Barnett Engineering B1474 Software Handshaking Using XON/XOFF, No Handshaking, Buffered

Software Handshaking Using XON/XOFF

Basic operation and timing using XON/XOFF software handshaking are as follows:

∙The sending computer begins sending data. The format must be 8 data bits, no parity, 1 stop bit.

∙Upon detecting data, the sending modem performs the following:

– checks the channel, if this feature is enabled. If the channel is free, it asserts its PTT to power up the radio’s transmitter. If the channel is busy, it returns XOFF to the sending computer, which halts the transmission of data. When the channel becomes free, the modem sends XON to the computer, which resumes sending data. The modem buffers the data while it continues with the following:

– presents a mark tone (carrier) onto the channel.

– sets its RTS/CTS delay timer (from 20 to 300 msec). RTS/CTS delay is a misnomer in this case as software handshaking does not use RTS and CTS signals. It is actually a buffering time between when the modem starts receiving data from the computer and when it starts transmitting. This 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, data is transmitted.

– The sending modem’s PTT drop delay timer is started at the end of the last transmitted character.

∙Once the PTT drop delay timer has expired, the modem releases the radio transmitter, turns off the modem carrier and returns to receive mode.

This configuration can buffer 500 msec worth of characters. To prevent internal data buffer overflow, it is important that the RTS/CTS delay timer be set to a value less than 500 msec and that the computer respond to XOFF immediately.

For software handshaking, set XON/XOFF signaling on, assert RTS (continuous) off, and buffering on. Do not connect hardware RTS and CTS. Set RTS/CTS delay and PTT drop delay as required.

No Handshaking, Buffered

Basic operation and timing with data buffering (no handshaking) are as follows:

∙The sending computer begins sending data. Data format must be 8 bits, no parity, 1 stop bit.

∙Upon detecting data, the sending modem performs the following:

B1474 BARNETT ENGINEERING LTD. PAGE 10