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Operation
3.4 Remote Command Interface Control
The PSM-500 Command Mode allows the use of an external controller or computer to monitor
and control the modem via a packet-based message protocol. This mode normally uses the RS–
485 connections allowing multiple modems (and other devices) to share the command link under
control of a single or multiple entities. An RS–232 connection is also usable for this application,
but lacks the RS-485's ability to work on a "party line" and is therefore limited to a single
controller and single modem, for example a computer to a modem. The packets use a unique
address for each controlled device, which is set using the modem's front panel. The message
packets themselves use a binary format for efficiency. The complete protocol is shown in
Appendix B.
The protocol consists of messages from the controller to the modem and response messages
from the modem back to the controller. The modem never initiates communications without
having first received a correctly addressed and formed message requiring a response.
Message packets to the modem can take two forms;
1. Messages requesting information in a response message or "Read";
2. Messages commanding a change in operating parameters or "Write".
Any write information is automatically saved to non-volatile memory and is still present on the
next power-up.
The packet of both incoming and outgoing messages take the same generic form. First are pad
and opening flag, then the destination and source addresses, followed by the command code
(and read or write mode), then necessary data. The message packet is closed with a closing flag
and check word to verify the packet integrity. The use of a source address allows multiple
controllers on a single control link.
3.4.1 System Unit Programming/Communications
The communications protocol is unique. This mode is termed "command mode" communications
in the following discussion and is normally accomplished via an RS–485 4-wire connection to the
modem at "Control" connector J6. Note that the transmit and receive pair of this interface are
separated to form a 4-wire basis. If a 2-wire connection is desired, the transmit A and B leads
may be connected to the Receive A and B leads respectively in the connector applied to J6.
This command mode communications protocol involves the sending of a standard message
packet from a controller requesting information or commanding a change. The PSM-500 modem
responds with a message packet containing the information or confirmation of change. The
Modem never initiates communications at any time except in response to a command or query
message from the station controlling devices.
The new features and capabilities of the PSM-500 modem over previous versions required
modification of the protocol such that older control programs would not work directly. The PSM-
500 however emulates the protocol used in the PSM-4900 and other M5 series modems within
the capabilities of those previous modems. Therefore, the PSM-500 may be placed into a system
with existing PSM-4900s and the control mechanisms do not have to be changed until newer
feature control is desired. This emulation capability also allows an SDMS type Ethernet interface
card designed for an M5 modem to be placed into an M500 series modem and still work. See
Appendix B for more information.
3.5 Modem Checkout
The following descriptions assume that the full system is in operation and that software is running
properly on the central processor.
Page 3-30
PSM-500/500L/500LT SCPC Satellite Modem
PSM-500/500L/500LT - Rev. 0.91
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