Notes – QL, Serial Ports & Modems

This little article is about some things I’ve learned and observed while using my QL and its serial ports. This info may be common knowledge, especially to seasoned QL communications veterans but for the benefit of others, here it is. First of all, Hermes is highly recommended for anyone using a QL and its serial ports. It allows the ports to reliably send and receive from 300 bps (bits-per-second) to 19200 bps. Hermes is a replacement for the QL’s second microprocessor, the 8049. Sending data out of the ports is not a problem for the QL at any speed. However, trying to receive data at the faster rates, certainly anything above 2400 always seemed to be a problem. With Hermes, the QL will reliably receive at rates up to 19,200 bps, but the modem must be able to respond to a signal from the QL to temporarily stop sending. This is known as a handshake. Most modems will also send a signal using another handshake line to tell the computer to temporarily stop if the modem can’t keep up with the incoming data. The QL has two handshake lines on each serial port, one that it uses to signal the modem to stop, and one which it monitors for the modem to tell it to stop. Note that these lines must be connected properly between the QL and modem in order for the handshaking to work!