The host-target link allows the debugger to communicate with the debug monitor running on the target computer. The link uses an RS-232C interface connected to a serial port on the host computer, and connected to a compatible serial port on the target computer.
The connecting cable must include a null modem. This is because both the host serial port and target serial port are configured to be connected to a terminal. The null modem is simply a cross over that wires the outputs from one port to the inputs of the other. Details of the wiring are given in Section F.1.
The RS-232 standard is given in Table F-1.
Table F-1. The RS-232 Standard
| DB-25 | DCE | DB-9 | |||
|---|---|---|---|---|---|
| 1 | AA | x | Protective Ground | ||
| 2 | TXD | 3 | BA | I | Transmitted Data |
| 3 | RXD | 2 | BB | O | Received Data |
| 4 | RTS | 7 | CA | I | Request To Send |
| 5 | CTS | 8 | CB | O | Clear To Send |
| 6 | DSR | 6 | CC | O | Data Set Ready |
| 7 | GND | 5 | AB | x | Signal Ground |
| 8 | CD | 1 | CF | O | Received Line Signal Detector |
| 9 | -- | x | Reserved for data set testing | ||
| 10 | -- | x | Reserved for data set testing | ||
| 11 | x | Unassigned | |||
| 12 | SCF | O | Secndry Rcvd Line Signl Detctr | ||
| 13 | SCB | O | Secondary Clear to Send | ||
| 14 | SBA | I | Secondary Transmitted Data | ||
| 15 | DB | O | Transmisn Signl Elemnt Timng | ||
| 16 | SBB | O | Secondary Received Data | ||
| 17 | DD | O | Receiver Signal Element Timing | ||
| 18 | x | Unassigned | |||
| 19 | SCA | I | Secondary Request to Send | ||
| 20 | DTR | 4 | CD | I | Data Terminal Ready |
| 21 | CG | O | Signal Quality Detector | ||
| 22 | 9 | CE | O | Ring Indicator | |
| 23 | CH/CI | I/O | Data Signal Rate Selector | ||
| 24 | DA | I | Transmit Signal Element Timing | ||
| 25 | x | Unassigned |
The wiring of a null modem cable is given in Table F-2.
Table F-2. Null Modem Wiring and Pin Connection
| 25 Pin | 9 Pin | 9 Pin | 25 Pin | |||
|---|---|---|---|---|---|---|
| FG (Frame Ground) | 1 | N/A | <----------> | N/A | 1 | FG |
| TD (Transmit Data) | 2 | 3 | <----------> | 2 | 3 | RD |
| RD (Receive Data) | 3 | 2 | <----------> | 3 | 2 | TD |
| RTS (Request To Send) | 4 | 7 | <----------> | 8 | 5 | CTS |
| CTS (Clear To Send) | 5 | 8 | <----------> | 7 | 4 | RTS |
| SG (Signal Ground) | 7 | 5 | <----------> | 5 | 7 | SG |
| DSR (Data Set Ready) | 6 | 6 | <----------> | 4 | 20 | DTR |
| DTR (Data Terminal Ready) | 20 | 4 | <----------> | 6 | 6 | DSR |