|Most of our boards have a 'Gateworks System Controller' implemented with an MSP430 MPU which emulates several I2C devices used for system control and monitoring (see [wiki:gsc]). The MSP430 is on our JTAG chain, along with the board specific CPU so that we can access both of these devices via JTAG. The MSP430 does not use standard JTAG and by default and the part comes up in 'Spy-By-Wire' mode where you must transition TRST and SRST properly to get the part into JTAG mode. Without doing so, it will 'break the chain'. Additionally, the MSP430 must be manually clocked when accessing its JTAG interface which the TDI signal is used for.
For these reasons the MSP430 must be the first device on the JTAG chain. On all Gateworks boards with a GSC a bypass resistor exists across the TDI and TDO pins of the GSC so that it can be optionally loaded with a 0ohm resistor to physically bypass the GSC from the JTAG chain. When this is done you can still access the rest of the chain, just not the GSC. The board will function normally you just wont be able to read/write GSC firmware via the GW16042 JTAG dongle and software. Refer the the individual product user manual for details about the resistor to bypass the GSC from the JTAG chain.