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rj45 to db9 --- NULL Modem??? 1
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franklin97355
Technical User
Which connection on the modem are you connecting to your PC? What is the pinout on the modem?
The answer is "42"
The answer is "42"
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Simple answer, swap the b and bw on pins 2 and 3 like this:
p1 p2 p3 p4
gw bw b g
Long answer:
I am assuming you are trying to connect a PC to RJ11 style programming serial port on a phone system. If this is true there is NO MODEM involved. All you are doing is connecting serial ports together. You need a correctly configured RJ45(or RJ11) to DB9 adapter and a length of straight through RJ11 phone cord (or patch cord you have cut and put a RJ11 on one end like you have done). You will plug one end of the phone cord into the serial port on the phone system and the other end into the RJ to DB adapter.
In such a system only 3 wires (lines) are used. TD (transmit data), RD (receive data) and GND (ground). TD and RD will most likely be on the center pins of the RJ to DB adapter and GRND will most likely be on the left pin when looking into the socket. If you connect your adapter and it does not work then reverse pins 2 and 3 on the DB9 (or RJ11) to swap TD and RD.
Here is my handy cheat sheet:
[tt]
Modular - Serial (RS-232)
II X II X
DB9 Co Fn D25 45 110 4pr
3 2 Rd TD 2 3 4 2 BL
2 3 Gr RD 3 2 5 1 wBL
7 8 Or RTS 4 5 2 4 OR
8 7 Br CTS 5 4 7 7 wBR
5 5 Bk GND 7 7 3 5 wGR
1 - Yl DCD 8 - 6 6 GR
4 6 Bu DTR 20 6 1 3 wOR
6 1/4 Wh DSR 6 8/20 8 8 BR
Straight at computer
Crossed at peripheral
4 wire uses TD, RD, GRD & sometimes DCD
[/tt]
The columns under II are the straight through pinout for the style of connector (DB9 or DB25).
The columns under X are the crossover (null modem) pinout for the style of connector (DB9 or DB25).
The Co column is the customary color for the signal type listed under Fn. The wire colors have been verified for AllenTel (and most other I have seen) RJxx to DBxx adapters.
Finally the columns 45 110 and 4pr are where these pins land on RJ45, 110 blocks and the colors of a 4pr cable.
In your case look down the Fn column and note the positions of TD, RD and GND. On a properly wired RJ45 to DB9 adapter they will be at pins 4, 5 and 3 respectively. If the RJ11 serial jack on the phone system is wired DCE then connect the Red, Green and Black wires to pins 3, 2 and 5 on the DB9 and shown in the II column. If the RJ11 serial jack on the phone system is wired DTE then connect the Red, Green and Black wires to pins 2, 3 and 5 on the DB9 and shown in the X column.
If the phone system manufacturer has used a non-standard pinout on the RJ11 serial jack then you will have to figure it out. In that case I usually figure out which pin is ground first then look for a pin with voltage and in a 3 wire system it is usually TD. Most serial communications I have come across in 15+ years stick to this pattern in the cheat sheet.
Good luck!
n.b. I know that there is actually no such thing as a DB9. The actual nomenclature of a D-sub connector used for serial connections on modern computers with 9 pins is a DE9. If you want to read more go here
However just like the whole RJ thing being accurate in this case obscures understanding.
p1 p2 p3 p4
gw bw b g
Long answer:
I am assuming you are trying to connect a PC to RJ11 style programming serial port on a phone system. If this is true there is NO MODEM involved. All you are doing is connecting serial ports together. You need a correctly configured RJ45(or RJ11) to DB9 adapter and a length of straight through RJ11 phone cord (or patch cord you have cut and put a RJ11 on one end like you have done). You will plug one end of the phone cord into the serial port on the phone system and the other end into the RJ to DB adapter.
In such a system only 3 wires (lines) are used. TD (transmit data), RD (receive data) and GND (ground). TD and RD will most likely be on the center pins of the RJ to DB adapter and GRND will most likely be on the left pin when looking into the socket. If you connect your adapter and it does not work then reverse pins 2 and 3 on the DB9 (or RJ11) to swap TD and RD.
Here is my handy cheat sheet:
[tt]
Modular - Serial (RS-232)
II X II X
DB9 Co Fn D25 45 110 4pr
3 2 Rd TD 2 3 4 2 BL
2 3 Gr RD 3 2 5 1 wBL
7 8 Or RTS 4 5 2 4 OR
8 7 Br CTS 5 4 7 7 wBR
5 5 Bk GND 7 7 3 5 wGR
1 - Yl DCD 8 - 6 6 GR
4 6 Bu DTR 20 6 1 3 wOR
6 1/4 Wh DSR 6 8/20 8 8 BR
Straight at computer
Crossed at peripheral
4 wire uses TD, RD, GRD & sometimes DCD
[/tt]
The columns under II are the straight through pinout for the style of connector (DB9 or DB25).
The columns under X are the crossover (null modem) pinout for the style of connector (DB9 or DB25).
The Co column is the customary color for the signal type listed under Fn. The wire colors have been verified for AllenTel (and most other I have seen) RJxx to DBxx adapters.
Finally the columns 45 110 and 4pr are where these pins land on RJ45, 110 blocks and the colors of a 4pr cable.
In your case look down the Fn column and note the positions of TD, RD and GND. On a properly wired RJ45 to DB9 adapter they will be at pins 4, 5 and 3 respectively. If the RJ11 serial jack on the phone system is wired DCE then connect the Red, Green and Black wires to pins 3, 2 and 5 on the DB9 and shown in the II column. If the RJ11 serial jack on the phone system is wired DTE then connect the Red, Green and Black wires to pins 2, 3 and 5 on the DB9 and shown in the X column.
If the phone system manufacturer has used a non-standard pinout on the RJ11 serial jack then you will have to figure it out. In that case I usually figure out which pin is ground first then look for a pin with voltage and in a 3 wire system it is usually TD. Most serial communications I have come across in 15+ years stick to this pattern in the cheat sheet.
Good luck!
n.b. I know that there is actually no such thing as a DB9. The actual nomenclature of a D-sub connector used for serial connections on modern computers with 9 pins is a DE9. If you want to read more go here
However just like the whole RJ thing being accurate in this case obscures understanding.
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