page. Your mileage may vary. Older Macintosh computers had lower voltages (~6-9v) than the RS-232 specification (~25v), and as a result, their range seems shorter to me.

Twisted Pair cable can help extend the range (and often has a lower capacitance...hmmm...), but the twists don't often help reduce line noise, because the various vendors use different RJ-45 wiring schemes. As a result, You often have your most susceptible leads (data) twisted together, versus twisted with a fairly static signal (like ground, or handshaking leads).

Shielded cable can help reduce noise, but the cost is higher than for normal CAT-5. You can usually achieve good noise isolation if you pay attention to WHERE your cables are placed. Avoid laying data cables parallel with AC power cables, or laying on/near transformers or fluorescent lighting ballasts (both are strong sources of electromagnetic radiation).

Protocol Analyzer Software & Tools

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Serial Protocol Analyzer Programs

Serial Protocol Analyzer Tools/Devices


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