Hi all,
I'm attempting to interface serial devices using standard Category 5 cables, since they are cheap, widely available, and can be manufactured in long lengths. To achieve this, I have obtained several adapter kits that convert D-Sub 9 and D-Sub 25 connectors to RJ45 connectors.
I am using the EIA-561 standard for wiring serial devices together, with a small modification. I am constructing the RJ45 adapters using the following pinout:
RJ45 pin............Signal
1 (blue)............DSR (Data Set Ready) 2 (orange)..........DCD (Carrier Detect) 3 (black)...........DTR (Data Terminal Ready) 4 (red).............GND (Signal Ground) 5 (green)...........RxD (Receive) 6 (yellow)..........TxD (Transmit) 7 (brown)...........CTS (Clear to Send) 8 (white)...........RTS (Request to Send)The difference between my pinout and EIA-561 is that I use Pin 1 for DSR (Data Set Ready) instead of RI (Ring Indicator).
Now, I wish to connect two PCs together using a null modem serial configuration (wherever an Ethernet port is not available). The nice thing about using EIA-561 is that I can make the null modem cable as long as I like by finding a longer Category 5 cable. I plan to use a straight-through EIA-561 adapter on one end, and a null-modem adapter on the other end, wired according to the following pinout:
RJ45 pin............Signal
1 (blue)............not connected 2 (orange)..........DTR (Data Terminal Ready) 3 (black)...........DCD (Carrier Detect) 4 (red).............GND (Signal Ground) 5 (green)...........TxD (Transmit) 6 (yellow)..........RxD (Receive) 7 (brown)...........RTS (Request to Send) 8 (white)...........CTS (Clear to Send)This will establish the following connections between both ends:
DSR -- not connected DCD -- DTR DTR -- DCD GND -- GND RxD -- TxD TxD -- RxD CTS -- RTS RTS -- CTS
I have actually built both of these adapters and they seem to be working, at least between a PC and an Apple IIGS.
However, I have a few questions about this configuration:
- Is there a consequence to omitting DSR in the null modem adapter? Will it reduce compatibility between particular devices?
- How important is the DSR signal for serial communications, especially in a null-modem configuration?
- Can I use two of my straight-through adapters in order to connect a modem to a PC? Will omitting RI cause any problems with modern modems?
- Should I have just wired Pin 1 in both adapters to be RI instead of DSR?
Thanks,
--A.J.