RS449 Technical Reference
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RS449 Interface
RS449, which is identical to V.11, relies on balanced differential signaling (RS422) to achieve longer range, higher speeds, and obtain some immunity against common-mode noise. The standard uses a 37-pin D-connector and is intended for synchronous wide area networking applications.
Each pair of differential signals are labeled as "A" and "B". The "A" wire always connects to "A" on the other interface, and "B" connects to "B".
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RS449 Speed vs. Distance
| Terminated |
Unterminated |
| Speed |
Distance |
Speed |
Distance |
| 10 MHz |
10 m |
1 MHz |
10 m |
| 2 MHz |
40 m |
100 kHz |
100 m |
| 1 MHz |
100 m |
56 kHz |
110 m |
| 100 kHz |
1 km |
10 kHz |
1 km |
RS449 Pinout
| Pin |
Signal Description |
Mnemonic |
| 1 |
Shield |
Frame ground |
| 2 |
Signal rate Indicator |
SI (unbalanced) |
| 3 |
Not used |
- |
| 4 |
Send Data A |
SDA |
| 5 |
Send Timing A |
STA |
| 6 |
Receive Data A |
RDA |
| 7 |
Request To Send A |
RTSA |
| 8 |
Receive Timing A |
RTA |
| 9 |
Clear To Send A |
CTSA |
| 10 |
Local Loopback |
LL (unbalanced) |
| 11 |
Data Mode A |
DMA (like DSR) |
| 12 |
Terminal Ready A |
TRA (like DTR) |
| 13 |
Receiver Ready A |
RRA (like DCD) |
| 14 |
Remote Loopback |
RLB (unbalanced) |
| 15 |
Incoming Call |
IC (unbalanced) (like RI) |
| 16 |
Signal Rate Selector |
SI (unbalanced) |
| 17 |
Terminal Timing A |
TTA |
| 18 |
Test Mode |
TM (unbalanced) |
| 19 |
Signal Ground |
SG |
| 20 |
Receive Common |
RC |
| 21 |
Not used |
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| 22 |
Send Data B |
SDB |
| 23 |
Send Timing B |
STB |
| 24 |
Receive Data B |
RDB |
| 25 |
Request to Send B |
RTSB |
| 26 |
Receive Timing B |
RTB |
| 27 |
Clear To Send B |
CTSB |
| 28 |
Terminal In Service |
IS (unbalanced) |
| 29 |
Data Mode B |
DMB |
| 30 |
Terminal Ready B |
TRB |
| 31 |
Receiver Ready B |
RRB |
| 32 |
Select Standby |
SS (unbalanced) |
| 33 |
Signal Quality |
SQ (unbalanced) |
| 34 |
New Signal |
NS |
| 35 |
Terminal Timing B |
TTB |
| 36 |
Standby Indicator |
SB (unbalanced) |
| 37 |
Send Common |
SC |
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| Balanced Driver (RS422)
The resistors Za and Zb are optional. I recommend 10 Ohms to bring the interface to 50 Ohms and to provide some protection from EMI. Note that the "A" signal is on the inverted output pin of the driver, about 50% of designers get this switched in their first design. This happens because some data books call the positive pin A and the negative pin B. The RS530 (RS422) "A" signal must be the inverted pin. This is also true of the receivers.
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| Balanced Receiver (RS422)
The resistor Zt is optional. I recommend 150 Ohms to reduce reflectance. Leave Zt out if you are trying to build a non intrusive receiver. Note that this interface, when left floating, will have an unpredictable output. Some engineers install pull up (to pin A) and pull down (to pin B) resistors of 10k Ohms to provide a known state when the cable is unplugged or connected equipment is turned off.
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| Unbalanced Driver
The typical unbalanced driver is the standard RS232 1488 IC, however this requires + and - 12 Volts. Many of the new interfaces are using the V.24 / RS232 driver chips with the charge pump built in, so they only need +5V to operate. These come in many configuration some include both drivers and recievers. Two parts that come to mind is the DS14C232 from TI or Max 232.
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| Unbalanced Receiver
The typical IC used for the reciever is the 1489, it only requires 5 volts to operate, but most new designs are using the charge pump IC which have both the receivers and drivers in the same chip.
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RS449 Breakout, Testing and ConversionADVICE manufactures tools that support the RS449 interface:
The UMATS is a breakout box, allowing you to make/break connections to diagnose and solve cable connection problems. Additionally, the UMATS allows conversion between RS449 and RS232, V.35 or RS530 among others.
The Exchanger is a full featured modem eliminator and/or interface converter that is designed for permanent installation. Click on the links for details.
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