Model 251 is an RTS controlled transmit optically isolated RS-232 to RS-485 interface converter that can convert RS-232 TXD and RXD signals to balanced half-duplex RS-485 signals. On the RS-232 side, Model 251 is powered by 20 pins (DTR) and 4 pins (RTS) control signal lines. DTR must always be set to a high level. RTS must be switched between high level (when transmitting) and low level (when receiving) so that Model 251 can operate normally.
The Model 251 is an RTS‑controlled RS‑232 to RS‑485 interface converter. It converts the TXD and RXD signals from RS‑232 into differential full‑duplex RS‑485 signals. On the RS‑485 side the unit is powered by an external +9 VDC@200 mA adapter, while on the RS‑232 side it draws power from the DTR (high) and RTS (control) pins.
The converter uses a DB‑25 connector for RS‑232 (TXD on pin 2, RXD on pin 3, RTS on pin 4, DTR on pin 20) and a 4‑pin terminal block for RS‑485 (+12 V, GND, 485‑, 485+). An LED indicates RS‑485 data activity. Supported data rates range from 300 KBPS down to 38.4 KBPS, with additional rates of 4800 BPS, 1200 BPS, and 57.6 KBPS. This manual is written in Chinese.
| Power supply (RS‑485 side) | +9 VDC @ 200 mA external adapter |
|---|---|
| Supported data rates | 300 KBPS – 57.6 KBPS, 4800 BPS, 1200 BPS, 38.4 KBPS |
| RS‑485 connector pins | Pin 1 +12 V, Pin 2 GND, Pin 3 485‑, Pin 4 485+ |
It needs an external +9 VDC@200 mA adapter for the RS‑485 side.
Pin 1 provides +12 V, Pin 2 is GND, Pin 3 is 485‑, and Pin 4 is 485+.
It supports rates from 300 KBPS down to 57.6 KBPS, as well as 4800 BPS, 1200 BPS, and 38.4 KBPS.