37 PIN D-SUB MALE at the DTE (Computer)
37 PIN D-SUB FEMALE at the DCE (Modem)
NOTOC
| Pin | Name | Dir | Description |
|---|---|---|---|
| 1 | GND | - | Shield Ground |
| 2 | SRI | IN | Signal Rate Indicator |
| 3 | n/c | - | Spare |
| 4 | SD | OUT | Send Data |
| 5 | ST | OUT | Send Timing |
| 6 | RD | IN | Receive Data |
| 7 | RTS | OUT | Request To Send |
| 8 | RR | IN | Receiver Ready |
| 9 | CTS | IN | Clear To Send |
| 10 | LL | OUT | Local Loopback |
| 11 | DM | IN | Data Modem |
| 12 | TR | OUT | Terminal Ready |
| 13 | RR | IN | Receiver Ready |
| 14 | RL | OUT | Remote Loopback |
| 15 | IC | IN | Incoming Call |
| 16 | SF/SR | OUT | Select Frequency/Select Rate |
| 17 | TT | OUT | Terminal Timing |
| 18 | TM | IN | Test Mode |
| 19 | GND | - | Ground |
| 20 | RC | - | Receive Twister-Pair Common |
| 21 | GND | - | Spare Twister-Pair Return |
| 22 | /SD | - | Send Data TPR |
| 23 | GND | - | Send Timing TPR |
| 24 | GND | - | Receive Timing TPR |
| 25 | /RS | - | Request To Send TPR |
| 26 | /RT | - | Receive Timing TPR |
| 27 | /CS | - | Clear To Send TPR |
| 28 | IS | IN | Terminal In Service |
| 29 | /DM | - | Data Mode TPR |
| 30 | /TR | - | Terminal Ready TPR |
| 31 | /RR | - | Receiver TPR |
| 32 | SS | OUT | Select Standby |
| 33 | SQ | IN | Signal Quality |
| 34 | NS | OUT | New Signal |
| 35 | /TT | - | Terminal Timing TPR |
| 36 | SB | IN | Standby Indicator |
| 37 | SC | - | Send Twister Pair Common |
Notes
- Direction is DTE (Computer) relative DCE (Modem)
- RS449 gets it’s blazing speed from the fact that, unlike RS232 which uses signals with reference to ground, it’s receivers look for the difference between two wires. Now the secret, by twisting these two wires any stray noise picked up on one wire will be picked up on the other, because both wires pick up the same noise the differential just shifts in voltage level with reference to ground, but does not change with respect to each other. Remember the receivers are only looking at the difference in voltage level of each wire to the other not to ground. This is what makes all the new wire interfaces work, V.35, RS530, 10baseT, etc. The biggest problem faced is how the cables are made. The differential signals for RS449 are labeled as either “A and B” or “+ and -”. In the case of RS449 wire A or + does not connect to B or -. Wire A always connects to A and B connects to B or + to + and - to -. If you do cross the wires you just inverted the data or clock in your interface. I have never seen any piece of equipment damaged from this, but they don’t work this way either.
Speed and Distance
(Max Recommended)
Terminated Circuits
- 10 MHz 10 Meters
- 6 MHz 17 Meters
- 2 MHz 40 Meters
- 1 MHz 100 Meters
- 100 KHz 1000 Meters
- 10 KHz 1000 Meters
Non- Terminated Circuits
- 1 MHz 10 Meters
- 100 KHz 100 Meters
- 56 KHz 110 Meters
- 10 KHz 1000 Meters
Cable Design
The design of your cable depends on what you are connecting together and the interfaces involved. There are two standard interfaces types “Data Terminal Equipment” (DTE) and “Data Communication Equipment” (DCE). At this point we will assume that both interfaces are RS449.
DTE to DCE (differential pairs must be twisted)
How to Build a RS422/RS449 Interface
RS449 (RS422) Differential Driver
The resistors Za and Zb are optional. I recommend 10 ohms to bring the interface to 50 ohms and to provide some protection from EMF. Here is a Tip: Note that the A or + signal is on the inverted output pin of the driver, most 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 RS449 (RS422) A or + must be the inverted pin. This is also true of the receivers.
RS449 (RS422) Differential Receiver
The resistor Zt is optional. I recommend 150 ohms to reduce reflectance. However If you are trying to build a non intrusive receiver I would leave this out. Note this interface when left floating will have an unpredictable output. Some engineers place pull up (to pin A) and pull down (to pin B) resistors of 10K to provide a know state when the cable is unplugged or connected equipment is turned off.
V.24 Driver
V.24 Receiver
