Small buffer devices

Some terminal devices have hardware limitations of the maximum buffer size that they can process. Additional limits can be imposed by software, either in the device or in the network.

Exceeding these limits can result in an error and failure to deliver a message that is too large for the device to process. Be aware of what these limits are, and of the alternatives available to your application program.

Different device types specify the limits in different ways. For input, the SEGSIZE or BUFSIZE specification in IMS is relevant, and, for output, the OUTBUF specification. For VTAM® terminals, the BIND RU size is based on these values. IMS supports message chaining for both input and output, but some terminals do not support chaining. In this case, message length is limited by sizes.

Your application program has control over the length of the output message, either directly or through the MFS definition being used to format the message. If you need to send a message that is longer than the maximum length supported by the target device, you must break the message into multiple messages, each of which is shorter than the maximum length for the device. The DL/I PURG call must be issued by your application program to do this. IMS delivers messages in the order that your application program inserts them; this is a good solution for devices that are not sensitive to message boundaries.