4 OPERATION
The B1474 operates from 12 to 30 VDC to provide an interface between two computer devices and the communications channel. In the descriptions below it is assumed that the sending device uses a DCE configuration.
Four different types of operation may be configured:
Hardware Handshaking
Basic operation and timing using hardware handshaking are as follows:
∙The sending device asserts its RTS and waits.
∙Upon receiving the RTS, the associated modem performs the following functions:
–checks the channel, if this feature is enabled. If the channel is free, it asserts its PTT to power up the radio’s transmitter.
–presents a mark tone (carrier) onto the channel.
–sets its RTS/CTS delay timer (from 20 to 1000 msec). RTS/CTS delay must be greater than the carrier detect delay at the receiving end.
∙The modem at the receiving end senses carrier on the line and asserts its CD. The received data is muted for 5 or 50 msec to ensure that transients do not affect the received data.
∙At the sending end, once the RTS/CTS delay timer has expired, the modem returns a CTS to the sending computer.
∙Data is transmitted; any format may be used.
∙Once all the data has been transmitted, the sending computer drops its RTS.
∙Upon sensing the dropped RTS, the sending modem does the following:
–drops CTS immediately.
–sets its PTT drop delay timer (0 to 250 msec).
∙Once the PTT drop delay timer has expired, the sending modem releases the radio transmit- ter, turns off the modem carrier and returns to receive mode.
For hardware handshaking, set XON/XOFF signaling off, assert RTS (continuous) off and buffering off. Set RTS/CTS delay and PTT drop delay as required.
Refer to Figure 1 for a summary of the modem timings.
B1474 | BARNETT ENGINEERING LTD. | PAGE 8 |