User`s manual

Electrical Interface Selection

Each of the eight ports on the COMHP 7804 can be individually configured as an RS-232, RS-422, or

RS-485 interface. This is selectable via the port DIP-switch, each is labeled with its port number (i.e. S1 = Port1, S2

= Port2 etc.)

Switch 1 (Silk M0) Switch 2 (Silk M1) Mode Select

OFF OFF 232

OFF ON 422

ON OFF 485 With Echo

ON ON 458 No Echo

Line Termination

Typically, each end of the RS-485 bus must have line-terminating resistors (RS-422 terminates at the receive end

only). A 120-ohm resistor is across each RS-422/485 input in addition to a 1K-ohm pull-up/pull-down combination

that biases the receiver inputs. Switch SW1 allows customization of this interface to specific requirements. Each

switch position corresponds to a specific portion of the interface. If multiple COMHP 7804 adapters are

configured in an RS-485 network, only the boards on each end should have switches T, P & P ON. Refer to the

following table for each position’s operation:

Switch Name Function

3 T Adds or removes the 120 ohm termination.

4 PU Adds or removes the 1K ohm pull-up resistor in the RS-422/RS-485 receiver circuit

5 PD Adds or removes the 1K ohm pull-down resistor in the RS-422/RS-485 receiver circuit

6 L Connects the TX- to RX- for RS-485 two-wire operation.

7 L Connects the TX+ to RX+ for RS-485 two-wire operation.

RS-485 ‘Echo’

The RS-485 ‘Echo’ is the result of connecting the receiver inputs to the transmitter outputs. Every time a character

is transmitted; it is also received. This can be beneficial if the software can handle echoing (i.e. using received

characters to throttle the transmitter) or it can confuse the system if the software does not. An RS-485 ‘No Echo’

option is selected by placing both Mode switches (M0, M1) in the ‘On’ position.

Figure 1 - RS-232 Mode

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COMHP 7804 CyberResearch

Serial Communication