When connecting the computer to a modem, a straight-through rs-232 cable should be used. No remapping or crossing of lines should be required.
When connecting a terminal (such as a vt100) to the computer, you must have a null modem between the terminal and the computer for data communications to work correctly.
There are number of choices you must make when deciding to use a null modem. You can use a commercial, off-the-shelf (COTS) null modem cable or COTS null modem adapter.
When purchasing COTS, check the line mappings as described below. Most COTS null modems will correctly make a distributed computing environment (DCE) appear as a data terminal equipment (DTE), or a DTE appear as a DCE. Avoid COTS null modems that describe crossing lines other than those described later in this topic. (For more information about see the EIA Standard EIA-232-D).
You can also make your own null modem cable or adaptor. A quality electronics store will have all of the parts you require. Unless you purchase in large quantities, however, a COTS product will likely be cheaper.
A common remapping of the RS-232 lines for 25 lines is as follows:
| line | line | |
|---|---|---|
| 1 | _ | 1 |
| 2 | _ | 3 |
| 3 | _ | 2 |
| 4 | _ | 8 |
| 5 | _ | 8 |
| 6 | _ | 20 |
| 7 | _ | 7 |
| 8 | _ | 4 |
| 8 | _ | 5 |
| 20 | _ | 6 |
| case | _ | case |
Where line numbers match the following functionality for 25 lines:
| DB-25 | Signal | DB-9 |
|---|---|---|
| 1 | frame ground | - |
| 2 | TD (transmit data) | 3 |
| 3 | RD (receive data) | 2 |
| 4 | RTS (request to send) | 7 |
| 5 | CTS (clear to send) | 8 |
| 6 | DSR (data set ready) | 6 |
| 7 | SG (signal ground) | 5 |
| 8 | DCD (data carrier detect (also known as DC, RLSD) | 1 |
| 9 | POS (reserved; usually positive DC test voltage) | |
| 10 | NEG (reserved; usually negative DC test voltage) | |
| 11 | (unassigned) | |
| 12 | SDCD (secondary data carrier detect) | |
| 13 | SCTS (secondary clear to send) | |
| 14 | STD (secondary transmit data) | |
| 15 | TC (transmit clock) | |
| 16 | SRD (secondary receive data) | |
| 17 | RC (receive clock) | |
| 18 | LL (local loopback) | |
| 19 | SRTS (secondary request to send) | |
| 20 | DTR (data terminal ready) | 4 |
| 21 | SQ (signal quality detect) | |
| 22 | RI (ring indicator) | 9 |
| 23 | - (data rate selector) | |
| 24 | SCTE (external clock transmit) | |
| 25 | TM (test mode; often used as BUSY) |
For nine-pin connections, use a DB9 to DB25 connector before using the above mappings.
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