# X COMM: Communication check Source: https://wndsoftware.com/mainframe-errors/3270-x-comm ### X COMM: Communication check X COMM followed by a number means input is inhibited because of a communication check. The link between the emulator and the host isn't working or isn't ready. For TN3270 sessions in IBM's emulators, such as Host On-Demand, the number (654, 657 or 659, for example) tells you which step of the connection failed. #### Other names for this code Also written as `X COMM`, `COMM 659`, `COMM 654`, `COMM 657`, `COMM 658`, `COMM 655`, `COMM 662`, `COMM 666`, `3270 communication check`, `COMM check TN3270`, `lightning bolt 3270 status line`. ### What it means A communication check means the problem is in the connection, not in the application or anything you typed. On the status line it shows as the word COMM, or as a lightning bolt symbol when the session uses a 3270 font, followed by a three-digit number. The keyboard is locked while it shows, because there is nothing to send your input to. For TN3270 and TN3270E sessions, IBM's emulator documentation lists these codes: - 654 : the TN3270E server refused the LU name. It may be in use, not defined, the wrong LU type, or the negotiation failed. - 655 : the socket is connected and waiting for Telnet negotiation to finish. It also appears when SSL is off but the port expects SSL. - 657 : the TCP/IP connection is being set up. With SSL it can mean the port isn't an SSL port or a signer certificate has expired. - 658 : the TCP/IP connection for TN3270E is initializing. - 659 : the TCP connection failed or was dropped by either side. - 662 : the server's certificate isn't trusted. - 663 : the name in the server's certificate doesn't match the server. - 664 : the secure connection couldn't be completed. - 665 : the server's certificate isn't valid yet. - 666 : the server's certificate has expired. ### Common causes - The host name, IP address or port in the session profile is wrong, or the TN3270 server isn't listening on that port (usually 657 or 659). - The session asks for a specific LU name that another session already uses or that isn't defined on the server (654). - SSL settings don't match the port. The session uses SSL against a plain port, or plain Telnet against an SSL port (655 or 657). - A firewall, VPN or idle timeout dropped the connection, or the server closed it (659). - The server's certificate isn't trusted on this PC, has expired, or has a different name from the one you connect to (662 to 666). ### How to find the problem - Read the number after COMM. It tells you which stage failed, and the list on this page gives the meaning. - Check the session's host name, port, LU name and security settings against what the mainframe team gave you. - Ask whether others connecting to the same server have the same code. If they do, the server or network side is down. - For certificate codes, view the server certificate and check its dates, issuer and host name. - If the code doesn't explain itself, turn on the emulator's communication trace and reproduce the problem. IBM suggests a transport-level trace for 654. ### How to fix it - For 657 and 659, check the address and port, check that you can reach the host, and reconnect. If it drops again after idle time, ask for keepalive settings or a longer timeout. - For 654, use a different LU name or a pool name, or ask the network team to free or define the LU. - For 655, match the SSL setting to the port. Most sites use one port for TLS and another for plain Telnet. - For 662 to 666, install the right signer certificate, connect with the name in the certificate, or ask the host team to renew an expired certificate. - After the connection is back, sign on again. Any screen you sent just before the check may not have reached the application. ### Read this error in WND Terminal For a visible X COMM message, [Explain this screen](https://wndsoftware.com/explain-this-screen) can help interpret the screen under IT’s configured AI policy. It does not establish the cause of a rejected host data stream. Preserve the exact status and follow the diagnostic steps in this article. [Try the WND Terminal sample with fictional data](https://wndsoftware.com/demo#launch) · [3270 emulator features and installation](https://wndsoftware.com/3270-terminal-emulator) · [5250 emulator for IBM i](https://wndsoftware.com/ibm-i) ### IBM documentation [IBM Personal Communications documentation](https://www.ibm.com/docs/en/personal-communications/15.0.0). Use the documentation for your installed release. This link opens the relevant IBM reference library or lookup instructions. Match the complete code and your installed product release; it is not a code-specific review. [Report a correction to WND Software](https://wndsoftware.com/contact#message). Technical reviewer: [Albert Joseph](https://wndsoftware.com/authors/albert-joseph), WND Software. Last reviewed 2026-10-18 by the WND Software team. Written in our own words. For the full reference, check IBM’s 3270 or 5250 display documentation. ### Related codes [X (input inhibited) Keyboard locked, input inhibited Read →](https://wndsoftware.com/mainframe-errors/3270-input-inhibited) [X SYSTEM Keyboard locked by the host Read →](https://wndsoftware.com/mainframe-errors/3270-x-system) [X clock Host or terminal busy, wait Read →](https://wndsoftware.com/mainframe-errors/3270-x-clock) [X -f Function not available Read →](https://wndsoftware.com/mainframe-errors/3270-x-minus-function) ### Questions #### What does COMM 659 mean? The Telnet TCP connection to the TN3270 server failed or was dropped. Check the address and port, then reconnect, and look for a firewall or idle timeout if it keeps happening. #### Why does COMM 657 stay on the screen? The emulator is still trying to set up the TCP/IP connection. If it never completes, the server may be unreachable, or an SSL session is pointed at a port that doesn't use SSL. #### Will pressing Reset clear a COMM check? No. The lock goes away only when the connection is working again. [3270 and 5250 screen errors](https://wndsoftware.com/mainframe-errors/terminal-errors) · [X clock](https://wndsoftware.com/mainframe-errors/3270-x-clock) · [X -f](https://wndsoftware.com/mainframe-errors/3270-x-minus-function) [← All error codes](https://wndsoftware.com/mainframe-errors/)